DETERMINING THE GROWTH PATTERN AND CONTROLLING FRUITING OF YOUNG
HIGHBUSH BLUEBERRIES
By
William Lindberg

A THESIS
Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of
Horticulture- Master of Science
2013

ABSTRACT
DETERMINING THE GROWTH PATTERN AND CONTROLLING FRUITING OF YOUNG
HIGHBUSH BLUEBERRIES
By
William Lindberg

Highbush blueberries grown in Michigan often require seven to ten years from planting
until peak fruit production. Preventing young plants from producing fruit for three years after
planting increases vegetative growth and reduces this time interval. Manual removal of floral
meristems is recommended, but labor intensive. Less costly methods to prevent fruiting are
needed. Studies were conducted across several years to determine the efficacy of plant
hormones in preventing fruiting and to determine the growth pattern of young highbush
blueberry cultivars currently being planted. The majority of shoot growth occurred from May to
July in the first and second growth flush. Floral buds were found on the first, second, and third
growth flushes, with the majority on the second flush in 2012 and on the first flush in 2013. The
pattern of floral bud distribution between growth flushes indicates a lengthy floral initiation time
interval. Foliar gibberellins (GA) applications reduced the number of floral buds up to 49%,
with the greatest reduction occurring when GA was applied multiple times from July to October.
Auxin applications showed inconsistent results. Cytokinin (BA) applications in the spring
reduced fruit set, but also reduced vegetative growth. Manual removal of floral structures
resulted in more vegetative shoots per plant and canopy growth compared to non-treated plants.

TABLE OF CONTENTS

LIST OF TABLES ................................................................................................................v
LIST OF FIGURES ..............................................................................................................vi
CHAPTER 1:
LITERATURE REVIEW ......................................................................................................1
Background .........................................................................................................................2
Factors Affecting Growth ...................................................................................................3
Shoot growth cycle .............................................................................................................6
Reproductive Development ................................................................................................6
Hormones ............................................................................................................................8
Photoperiod .........................................................................................................................11
Temperature ........................................................................................................................12
Fruit Thinning ....................................................................................................................12
LITERATURE CITED ......................................................................................................15
CHAPTER 2:
APPLICATION OF GA AND AUXIN IN YOUNG HIGHBUSH BLUEBERRY TO PREVENT
FLORAL INITIATION .........................................................................................................24
Abstract ..............................................................................................................................25
Introduction ........................................................................................................................25
Materials and Methods .......................................................................................................27
Study 1 ................................................................................................................................27
Study 2 ................................................................................................................................28
Study 3 ................................................................................................................................28
Study 4 ................................................................................................................................29
Results ................................................................................................................................29
Study 1 ................................................................................................................................30
Study 2 ................................................................................................................................30
Study 3 ................................................................................................................................30
Study 4 ...............................................................................................................................31
Discussion ..........................................................................................................................31
LITERATURE CITED .......................................................................................................35
CHAPTER 3:
IMPACT OF THINNING ON FRUIT SET AND VEGETATIVE GROWTH IN YOUNG
HIGHBUSH BLUEBERRIES ..............................................................................................39
Abstract ..............................................................................................................................40
Introduction ........................................................................................................................40
Materials and Methods ........................................................................................................41
Study 1 ................................................................................................................................42
Study 2 ................................................................................................................................42
iii

Study 3 ................................................................................................................................42
Results ................................................................................................................................43
Discussion ..........................................................................................................................44
LITERATURE CITED .......................................................................................................47
CHAPTER 4:
THE SHOOT GROWTH PATTERN AND DISTRIBUTION OF FLOWER BUDS IN YOUNG
HIGHBUSH BLUEBERRIES GROWN IN MICHIGAN ....................................................50
Abstract ..............................................................................................................................51
Introduction ........................................................................................................................51
Materials and Methods ........................................................................................................54
Study 1 ................................................................................................................................54
Study 2 ................................................................................................................................55
Results ................................................................................................................................55
Discussion ..........................................................................................................................57
LITERATURE CITED .......................................................................................................62
APPENDIX:
LITERATURE CITED ..........................................................................................................80

iv

LIST OF TABLES

Table 2.1: Effect of GA applied at 400 mg.L-1 during 2009 on floral meristem numbers in 2010.
Data are means of three cultivars (Aurora, Elliott, Liberty) and two GA forms (GA3, GA4+7)
................................................................................................................................................69
Table 2.2 ANOVA results for 2009 study based on differences in the number of flower buds per
plant in response to different effects ......................................................................................69
Table 2.3 Effect of GA4+7 application time and concentration (mg.L-1 a.i. as ProVide) during
2010 on plant development in 2011. Data are means of four cultivars (Elliott, Draper, Aurora,
Liberty). .................................................................................................................................69
Table 2.4 ANOVA results for 2010 study based on differences in the number of flower buds per
plant in response to different effects ......................................................................................70
Table 2.5: Effect of auxin applications on floral induction in ‘Liberty’ and ‘Chandler’ highbush
blueberries grown near Coopersville, MI ..............................................................................70
Table 3.1: Effect of thinning treatments on ‘Bluecrop’ highbush blueberry fruit set and canopy
growth in Holt, MI during 2012 .............................................................................................71
Table 3.2: Effect of thinning treatments on number of vegetative shoots in ‘Bluecrop’ highbush
blueberry grown in Holt, MI during 2012 .............................................................................71
Table 3.3: Effect of BA and carbaryl foliar sprays to individual shoots in May to June on fruit
set in ‘Jersey’ highbush blueberries grown in Holt, MI during 2012. ...................................72
Table 3.4: Effect of manual removal of flowers at different rates on 1 year old ‘Duke’ highbush
blueberries grown in Holt, MI in 2013 ..................................................................................72
Table 4.1: Shoot growth and floral bud numbers associated with the primary, secondary and
tertiary growth flushes of highbush blueberry cultivars grown in Coopersville, MI. (‘Liberty’)
or Holt, MI. (‘Duke’, ‘Draper’) in 2012 ................................................................................76
Table 4.2: Shoot growth and floral bud numbers associated with the primary, secondary and
tertiary growth flushes of highbush blueberry cultivars grown in Coopersville, MI. (‘Liberty’)
or Holt, MI. (‘Duke’, ‘Draper’) in 2013 ................................................................................76
Table 4.3: Shoot growth and floral bud numbers associated with the primary, secondary and
tertiary growth flushes during 2012 of ‘Bluecrop and Elliott’ highbush blueberry cultivars grown
in Holt, MI. by inspecting dormant branches ........................................................................77
Table 4.4: Growing degree days for 2012 and 2013 at weather stations in West Olive and Holt,
MI...........................................................................................................................................77
Table 4.5: Average maximum air temperatures from May to October in 2012 and 2013 at West
Olive, MI and at Holt, MI ......................................................................................................78
v

Table 4.6: Number of flower buds on primary shoots with or without subsequent growth flushes
in highbush blueberries grown at Coopersville, MI (Liberty) or Holt, MI (Duke, Draper) during
2012........................................................................................................................................78
Table 4.7: Number of flower buds on primary shoots with or without subsequent growth flushes
in highbush blueberries grown at Coopersville, MI (Liberty) or HTRC (Duke, Draper) during
2013........................................................................................................................................79

vi

LIST OF FIGURES

Figure 4.1: Diagram of the shoot growth cycle of highbush blueberries exhibiting (a) first
growth flush (dashed line) during June, (b) second growth flush (dotted line) during July, and (c)
third growth flush (dash dot line) during August. Solid line is previous year’s growth. Circles
are approximate position and numbers of flower buds on the first, second, and third growth flush.
................................................................................................................................................73
Figure 4.2: Timing of 2012 shoot growth within the first, second, or third flush in three young
highbush blueberry cultivars grown in Holt (Duke, Draper) or Coopersville, MI (Liberty) .
................................................................................................................................................74
Figure 4.3: Timing of 2013 shoot growth within the first, second, or third flush in three young
highbush blueberry cultivars grown in Holt (Duke, Draper) or Coopersville, MI (Liberty) .
................................................................................................................................................75

vii

CHAPTER 1:
LITERATURE REVIEW

1

Background:
Blueberries are members of the family Ericaceae and are closely related to lingonberry
(Vaccinium vitis-idaea), cranberry (V. macrocarpon), and sparkleberry (V. arboreum) (Song and
Hancock, 2011). Blueberries are a diverse group of many species of economic importance, such
as highbush (V. corymbosum), rabbiteye (V. ashei), and lowbush (V. angustifolium) blueberry
(Scherm et al., 2001). Domestic highbush blueberries originated through the breeding program
of Fredrick Coville at the start of the twentieth century (Moore, 1965). Significant crosses
between blueberry species have also been made. Crosses between highbush and lowbush
blueberry generated half-high blueberries (V. corymbosum x V. angustifolium), which can
survive in colder winter environments than highbush blueberries (Albert et al., 2010). Crosses
have also been made between the northern highbush blueberry and native southern blueberry
species, including V. darrowii, to produce southern highbush cultivars, which have low chill
requirements and are well adapted to conditions in the southern United States (Hancock, 2006;
Ratnaparkhe, 2007).
The demand for blueberries has steadily increased, as American consumption went from
0.18 pounds per capita in 1980 to 0.95 pounds in 2009 (USDA, 2012). Due to increased
demand, global production grew 30% from 2005 to 2010 (USDA, 2012). Blueberries have high
concentrations of antioxidants and are widely perceived as providing health benefits (Giacalone
et al., 2011; Yang et al., 2011). North America leads total production, but significant yields also
come from South America, Europe, Australia, and Asia (Brazelton and Strik, 2007).
The majority of domestic highbush blueberry production comes from the Pacific
Northwest, Michigan, and New Jersey (Finn et al., 2003; Strik and Yarborough, 2005). From
2010 to 2012 Michigan blueberry production averaged 89 million pounds, representing 35% of

2

United States production (USDA, 2012). However, 40% of Michigan’s acreage contains older
cultivars (USDA, 2012). With increasing market competition, growers may need to replant
fields with modern cultivars, which can yield higher quality and quantities of fruit. However,
young plants may take seven to ten years to reach maximum fruit production in Michigan’s
climate (Strik, 2007). If the establishment times were reduced, Michigan growers would be more
likely to replace old cultivars with improved varieties and increase their advantage over
competing regions. New management strategies are needed to decreases the time to full fruit
production of young highbush to make replanting with new cultivars feasible.

Factors Affecting Growth:
Highbush blueberry plants in other regions, like the Pacific Northwest, often grow faster
than those in Michigan. For example, three years after planting highbush blueberry plants of the
same cultivar in Oregon were 80% taller, 104% wider, and had a 36% higher survival rate than
plants grown in Michigan (Finn et al., 2003). Growth differences likely resulted from differing
environmental factors. (Finn et al., 2003; Hancock et al., 1992; Moon et al., 1987). Michigan
summer temperatures often exceed 30º C, while in Oregon temperatures rarely reach 30º C. The
photosynthetic capabilities of highbush blueberries are inhibited and carbon assimilates were
reduced 21 to 50% as temperatures increased from 20° C to 30º C (Hancock et al., 1992).
During a study by Finn et al. (2003) winter temperatures in Oregon remained fairly steady at 0º
C, while Michigan temperatures often fell under 0º C and to as low as -20º C. Winter injury can
significantly limit blueberry establishment (Moore, 1994). In addition, the length of the growing
season is different between these regions. Based on a growing degree model from March to
October, Michigan receives about 5% fewer growing degree days than Oregon (NWS, 2013).

3

Finn et al. (2003) also observed bloom occurring three weeks earlier in Oregon than in Michigan.
The combination of greater winter damage, decreased photosynthesis due to higher summer
temperatures, and a shorter growing season may account for reduced plant growth in Michigan.
Blueberries have a shallow fibrous root system, which runs parallel to the surface and
lacks root hairs (Gough, 1980). These traits reduce water absorption capabilities and make
blueberries sensitive to drought (Gough, 1980). Irrigation with drip lines, micro sprinklers, and
overhead irrigation are commonly used in blueberry production to promote growth (Bryla et al.,
2011).
Soils may also influence the growth of highbush blueberries. Unlike many plants,
blueberries grow best in highly acidic soils with a pH of 4.0 to 5.0 (Haynes and Swift, 1985). In
addition to soil pH, soil nitrogen levels can alter plant growth. Blueberries can access both
NH4+and NO3- from the soil and there is conflicting evidence on whether one form is preferred
(Peterson et al., 1988; Rosen et al., 1990). Soil microorganisms can also alter blueberry growth.
Plant parasitic nematodes are ubiquitous in nature and are a threat to agricultural crops (Lilley et
al., 2012). In apples (Malus domestica), replant disease can commonly occur, resulting in
stunted plant growth on a replanted site (Braun et al., 2010). Highbush blueberries planted on
previously cultivated soils had reduced root systems and plant growth, compared to those on
virgin soils (Blasing, 1988). Differences in growth were not attributed to soil nutrition, but may
have resulted from plant pathogens (Blasing, 1988).
Highbush blueberries may produce fruit within a year of planting (Strik and Buller,
2005). However, plants that are prevented from fruiting often produce more vegetative growth.
Highbush blueberry plants that had floral meristems removed during the first two years after
planting had 44% greater root mass and 60% greater shoot mass (Strik and Buller, 2005).

4

Southern highbush cultivars also produced more vegetative mass when fruiting was prevented
for the first two years after planting (Williamson and NeSmith, 2007).
Fruiting may reduce growth due to the allocation of photosynthates away from
vegetative sinks. Although fruits can photosynthesize, they depend on the other sources for the
majority of carbon assimilates (Birkhold et al., 1992). Young highbush blueberry plants allocate
over half of total photosynthates towards flowers and fruiting (Pritts and Hancock, 1985).
Vegetative growth, leaf area, and internal carbohydrate concentrations were reduced with
increasing floral bud density in southern highbush blueberries (Maust et al., 1999). Similar
trends probably occur in northern highbush blueberries.
Fruiting can also draw mineral nutrients away from vegetative growth. In Banksia
hookeriana, 44% of total nitrogen and 65% of total phosphorus were within reproductive
structures (Witkowski and Lamont, 1996). Similar competition probably exists in blueberries, as
both vegetative and reproductive development depends on stored nitrogen reserves (Birkhold et
al., 1992). In Michigan, highbush blueberries are often grown on sandy soils that are low in
organic material and cation exchange capacity (Hanson and Retamales, 1992). These soils have
increased leaching and a limited supply of available nutrients, which may increase competition
between vegetative and reproductive development. Increasing fertilization applications may not
help, due to the small root zone of young highbush blueberries and the possibility of killing roots
with high salt contents from extra fertilizer applications (Bañados et al., 2012).
To prevent the competition between vegetative and reproductive growth for nutrients and
photosynthates, the manual removal of floral buds is recommended (Pritts and Hancock, 1992).
However, this process is highly labor intensive and costly (Strik and Buller, 2005). A more
efficient method is needed to prevent early fruiting in highbush blueberries, which would reduce

5

the interval from planting to peak fruit production and allow growers the opportunity to plant
modern cultivars.
Shoot growth cycle:
Shoots in highbush blueberries can originate from two types of buds. Most shoot come
from axillary buds on the previous year’s growth, but some may develop from adventitious or
latent buds on the crown or roots of the plant (Bañados 2006; Gough et al., 1978). The shoot
growth pattern of highbush blueberry is episodic and sympodial and is similar to the American
elm (Ulumus americana) and lilac (Syringa vulgaris) (Gough et al., 1978). Each growth flush
terminates with the abortion of the apical meristem (Gough et al., 1976). Two to five weeks after
apical meristem abortion, a lateral meristem may commence shoot growth and form the next
growth flush (Gough et al., 1978). This cycle may repeat itself, with the majority of shoot
growth occurring in the late spring and early summer (Gough et al., 1976). The number of
growth flushes is dependent on cultivar, environmental conditions, and plant health (Gough et
al., 1976). Gough et al. (1976) found varieties that ripen fruit later in the season tended to have
fewer growth flushes than early fruiting varieties.
Reproductive Development:
Reproductive development starts with floral induction, a process where signals to
transition from a vegetative bud to a floral bud are produced and transported to the meristem
(Samach and Smith, 2013). After the signal is perceived, floral initiation occurs, which is the
transition from a vegetative meristem to an inflorescence meristem (Samach and Smith, 2013).
Based on histological examinations Gough et al. (1978) found the first morphological evidence
of floral initiation was the flattening of the meristem and formation of sepal primordial, which
occurred six to ten weeks after full bloom during the fruit harvest period (Gough et al., 1978; Ye

6

et al., 2005). Floral primordia development continued and by October all flower parts formed
(Gough et al., 1978). In March, meristematic division’s resumed as floral buds concluded their
development (Gough et al., 1978). Pollen grains formed in April and bloom occurred in May
(Gough et al., 1978). After pollination, blueberry fruit development follows a double sigmoidal
curve pattern; harvest of mature fruit occurs from July through September, depending on
environmental conditions and cultivar (Finn and Luby, 1986).
Floral induction and initiation are complex physiological processes, controlled by many
regulatory factors (Bowman et al., 2012). In Arabidopsis, floral induction is controlled in part by
FT genes, which produce proteins in the leaves that are transported and accumulate in the
meristem, where they interact with other factors, and lead to floral initiation (Taoka et al., 2011;
Turck et al., 2008). Once floral initiation occurs, floral identity genes, such as LFY, AP1, and
CAL, specify formation of floral organ formation (Piñeiro and Coupland, 1998).
In addition to genetic factors, other factors affect floral initiation. Arabidopsis mutants
unable to metabolize starch had delayed floral initiation, suggesting floral initiation and starch
metabolism may be related (Eimert et al., 1995). In avocado (Persea americana) the level of
floral initiation was dependent on endogenous carbohydrate levels (Scholefield et al., 1985).
Mineral nutrient levels also may alter floral initiation in plants. Excess nitrogen leads to
increases in floral initiation per stem area in rabbiteye blueberries (Darnell et al., 1992). In
tobacco (Nicotiana tabacum), floral initiation occurred once levels of nitrogen and carbohydrates
increased in the apical meristem (Rideout et al., 1992). In sunflower (Helianthus annuus),
nitrogen applications at the start of the floral initiation period increased the number of floral
meristems (Steer and Hocking, 1983). In highbush blueberries, a drought stress during the floral
initiation period resulted in fewer floral meristems (Mingeau et al., 2001).

7

Hormones:
Plant hormones are organic molecules that regulate activities in plants (Nemhauser et al.,
2006). Gibberellins (GA) are the largest class of plant hormones and include numerous forms
that differ in their tetracyclic dierpene structure (Iqbal et al., 2011). Floral initiation is promoted
by GA in some species, and repressed in others (Sharp et al., 2010). In many woody plants,
decreases in GA levels are required prior to floral initiation occurring (Sharp et al., 2010). In
biennial producing apple plants, seeds within developing fruit may inhibit floral initiation by
producing and secreting GA into the shoot (Dennis and Neilsen, 1999).
Application of GA can affect the number of flower buds formed. In Rhododendron the
application of GA prevented floral initiation, while inhibitors of GA synthesis increased the
process (Sharp et al., 2010). GA applications reduced floral initiation in apples, sweet cherries
(Prunus avium), mangos, (Mangifera indica) and apricots (Prunus armeniaca cv. Royal)
(Bradley and Crane, 1960; Luckwill, 1970; Turnbull et al., 1996; Lenahan et al., 2006). The
response of highbush blueberries to GA have varied from reduction in reducing the number of
flower buds 0 to 95% relative to non-treated plants (Retamales et al., 2000; Black and Ehlenfeldt,
2007). The application of paclobutrazol, which blocks GA production, increased the number of
floral meristems in highbush and lowbush blueberries (Lewis and Hu, 1993; Ehlenfeldt, 1998).
Several factors could explain the inconsistent results of experiments of highbush
blueberries to GA applications. In sour cherry leaves, GA was absorbed for only the first three
hours after application. The absorption rates was dependent on temperature, with greater
absorption at 25°C than at 15°C or 35°C (Knoche et al., 1992). Different temperatures during
the application of GA could change the absorption rates and thereby alter effectiveness.

8

Timing of GA applications may also alter plant response. GA applications did not
prevent floral development in Citrus sinensis after sepal primordia had formed (Lord and Eckard,
1987) or in Fuchsia hybrids after floral primordial were initiated (Sachs et al., 1967). The period
of floral initiation in highbush blueberries is unclear, but is thought to occur approximately six to
nine weeks after full bloom (Gough et al., 1978). Mainland and Eck (1969) applied GA to
highbush blueberries during full bloom and reduced the number of floral buds, while Black and
Ehlenfeldt (2007) found the greatest reduction in induction when GA was applied in September.
Retamales et al. (2000) also found reductions in the number of floral buds when applying GA 18
weeks after full bloom. An ideal application timing of GA for highbush blueberries is not
known.
The variable response of highbush blueberries to GA may also be a result of the
environment in which plants were grown, as greenhouse grown plants often show greater
response than field grown plants. Mainland and Eck (1969) found GA applications significantly
reduced the number of floral buds in greenhouse treated plants, but not in field grown plants.
Retamales et al. (2000) also found significant reductions in the number of flower buds in nursery
plants, but not in field grown plants. Mesquite plants (Prosopis juliflora), grown in greenhouses
had less cuticle development compared to those in the field, which likely resulted from changes
in ultra violet irradiation, soil moisture, humidity, or wind (Hull, 1958). Similar trends in leaf
cuticle development could exist in blueberries. This could alter GA absorption rates and account
for variable responses in previous studies.
Plant responses are also affected by the type of GA applied. There are over one hundred
GA forms found in plants, but only a portion are biologically active (MacMillian, 2002). GA
with hydroxylation of the C3 position and double bonds at the C(1,2) or C(2,3) were most

9

effective in preventing floral initiation in cherries (Oliveira and Browning 1993). In apples, GA4
had no effect or increased floral initiation, while GA3 or GA(4+7) prevented the process
depending on concentration and application timing (Li et al., 1995; Looney et al., 1985; Meador
and Taylor 1987; Tromp 1982). In Clerodendrum thomsoniae, GA3 prevented floral initiation
whereas GA7 had the opposite effect (Koranski et al., 1979). In highbush blueberries, GA3 and
GA(4+7) were both effective in preventing floral initiation (Retamales et al., 2000; Black and
Ehlenfeldt, 2007).
Auxins are another class of plant hormones that can influence floral initiation (Muday
and DeLong 2001). In pineapple (Ananas comosus) 0.001% auxin, applied prior to floral
initiation increased the number of floral meristems, whereas 0.1% prevented floral development
(Clark and Kearns, 1942). In Cocklebur (Xanthium) low auxin concentrations (50 mg.L-1) had
no effect on floral initiation, but high concentrations (500 mg.L-1) prevented the process (Bonner
and Thurlow, 1949). In Chenopodium rubrum auxin applied prior to inductive photoperiods
prevented floral initiation, while applications after inductive photoperiods increased initiation
(Seidlova and Khatoon 1976). In pecans (Carya illnoinensis) auxin applications prevented
pistillate flower formation and an auxin transport inhibitor increased floral initiation (Wood,
2011).
Plant hormones may also co-regulate floral initiation. The application of GA in apple and
bayberry (Morella pensylvanica) prevented floral initiation and increased the synthesis and
transport of auxin (Callejas and Bangerth, 1997; Li et al., 2003). Developing highbush blueberry
fruit treated with GA showedz a tenfold increase in endogenous auxin (Mainland and Eck 1971).

10

Floral initiation may require an optimal level of endogenous auxin, which can be altered by GA
applications (Li et al., 2003). Auxins may also regulate the production of GA. In peas (Pisum
sativum L.) application of an auxin increased the expression of a key enzyme in GA biosynthesis
(Ngo et al., 2002).
Photoperiod:
Photoperiod, the length of the light period, is an important signal for plants (Wilkie et al.,
2008). Plants are classified based on their response to photoperiod as either short day, long day,
or day neutral (Wilkie et al., 2008). Flowering of short day plants is promoted under long dark
periods, while short dark periods promote flowering in long day plants; flowering in day neutral
plants is not affected by photoperiod (Mockler et al., 1999).
Blueberries are considered short day plants (Darnell, 1991). Lowbush blueberries placed
under 16 h photoperiods produced no floral meristems, while those in 14 h or less photoperiods
did (Hall et al., 1961). In northern highbush blueberries, floral buds formed in 8 h photoperiods,
but not in 16 h photoperiods (Bañados and Strik, 2006). Southern highbush blueberries produced
floral meristems in 8 h photoperiods but not in 16 h or 8 h photoperiods with a night interruption
(Spann et al., 2003). In rabbiteye blueberries the effects of photoperiod on floral initiation was
cultivar specific and quantitative, not qualitative, showing that flower buds could form under
non-inductive photoperiods (Darnell, 1991). These studies demonstrate that in very short or long
photoperiods formation of floral buds is strongly regulated. However, day lengths under field
conditions in Michigan (latitude 43º N) do not reach these levels, but range from 15 to 13 h in
July and August when floral initiation is thought to occur. Short day photoperiods (< 11 hr) do
not occur in Michigan until October. Effects of photoperiod on highbush blueberry development
appear variable, with floral buds forming even under long day conditions potentially as a result

11

of other pathways co-regulating the process. Pescie et al. (2011) found that southern highbush
blueberries growing in Argentina initiate floral buds in both long (15 h) and short (8 h)
photoperiods.
Temperature:
Temperature can also alter floral development. Some plants, such as winter wheat,
flower in response to extended periods of cold temperatures, a phenomenon known as
vernalization (Simpson and Dean, 2002). Plants may respond differently to lowered ambient
temperatures during floral initiation periods. Grape (Vitis vinifera) plants grown at 30º C had
increased numbers of floral primordia, while those at 20º C produced none (Buttrose, 1969).
Both ambient temperature and photoperiod influenced floral initiation in short-day strawberries
(Fragaria ananassa) (Ito and Saito, 1962). Strawberry plants held at 9º C initiated floral
meristems, while plants at 30º C did not regardless of photoperiod (Ito and Saito, 1962). Plants
grown between 17º C and 24º C initiated floral meristems at photoperiods of 4 to 12 h, but did
not at photoperiods of 16 h or more (Ito and Saito, 1962).
Likewise, both photoperiod and temperature may regulate floral initiation in blueberries.
Lowbush blueberries in 11 h photoperiods initiated more floral meristems at 21º C, than at 10º C
(Hall and Ludwig 1961). Southern highbush blueberries in 8 h photoperiods initiated flower
buds at 21º C, but produced 50% fewer flower buds at 28º C (Spann et al., 2004). Variable
weather patterns could alter the timing of floral initiation from year to year. In Michigan’s
climate summer temperatures often reach 30° C and could suppress or shift the timing of floral
bud initiation.
Fruit Thinning:

12

Another strategy to prevent fruit production on young plants is to remove or thin flowers
and fruit. Plants may naturally abort flowers or fruits, depending on the species (Bangerth 2000).
A lack of carbohydrates may lead to fruit thinning, as apple trees thinned their crop load as sugar
levels fell below a threshold level (Beruter and Droz, 1991). However, other studies found no
relationship between carbohydrate reserves and thinning in apples (Abruzzese et al., 1995).
Another possibility is that changes in hormone concentrations result in thinning (Wertheim
2000). Auxin translocated from the leaf may prevent the activation of the abscission zone
(Bangerth 2000; Schroder et al., 2013). If auxin is not translocated, ethylene will activate the
abscission zone and lead to fruit or flowering abscission (Bangerth 2000). In a similar theory,
termed the auxin-auxin balance, auxin transported from specific regions of the leaf regulates
thinning (Schroder et al., 2013).
Artificial thinning is used in many crops to balance vegetative and reproductive growth.
In apple, thinning improves the size and quality of the remaining fruit, prevents alternate bearing,
and reduces harvest costs (Ebert and Bangerth, 1982; Schroder et al., 2013). Chemical thinning
can be accomplished by applying agents that damage floral structures or alter hormone levels
(Wertheim 2000; Williams 1994). Endothall, carbaryl, and hydrogen cyanamide reduced fruit
numbers in apples (Greene 2002; Williams et al., 1995; Fallahi et al., 1992). Hydrogen
cyanamide, cytokinin, and carbaryl reduced fruit set in rabbiteye blueberries (Williamson and
NeSmith, 2007; Cartagena et al., 1994), while in highbush blueberries ammonium thiosulfate,
BA, and N-(2-chloro-4-pyridyl)-N-phenyl urea reduced fruit set, but also inhibited vegetative
growth (Koron and Stopar, 2004). Several factors can affect the efficacy of thinning agents,
including application coverage, temperature, humidity, concentration, and plant developmental
stage.

13

Finding more efficient methods to prevent early fruiting and increase plant growth is
important for Michigan’s blueberry industry. Early fruit production can result in the
transmission of pollen borne viruses, reduced plant size and vigor due to competition for
photosynthates and nutrients, and an increased time period until peak fruit production. Inhibiting
the transition from a vegetative to floral meristem or removing flowers or fruit may be viable
options to increase plant growth of young highbush blueberries.

14

LITERATURE CITED

15

LITERATURE CITED

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CHAPTER 2: APPLICATION OF GA AND AUXIN IN YOUNG HIGHBUSH
BLUEBERRY TO PREVENT FLORAL INITIATION

24

Abstract:
Preventing young blueberry (Vaccinium corymbosum L.) plants from fruiting can increase their
vegetative growth. Gibberellins (GAs) and auxins have prevented floral induction in other
species. The purpose of this work was to determine the inhibitory effects on floral initiation with
GAs or auxins in new plantings of highbush blueberries. Foliar sprays were applied in several
experiments with different application intervals and concentrations; efficacy was determined by
counting the number of floral meristems formed. Two GA forms (GA3, GA4+7) and
concentrations (200, 400 mg.L-1) were equally effective in preventing floral induction. GA
applications in July and August were more inhibitive than those in September and October. Four
or eight sprays applied at 1 to 2 week intervals was effective to inhibit initiation by as much as
49% compared to non-treated controls. Auxin treatments did not consistently reduce floral
initiation.

Introduction:
The global production of highbush blueberries (Vaccinium corymbosum) has increased
400% from 1975 to 2004 (Brazelton and Strik 2007). In short season growing regions like
Michigan, blueberry plants may not reach full fruit production for 7 to 10 years after planting
(Finn et al.,2003; Strik, 2007). Slow establishment rates deter growers from removing older
plants, while newer cultivars may produce higher yields and superior fruit (Hancock et al., 2008).
Growth of new plants can be accelerated by preventing fruit production, as young
highbush blueberries allocate over 50% of photosynthates to fruiting (Pritts and Hancock, 1985).
Manually removing blueberry floral buds for two years after planting increased shoot length and
mass (Strik and Buller, 2005) and reduces the exposure of plants to pollen borne diseases
25

(Bristow and Martin, 1998). Manual removal of flower buds is recommended (Pritts and
Hancock, 1992), but is labor intensive.
Gibberellin (GA) applications reduced floral induction in apples, sweet cherries, and
mangos (Lenahan et al., 2006; Lobos and Yuri, 2006; Luckwill, 1970; Turnbull et al., 1996), but
the response of blueberries has been variable and cultivar dependent, ranging from 0-95% floral
inhibition (Black and Ehlenfeldt, 2007; Retamales et al., 2000). Previous studies in blueberries
indicated that floral induction was inhibited by GA3 and GA4+7 at concentrations of 150 to 400
mg.L-1 (Black and Ehlenfeldt, 2007; Retamales et al., 2000). Single applications inhibited floral
induction (Retamales et al., 2000), but multiple sprays applied over several weeks were most
inhibitive (Black and Ehlenfeldt, 2007).
GAs are expected to be most inhibitory when applied prior to floral initiation.
Applications did not prevent floral development in Citrus sinensis after sepal primordia had
formed (Lord and Eckard, 1987). Blueberry floral initiation is thought to occur from July to
September (Bañados and Strik, 2006; Gough et al., 1978), but the timing may be influenced by
shoot growth. Blueberries produce episodic and sympodial vegetative flushes. Tamada (1997)
concluded from anatomical changes that floral induction on primary shoots of ‘Jersey’ bushes in
Japan (latitude 35.6 N) progressed for two to six weeks after apical abortion, which corresponded
to mid-July to the end of August (day length of 14.1 to 12.8 h). If this model is true for all
shoots, floral induction could occur from July to October on multiple growth flushes.
However, floral initiation is also influenced by environmental factors. In growth chamber
studies, floral induction was strongly suppressed by 14 to 16 h photoperiods (Hall et al., 1963;
Bañados and Strik, 2006). The amount of floral initiation was proportional to the duration of

26

exposure to short days (Darnell, 1991) and was suppressed by high temperatures (Spann et al.,
2004).
Auxin may also affect floral initiation alone or in combination with GAs. In pineapple
(Ananas comosus) low concentrations of auxin (10 mg.L-1) increased floral initiation, whereas
high concentrations (1,000 mg.L-1) prevented it (Clark and Kearns, 1942). High concentrations
of auxin also prevented floral development in cocklebur (Xanthium) (Bonner and Thurlow,
1949). In pecans (Carya illnoinensis) auxin applications prevented normal floral initiation by
disrupting carpel formation (Wood, 2011). In apple and bayberry, GAs prevented floral initiation
and increased the synthesis and transport of auxin (Callejas and Bangerth, 1997; Li et al., 2003).
Developing highbush blueberry fruit treated with GAs contained ten times more endogenous
auxin than non-treated fruit (Mainland and Eck, 1971). Auxins may also regulate the production
of GA in some species (Ngo et al., 2002). In summary, both GA and auxin have shown to inhibit
floral initiation in some species, while the results regarding young highbush cultivars currently
being planted remain absent. The purpose of this work was to determine the inhibitory properties
of GAs and auxins in preventing floral initiation in young highbush blueberries currently being
widely planted and to determine an ideal application interval and concentration.

Materials and Methods:
Study 1
Commercial blocks of Vaccinium corymbosum cultivars ‘Elliott’, ‘Liberty’, and ‘Aurora’
plants established in 2009 in Gobles, MI (42 º22’ N, 85 º54’ W) were used. Plots containing five
plants of similar size were assigned one of seven treatments, with six replicates in a randomized
complete block design. Treatments consisted of a water-sprayed control and GA4+7 (ProVide,

27

Valent BioSciences Corp. Libertyville, IL) or GA3 (ProGibb, Valent BioSciences Corp.
Libertyville, IL) applied early (10 and 17 Aug. 2009), mid-season (26 Aug. and 3 Sept.), or late
(13 and 21 Sept.). Sprays were applied at 400 mg.L-1 active ingredient (a.i.) with a handheld
sprayer to the point of runoff. The number of floral buds per plant was recorded once they
began to swell in April 2010.
Study 2
This experiment was conducted in commercial fields near Lacota, Michigan (42° 24' N,
86° 07' W) that were planted in 2008 (‘Elliott’) or 2009 (‘Draper’, ‘Aurora’, ‘Liberty’). Plants
with similar size and vigor were assigned one of six treatments, in a randomized complete block
design with eight single-plant replicates. Treatments were a water-sprayed control and 400 mg.L1

a.i. GA4+7 applied at two week intervals either early (21 July. to 1 Sept, 2010), late (8 Sept. to

20 Oct., 2010), or early and late (21 July to 20 Oct. 2010). The sixth treatment consisted of
sprays on the early and late dates at 200 mg.L-1 a.i. All treatments were applied to the foliage
until the point of runoff.
Flower bud number per plant was recorded in April, 2011. On 13 May 2011, the length
of dead branches was recorded as a measure of winter cold injury and the number of flowers per
bud was determined on five randomly selected buds per plant. On 7 July 2011, berry number per
bush was recorded and bush height and width in the least and greatest dimensions were
measured. Canopy volume was estimated as the product of these dimensions.
Study 3
A three-year-old planting at Michigan State University Horticultural Teaching and
Research Center in Holt, Michigan (42º 67’ N, 84º 48’ W) was used. Three treatments were
replicated on eight single-bush plots: 1) non-treated control; 2) GA4+7 at 400 mg.L-1 applied
28

weekly from 25 July to 30 Aug. 2011 (early); 3) GA4+7 at the same rate applied weekly from 2
Sept. to 9 Oct. 2011 (late). Treatments were repeated in separate sections of ‘Liberty’ and
‘Draper’. Flower buds per plant were counted in Nov. 2011 after leaves abscised. Average shoot
length was also recorded by randomly selecting one main branch per plant and measuring the
length of each shoot.
Study 4
Commercial fields of ‘Liberty’ and ‘Chandler’ planted in 2011 and 2012 near
Coopersville, MI (43º 06’ N, 85º 93’ W) were used. Plants of similar size and vigor were
assigned one of nine treatments with six single-plant replicates in a randomized complete block
design. Treatments were a nontreated control and 1-naphthalenacetic acid (Fruitone, AMVAC,
Los Angeles, CA) applied at biweekly intervals early (14 July to 11 Aug. 2012) or late (25 Aug.
to 22 Sept. 2012) at 0.2, 2.0, 20, or 200 mg.L-1 a.i. All treatments were applied with a hand held
sprayer to the foliage until the point of runoff. The number of floral buds per plant was recorded
as previously described. Plants with obvious deer browsing injury were excluded during data
collection.
Data from all studies was analyzed with SAS 9.3 (SAS Institute, Cary, NC) as
randomized complete block designs with cultivar as the main factor and form and/or timing as
sub-plot factors. PROC Glimmix was used to determine statistical significance for floral
meristems and flowers per meristem, while PROC Mixed was used to determine significance for
shoot length and canopy volume. When significant interactions (P<0.05) were found, means
separation was performed with PDIFF in LSMEANS statement.

Results:

29

Study 1
A study was performed to determine the efficacy of different GA forms and applications
intervals on the inhibition of floral initiation in several highbush blueberry cultivars. Flower bud
numbers were affected by GA application time, (Table 2.1) but not by GA form (GA3 or GA4+7)
or the interactions between GA time, form, or cultivar (Table 2.2). All GA application times
reduced flower bud number relative to the control, but the middle timing (26 Aug., 3 Sep.)
provided a greater inhibition (43% reduction) than the early timing (22%) or late timing (21%).
Study 2
From the results of Study 1 a subsequent study was performed to determine if longer GA
application intervals would lead to greater reductions in floral initiation in recently planted
highbush blueberry cultivars. In addition, possible differences in response between GA
concentrations was also tested. The time of GA application affected flower bud and berry
numbers, while GA rate affected the number of flowers per bud (Table 2.3). The effects of
concentration and all interactions between cultivar, time, and concentration were not significant
(Table 2.4). The greatest reductions in flower bud and berry numbers resulted from earlier
applications (49% reduction) and early plus late timings (42%). The 400 mg.L-1 rate reduced the
number of flowers per bud relative to the control. Treatments did not affect the length of dead
branches per bush (overall mean 32 cm) or canopy volume (39 m3).
Study 3
Based on the results from study 2, a subsequent study was performed to determine if
greater frequency of GA applications would affect the suppression of floral initiation in highbush
blueberry cultivars. The analysis of variance indicated that flower bud numbers were
significantly reduced by weekly applications of GA4+7 in July and August (121 per plant)

30

compared to numbers on non-treated plants (218) and those treated in September and October
(194). The interaction of GA application time and cultivar was not significant, indicating that
‘Liberty’ and ‘Draper’ responded similarly. Average shoot length (8.4 cm) was not affected by
the treatments.
Study 4
Flower bud numbers were significantly affected by the interaction of cultivar, application
timing, and concentration (Table 2.5). Auxin applications had inconsistent effects on flower bud
numbers. The highest rate of auxin applied from August to September decreased the number of
flower buds in ‘Liberty’, (10 per plant) but not in ‘Chandler’ (19) relative to the controls. Other
timings and concentrations did not reduce floral induction.

Discussion:
These results are similar to those reported by Black and Ehlenfeldt, 2007 and Retamales
et al. 2004 in that GAs inhibit floral initiation in highbush blueberries. Although two sprays at
two week intervals reduced flower bud numbers (Table 2.1), maximum inhibition (49%) was
achieved from eight applications applied from late July through October (Table 2.3). Weekly
sprays (Study 3) did not reduce flower buds more than bi-weekly applications over a similar time
frame. Black and Ehlenfeldt (2007) also found multiple applications from July through
September most effective, but were able to achieve as high as 95% inhibition. Cultivars
responded similarly to GA, even though they ranged in harvest times from relatively early
(‘Draper’) to very late (‘Aurora’, ‘Elliott’).
Timing of GA sprays affected floral initiation in each study. Highbush blueberries are
short day plants; floral induction was inhibited by 14-16 h photoperiods and promoted in 10 h

31

photoperiods (Hall et al., 1963; Bañados and Strik, 2006). Black and Ehlenfeldt (2007) suggested
that optimum timing for GA may be during inductive photoperiods. However, in southern
highbush blueberries, floral initiation occurred in both long and short day photoperiods (Pescie,
2011). In current studies, photoperiods were longer than 14 hr until 7 Aug. and 1 Aug. (Black
and Ehlenfeldt; 2007). Although treatments included multiple sprays over different time periods,
the most effective was mid-August to early September (Tables 2.1, 2.3), consistent with previous
reports (Black and Ehlenfeldt, 2007; Retamales et al., 2000). Interestingly, GA applied prior to
inductive day lengths also reduced flower bud numbers (Black and Ehlenfeldt, 2007; Retamales
et al., 2000). In addition, greenhouse grown blueberries treated with GA during bloom (to induce
parthenocarpy) produced fewer flower buds (Mainland and Eck, 1969). GA sprays could result
in elevated tissue levels and activity for several weeks after application (Samach and Smith,
2013). The impact of photoperiods ranging from 14 to 12 h on highbush blueberries floral
initiation is somewhat unclear, but may partially control floral initiation in field conditions.
The fact that multiple GA sprays over several weeks provided the greatest reduction in
flower bud numbers suggests that induction occurs over a lengthy time period. Blueberries can
produce multiple shoot flushes, each terminating in apical abortion. In Michigan, most primary
flushes terminate growth in June, while later flushes may grow into September, depending on the
cultivar, environment, and plant health (Gough et al., 1978). Tamada (1997) suggested that
blueberries initiate flower buds only for two to six weeks after apical abortion. If this is true,
plants may initiate flower buds from July to October, depending on the timing of growth flushes.
It is unclear how photoperiod affects this timing, since many primary shoots terminate growth in
June, several weeks before photoperiods become inductive.

32

GA3 and GA4+7 were equally effective in preventing floral initiation (Table 2.2), which
is consistent with earlier blueberry studies (Retamales et al., 2000; Black and Ehlenfeldt 2007).
Over 100 forms of GAs occur naturally, but only a few are biologically active (MacMillan,
2002). GA structures with a hydroxylation of the C3 position and double bonds of C(1,2) or
C(2,3) were most effective at preventing floral induction in cherry (Oliveira and Browning 1993).
GA3 and GA7 have a C3 hydroxylation and C-1,2 double bond, while GA4 has C3 hydroxylation
(Oliveira and Browning 1993).
Concentrations of 200 or 400 mg.L-1 GA were effective in preventing floral initiation
(Table 2.4). This is supported by Retamales et al. (2000) who found rates of 150 or 300 mg.L-1
equally effective. Black and Ehlenfeldt (2007) found rates of 400 mg.L-1 more effective than
200 mg.L-1, but no different than 600 mg.L-1. In apples, inhibition of floral initiation was
dependent on GA concentrations in a linear manner (Greene, 1993).
In our preliminary study, auxin applications did not consistently reduce floral initiation
(Table 2.5). In ‘Liberty’ the highest rate applied from August to September reduced floral
initiation compared to the non-treated control, but other rates and timings had no effect, and
‘Chandler’ showed no floral inhibition to any auxin treatment. It is unclear why auxin had
minimal effect on induction in this study, but substantial effects were observed in other species
(Clark and Kearns, 1942; Bonner and Thurlow, 1949). Salisbury (1955) suggested that auxin
needs to be applied prior to inductive photoperiods to prevent floral initiation. In the current
study, photoperiods were longer than 14 h during the early treatment, but shorter than 14 h
during the late treatment period.
33

In summary, these studies indicate that GA partly inhibits floral initiation in young
highbush blueberries currently being widely planted. The effect is consistent across several
cultivars, but multiple sprays over several weeks are needed for maximum effect. Four to eight
applications may be impractical since GA products are relatively expensive. The growth benefits
from partially inhibiting fruiting needs to be determined. This preliminary study indicated that
auxin did not inhibit flowering, but further studies are needed to determine its commercial
potential.

34

LITERATURE CITED

35

LITERATURE CITED

Bañados, M.P. and B.C. Strik. 2006. Manipulation of the annual growth cycle of blueberry using
photoperiod. Acta Hort. 715:65-72.
Black, B. L. and M. K. Ehlenfeldt. 2007. Foliar applications of GA4+7 reduce flowering in
highbush blueberry. HortScience 42:555-558.
Bonner, J. and J. Thurlow. 1949. Inhibition of photoperiodic induction in xanthium by applied
auxin. Bot. Gaz. 110:613-624.
Brazelton, D. and B. C. Strik. 2007. Perspective on the U.S. and global blueberry industry. J
Amer. Pomol Soc. 61:144-147.
Bristow, P. R. and R. R. Martin. 1998. Transmission and the role of honeybees in field spread of
blueberry shock Ilarvirus, a pollen-borne virus of highbush blueberry. Virology 89:124130.
Callejas, R. and F. Bangerth. 1997. Is auxin export of apple fruit an alternative signal for
inhibition of flower bud induction? Acta Hort. 463:271-277.
Clark, H. E. and K. R. Kerns, 1942. Control of flowering with phytohormones. Science 95:536537.
Darnell, R. L. 1991. Photoperiod, carbon portioning, and reproductive development in rabbiteye
blueberry. J. Amer. Soc. Hort. 116:856-860.
Finn, C.E., J.F. Hancock, T. Mackey, and S. Serce. 2003. Genotype x environment interactions
in highbush blueberry (Vaccinium sp L.) families grown in Michigan and Oregon. J.
Amer. Soc. Hort. Sci. 128:196-200.
Greene, D. W. 1993. Effects of GA4 and GA7 on flower bud formation and russet development
on apple. J. Hort. Sci. 68:171-176.
Gough R. E., V. G. Shutak and R. L. Hauke. 1978. Growth and development of highbush
blueberry II Reproductive Growth, Histological Studies. J. Amer. Soc. Hort. Sci.
103:476-479.
Hall, I.V., D. L. Craig, and L. E. Aalders. 1963. The effect of photoperiod on the growth and
flowering of the highbush blueberry (Vaccinium corymbosum L.). Proc. Amer. Soc.
Hort. Sci. 82:260-263.

36

Hancock, J. F., P. Lyrene, C. E. Finn, N. Vorsa, and G. A. Lobos. 2008. Blueberry and
cranberry. In Hancock J.F (ed). temperate fruit crop breeding: germplasm to genomics.
Kluwer Academic Publishers, Dordrecht, The Netherlands.
Li, X., S. Li, and J. Lin. 2003. Effect of GA3 spraying on lignin and auxin contents and the
correlated enzyme activities in bayberry (Myrica rubra Bieb.) during flower-bud
induction. Plant Sci. 164:549-556.
Lenahan, O.M., M.D. Whiting, and D.C. Elfving. 2006. Gibberellic acid inhibits floral bud
induction and improves bing sweet cherry fruit quality. HortScience 41:654-659.
Lobos, G. A. and J. A. Yuri. 2006. Flower induction and differentiation of four apple cultivars in
Chile. Agr. Tecnica 66:141-150.
Lord, E. M. and K. J. Eckard. 1987. Shoot development in citrus sinensis L. (Washington Navel
Orange). II. alteration of developmental fate of flowering shoots after GA3 treatment.
Bot. Gaz. 148:17-22.
Luckwill, L.C. 1970. The control of growth and fruitfulness of apple trees. In: Luckvill, L. C.,
Cutting, C. V. (Eds.), Physiology of tree crops, Academic Press, Nueva York, EEUU.
MacMillan, J. 2002. Occurrence of gibberellins in vascular plants, fungi, and bacteria. J. Plant
Growth Regulat. 20:87-442.
Mainland, C. M. and P. Eck. 1971. Endogenous auxin and gibberellin-like activity in highbush
blueberry flowers and fruit. J. Amer. Soc. Hort. Sci. 96:141-145.
Ngo, P., J. A. Ozga, and D. M. Reinecke. 2002. Specificty of auxin regulation of gibberellin 20oxidase gene expression in pea pericarp. Plant Mol. Bio. 49:439-448.
Oliveira, C. M. and G. Browning. 1993. Gibberellin structure-activity effects on flower
initiation in mature trees and on shoot growth in mature and juvenile Prunus avium. Plant
Growth Regulat. 13:55-63.
Pescie, M., M. Lovisolo, A. Magistris, B. Strik, and C. Lopez. 2011. Flower bud initiation in
southern highbush blueberry cv. O’Neal occurs twice per year in temperate to warmtemperate conditions. J. Applied Hort. 13:8-12.
Pritts, M. P. and J. F. Hancock. 1985. Lifetime biomass portioning and yield component
relationships in the highbush blueberry, Vaccinium corymbosum L. (Ericaceae). Amer.
J. of Bot. 72:446-452.
Pritts, M.P. and J. F. Hancock (eds.) 1992. Highbush blueberry production guide. NRAES-55,
Ithaca, N.Y.

37

Retamales, J. B., E. J. Hanson, and M. J. Bukovac. 2000. GA3 as a Flowering Inhibitor in
Blueberries. Acta Hort. 527:147-151.
Salisbury, F. B. 1955. The dual role of auxin in flowering. Plant Physiol. 30:327-338.
Samach, A. and H. M. Smith. 2013. Constraints to obtaining consistent annual yields in
perennials. II: environment and fruit load affect induction of flowering. Plant Sci. 207:
168-176.
Spann, T., M., J. G. Williamson, and R. L. Darnell, 2004. Photoperiod and temperature effects
on growth and carbohydrate storage in southern highbush blueberry interspecific hybrid.
Amer. Soc. Hort. Sci. 129:294-298.
Strik, B. C. 2007. Horticultural practices of growing highbush blueberries in the ever-expanding
U.S. and global scene. J. Amer. Pomol. Soc. 61:148-150.
Strik, B. and G. Buller. 2005. The impact of early cropping on subsequent growth and yield of
highbush blueberry in the establishment years at two planting densities is cultivar
dependent. HortScience 40:1998-2001.
Tamada T. 1997. Flower bud differentiation of highbush and rabbiteye blueberries. Acta Hort.
446:349-356.
Turnbull, C. G., K. L. Anderson, and E. C. Winston. 1996. Influence of gibberellins treatment
on flowering and fruiting patterns in mango. Aust J. Exp. Agric. 36:603-611.
Wood, B. W. 2011. Influence of plant bioregulators on pecan flowering and implications for
regulation of pistillate flower initiation. HortScience 46:870-877.

38

CHAPTER 3: IMPACT OF THINNING ON FRUIT SET AND VEGETATIVE
GROWTH IN YOUNG HIGHBUSH BLUEBERRIES

39

Abstract:
Highbush blueberries often require seven to ten years to reach full fruit production in
Michigan’s climate. Thinning fruit has been used in other plants to balance reproductive and
vegetative growth. The purpose of this study was to determine the efficacy of removing flowers
and fruit to promote vegetative growth in highbush blueberries. Experiments were performed in
2012 and 2013 and the effect on fruiting and vegetative growth was measured. In 2012, foliar
applications of 6-benzyladenine (BA) reduced fruit set, but also decreased the number of
vegetative shoots relative to the control, while applications of carbaryl had no affect; manually
thinning reduced fruit set and doubled the number of shoots and canopy growth. In 2013,
manually thinning treatments of 0, 50, or 100% of flower buds did not affect shoot growth.
Flower thinning can be used to enhance vegetative growth in young highbush blueberries, but the
impact is also dependent on how heavily plants are fruiting. Further studies are needed to
determine whether other agents can reduce fruit set without adversely affecting vegetative
growth.

Introduction:
Highbush blueberries can produce fruit within a year of planting, but often do not reach
maximum fruit production for an additional six to nine years (Strik, 2007; Finn et al., 2003;
Black and Ehlenfeldt, 2007). Preventing fruiting on young plants can increase growth, decrease
the exposure of plants to pollen borne diseases, and reduce the interval from planting to peak
fruit production (Strik and Buller, 2005; Pritts and Hancock, 1992). Highbush blueberry plants
that had their floral meristems manually removed for the first two seasons after planting had 44%
greater root mass and 60% greater shoot mass (Strik and Buller, 2005). Added growth may
40

result from a reduction in competition for carbon assimilates, as fruiting accounts for half of
available photosynthates in young highbush blueberries (Pritts and Hancock, 1985). The removal
of floral meristems on young plants is recommended (Pritts and Hancock, 1992), but highly labor
intensive (Strik and Buller, 2005). More efficient strategies are desired to prevent fruiting in
young highbush blueberries.
Fruit thinning is a complex process involving both environmental and internal factors
(Dennis 2000; Schroder et al., 2013). Plants may abort a portion of their flowers depending on
environmental conditions (Bangerth 2000). The mechanisms controlling thinning are unclear,
but may involve assimilate supply, sink activity, seed number, ethylene production, and polar
auxin transport (Schroder et al., 2013). Artificial thinning, by either chemical or mechanical
means, has been used to balance vegetative and reproductive growth in other plants (Schroder et
al., 2013). In apples, thinning improves the size and quality of the remaining fruit and prevents
alternate bearing cycles (Ebert and Bangerth, 1982). Chemical treatments of hydrogen
cyanamide, cytokinin, and carbaryl successfully reduced fruit set in rabbiteye blueberries
(Williamson and NeSmith, 2007, Cartagena et al., 1994), while in highbush blueberries
ammonium thiosulfate, BA, and N-(2-chloro-4-pyridyl)-N-phenyl urea reduced fruit set, but also
inhibited vegetative growth (Koron and Stopar, 2006). The purpose of this study was to
determine the efficacy of different agents for removing flowers and fruit and promoting
vegetative growth on highbush blueberries.

Materials and Methods:
All studies were conducted at Michigan State University’s Horticultural Teaching and
Research Center in Holt, Michigan (42º 67’ N, 84º 48’ W).

41

Study 1
Study one was conducted in 2012 using four-year-old ‘Bluecrop’ plants of similar size
and vigor. A completely randomized block design was used, which included four treatments
with eight single bush replicates. Treatments included foliar sprays of BA (MaxCell, Valent
Bioscience Corp., Libertyville, IL) at 400 mg.L-1 or carbaryl (Sevin SL, Bayer Environmental
Sciences, Research Triangle Park, NC) at 800 mg.L-1. Sprays were applied with handheld
sprayer till the point of runoff. Other treatments included a non-treated control and a manual
removal (hand thinning) of all fruiting structures. Plants were treated on 24, May, about two
weeks after petal fall.
To measure fruit set, flowers were counted on four randomly selected shoots per plant on
2, April and viable berries were counted on the same branches on 18, June, just before maturity.
Fruit set was calculated by dividing berry number by flower number. The number of vegetative
shoots per plant was counted on 13, June and 21, Aug. Canopy volume growth was estimated by
recording the height and width of the plants in the greatest and smallest dimensions to the nearest
inch on 22, May and 21, Aug.
Study 2
Study 2 was conducted in 2012 using mature plants from the cv. ‘Jersey’. Experimental
units consisted of 10 to 30 cm long branches, which had flower buds. Seven uniform branches
were randomly selected on each of ten bushes, and seven treatments were assigned to one branch
on each bush. Treatments included carbaryl, BA, and a non-treated control; all applications were
applied as previously described on 17, 24, 30 May, with petal fall on 14, May. Fruit set was
determined as previously described on 18, June.
Study 3

42

Study 3 was performed in 2013 with one-year-old ‘Duke’ plants of similar size and vigor.
Plants were placed into a randomized complete block design, consisting of three treatments and
eight single plant replicates. Treatments were a non-treated control, a 50% hand thinning of all
floral structures, or a 100% hand thinning of floral structures applied on 14, May once floral
buds had opened. Netting was placed over all bushes to prevent bird damage. Berries were hand
harvested, counted, and weighed on 8, 14, and 22, of July. The number and length of vegetative
shoots per plant were recorded on 26, Aug.
Data from all studies were analyzed with SAS 9.3 (SAS Institute, Cary, NC), as a
randomized complete block design. For study 1 and 3, treatment was the main factor and for
study 2 treatment was the main factor and timing was a sub-plot factor. PROC Glimmix was
used to determine statistical significance for fruit set, number of vegetative shoots, and berry
number, while PROC Mixed was used to determine statistical significance for canopy volume,
fruit weight, and shoot length. When treatment effects were significant based on ANOVA table
(P<0.05), differences between treatment means were analyzed by LSMEANS test.

Results:
Experiments were performed to determine the efficacy of thinning treatments to reduce
fruit set in highbush blueberries. Hand thinning and BA treatments significantly reduced fruit set
compared to the control, while carbaryl did not (Table 3.1). Canopy growth under hand thinning
treatment was significantly greater compared to all other treatments, an approximate 150%
increase compared to the control. Treatments also significantly affected the number of
vegetative shoots per plant (Table 3.2). BA treatment resulted in a reduction in the number of
vegetative shoots per plant compared to all other treatments on 13, June. Hand thinning resulted

43

in an increase in the number of vegetative shoots per plant immediately after treatment and a
near doubling at the end of the experiment compared to control.
Study 2 was performed to determine the efficacy of thinning treatments applied to
individual shoots at different time intervals. Treatment timing, form, and the interaction of
timing and form did not significantly affect fruit set for study 2 (Table 3.3).
Study 3 was performed to determine the effect of different thinning rates on the
vegetative growth of young highbush blueberries. For study 3 treatments affected the number of
berries per plant, but not the number of shoots, shoot length, or average berry weight (Table 3.4).
Plants receiving the 100% thinning treatment had significantly fewer berries per plant than those
under the 50% thinning or non-treated control.

Discussion:
In 2012, BA significantly reduced fruit set when applied to the entire canopy and
decreased the number of shoots per plant (Tables 3.1 and 3.2). These results are similar to those
of Koron and Stopar (2006), who found that two forms of cytokinins, BA and CPPU, reduced
fruit set and stunted shoot growth in highbush blueberry. Koron and Stopar (2006) also used
similar timings and concentrations of BA and found that concentrations of 200 mg.L-1 BA
effectively reduced fruit set, but had negative impacts on growth. At concentrations of 20 mg.L-1
BA had no effect on either fruit set or shoot growth. It appears that the act of stressing the plant
to induce thinning, also negatively impacts shoot growth and thereby negates possible benefits
for increased shoot growth.
Applications of cytokinins to rabbiteye blueberries in similar concentrations as the
current study reduced fruit set, without inhibiting vegetative growth (Cartagena et al., 1994).

44

Differences in response may have resulted from species specific reactions or bud break patterns.
Vegetative buds in highbush blueberries begin growing before floral buds open (Bell, 1950),
whereas rabbiteye blueberries flowering and fruit set occur prior to vegetative growth in (Maust
et al., 1999). This difference in timing may have exposed highbush blueberry vegetative shoots
to higher concentrations of cytokinins resulting in injury.
Foliar treatments to individual shoots did not affect fruit set (Table 3.3). Similar results
were found in apples, as BA applied to leaves and canopy was more effective at thinning than
those applied to fruit alone (Greene et al., 1992; Schroder et al., 2013). This may be a result of
insufficient coverage or absorption. Thinning may be regulated by polar auxin movement from
the apical region of the leaf (Schroder et al., 2013). If auxin is expressed in high enough
concentrations, activation of the abscission zone ensues leading to abscission (Schroder et al.,
2013). In roots, cytokinins regulates polar auxin transport by altering expression of auxin
transport components (Ruzicka et al., 2009). Similar processes could regulate auxin transport in
leaves. BA applied to individual shoots may not increase auxin transport to sufficient levels
compared to when BA was applied to the entire canopy. This could explain inconsistent thinning
effects of canopy applied versus shoot applied BA.
In 2012, hand thinning significantly increased the number of vegetative shoots per plant
and canopy volume growth compared to control plants (Tables 3.1 and 3.2).

These results are

in agreement with Strik and Buller (2005), who found removing floral meristems for two
consecutive years promoted shoot growth. Maust et al. (1999) also found an inverse relationship
between floral meristem density and canopy establishment in southern highbush blueberries.
However, in 2013 hand thinning of 0, 50, or 100% of floral structures did not affect shoot
number or length per plant (Table 3.4). This may be due to a small number of floral meristems

45

(12) or berries (10) per plant. Reduced fruiting stress, would result in a reduced response to
manual thinning. In addition, plants in current study appeared to exhibit nitrogen deficiency,
which would hinder growth and obscure differences between treatments.
Preventing fruiting on young highbush blueberries can result in greater plant growth.
This study demonstrated that thinning flowers can be an effective way to increase the number of
vegetative shoots and canopy growth. Thinning should be done prior to or soon after pollination,
as fruit pericarp tissue begins rapid growth and competes with vegetative growth for resources
(Finn and Luby, 1986). The potential for increased vegetative growth is also dependent upon the
number of floral buds set and cultural factors. From the current study, plants that averaged ten
flower buds did not respond to thinning treatment. Growers may use this information and
incorporate into management model, in which plants that average ten or fewer flower buds
would not require thinning treatment. Chemical thinning could be a labor saving alternative to
manually thinning, but further studies are needed to determine if other agents, concentrations, or
timings may reduce fruit set without negatively affecting vegetative growth.

46

LITERATURE CITED

47

LITERATURE CITED

Bangerth, K. F. 2000. Abscission and thinning of young fruit and their regulation by plant
hormones and bioregulators. Plant Growth Regulat. 31:43-59.
Bell, H. P. 1950. Determinate growth in the blueberry. Can. J. Res. 28:637-644
Black, B. L. and M. K. Ehlenfeldt. 2007. Foliar applications of GA4+7 reduce flowering in
highbush blueberry. HortScience 42:555-558.
Cartagena, J. R., F. B. Matta, and J. M. Spiers. 1994. Chemical fruit thinning of Vaccinium
ashei Reade. J. Amer. Soc. Hort. Sci. 119:1133-1136.
Dennis, F. G. 2000. The history of fruit thinning. Plant Growth Regulat. 31:1-16.
Ebert, A., F. Bangerth. 1982. Possible hormonal modes of action of three apple thinning agents.
Scientia Hort. 16:343-356.
Finn, C.E., J.F. Hancock, T. Mackey, and S. Serce. 2003. Genotype x environment interactions
in highbush blueberry (Vaccinium sp L.) families grown in Michigan and Oregon. J.
Amer. Soc. Hort. Sci. 128:196-200.
Finn, C.E. and J. J. Luby. 1986. Inheritance of fruit development interval and fruit size in
blueberry progenies. J. Amer. Soc. Hort. Sci. 111: 784-788.
Greene, D. W., W. R. Autio, J. A. Erf, Z. Y. Mao, 1992. Mode of action of benzyladenine when
used as a chemical thinner on apples. J. Amer. Soc. Hort. Sci. 117:775-779.
Koron, D. and M. Stopar. 2004. Effect of thinners on yield, fruit size and ripening time of
highbush blueberry. Acta Hort.715:273-278.
Maust, B. E., J. G. Williamson, and R. L. Darnel., 1999. Flower bud density affects vegetative
and fruit development in field grown southern highbush blueberries. HortScience
34:607-610.
Pritts, M. P. and J. F. Hancock. 1985. Lifetime biomass portioning and yield component
relationships in the highbush blueberry, Vaccinium corymbosum L. (Ericaceae). Amer.
J. of Bot. 72:446-452.
Pritts, M.P. and J. F. Hancock (eds.) 1992. Highbush blueberry production guide. NRAES-55,
Ithaca, N.Y.

48

Ruzicka, K., M. Simaskova, J. Duclercq, J. Petrasek, E. Zazimalova, S. Simonb, J. Frimla, M.
Van Montagua, and E. Benekovaa. 2009. Cytokinin regulates root meristem activity via
modulation of the polar auxin transport. Proc. of the Natl. Acad. of Sci. 106:4284-4289.
Schroeder, M. and H. Link. 2013. Correlative polar auxin transport to explain the thinning mode
of action of benzyladenine on apple. Scientia Hort. 153:84-92.
Strik, B. C. 2007. Horticultural practices of growing highbush blueberries in the ever-expanding
U.S. and global scene. J. Amer. Pomol. Soc. 61:148-150.
Strik, B. and G. Buller. 2005. The impact of early cropping on subsequent growth and yield of
highbush blueberry in the establishment years at two planting densities is cultivar
dependent. HortScience 40:1998-2001.
Williamson, J. G. and D.S. NeSmith. 2007. Evaluation of flower bud removal treatments on
growth of young blueberry plants. HortScience 42:571-573.

49

CHAPTER 4: THE SHOOT GROWTH PATTERN AND DISTRIBUTION OF FLOWER
BUDS IN YOUNG HIGHBUSH BLUEBERRIES GROWN IN MICHIGAN

50

Abstract:
The patterns of shoot growth and floral initiation in young highbush blueberries grown in
Michigan are not fully understood. Highbush blueberries exhibit episodic and sympodial
growth, which commences in the spring and ceases with the abortion of the apical meristem.
Two to six weeks later a distal, lateral meristem may commence growth, forming the next
growth flush. This process may be repeated throughout the season. The purpose of this
experiment was to describe the chronology of shoot growth flushes over the growing season and
to determine the distribution of flower buds between growth flushes. In 2012 and 2013, the time
of shoot growth initiation and termination, final shoot length, and flower bud numbers were
recorded on eight plants of recently planted ‘Duke’, ‘Draper’, and ‘Liberty’. The majority of
shoot growth occurred from May to July in the first and second flushes. In 2012, the majority of
flower buds developed on secondary shoots, whereas in 2013 the majority of flower buds were
located on primary shoots. Primary shoots that initiated later growth flushes produced fewer
flower buds than those that did not. This pattern of shoot growth creates a lengthy floral
initiation time period, based on the when shoot growth ceases and whether subsequent growth
flushes occur.

Introduction:
Highbush blueberries in short growing regions may need seven to ten years to reach
maximum fruit production (Strik, 2007) whereas bushes in longer growing seasons often exhibit
faster establishment (Finn et al., 2003). Preventing young blueberry plants from fruiting can
increase their vegetative growth (Strik and Buller, 2005; Williamson and NeSmith, 2007).
Flowers and flower buds are often removed manually from young plants, but this is time

51

consuming (Strik and Buller, 2005). Inhibiting flower bud formation could be a low-cost
alternative to manual removal. Gibberellin (GA) applications have prevented flower bud
formation in other species (Lenahan et al., 2006; Turnbull et al., 1996; Bradley and Crane, 1960),
but results have been inconsistent in highbush blueberries (Black and Ehlenfeldt, 2007;
Retamales et al., 2000). GA effectiveness appears to be dependent on application timing, as GA
was ineffective once the transition from a vegetative to a reproductive meristem occurred (Lord
and Eckard, 1987; Sachs et al., 1967). However, this timing in highbush blueberry is not well
understood.
Reproductive development begins with floral induction, as signals are transported from
the leaves to the vegetative meristem (Samach and Smith, 2013). When these signals are
perceived, floral initiation, the transition from a vegetative meristem producing leaves and shoots
to a reproductive meristem, producing sepals, petal, stamens, and carpels, occurs and can be
observed as the formation of floral primordia (Bowman et al., 2012; Samach and Smith, 2013).
Based on histological examination, Gough et al. (1978) found that floral initiation in highbush
blueberries occurred six to nine weeks after full bloom. Tamada (1997) also found floral
initiation occurring about nine weeks after full bloom and linked the timing of initiation to shoot
growth patterns. Floral initiation began two weeks after the current shoot ceased growth and was
completed after one month (Tamada, 1997).
Temperature and photoperiod also influence floral development in blueberries. Southern
highbush blueberries (Vaccinium corymbosum interspecific hybrid) formed flower buds at 21°C,
but numbers were greatly reduced at 28°C (Spann et al., 2004). Photoperiod also influences
floral development in blueberries. Lowbush blueberries (Vaccinium angustifolium) in 16 h
photoperiods produced no floral meristems, while those in 12, 10, or 8 h photoperiods did (Hall

52

et al., 1961). Likewise, northern highbush blueberries produced floral buds at photoperiods of 8
h, but not 16 h (Bañados and Strik, 2006). Southern highbush blueberries in 8 h photoperiods
produced floral meristems, while those in 16 h or 8 h photoperiods plus a night interruption did
not (Spann et al., 2003). These studies indicate that floral bud induction is strongly regulated by
photoperiod. However, natural photoperiods in Michigan (latitude 43° N) during July and
August, when floral initiation is thought to occur, range from 15 to 13 h. Other studies have
shown more variable results of floral initiation in response to photoperiod. Pescie et al. (2011)
found southern highbush blueberries growing in Argentina exhibiting two distinct floral
initiation periods, with flower buds developing under both long day (16 h) and short day
photoperiods (8 h). This study was performed in Buenos Aires (34°S, 59°W), which may have
different environmental conditions than plants grown in Michigan. These studies indicate that
floral development in field grown blueberries likely occurs even under long day conditions.
The shoot growth pattern of highbush blueberries is episodic and sympodial (Gough et
al., 1978). Most shoots originate from axillary buds on the previous seasons growth, but some
shoots develop from adventitious buds near the crown or roots (Bañados 2006). Growth flushes
terminate in the abortion of the apical meristem (Gough et al., 1978). This stage is referred to as
black tip because the blackened shoot apex and subtending leaf and stem tissue are easily visible
(Gough et al., 1978). After two to five weeks, a distal meristem may begin growing and form a
second shoot or flush (Gough et al., 1978). The number of growth flushes is dependent upon
plant health, environmental conditions, and cultivar, with later fruiting varieties producing fewer
growth flushes than early and mid-season varieties (Gough et al, 1976). Growth flushes may
develop flower buds at different times, with later flushes producing fewer flower buds (Bañados,
2006). Gough et al. (1976) observed highbush blueberries in Rhode Island to have flower buds

53

interspersed on all flushes and higher numbers on thicker diameter shoots and observed up to
five growth flushes on a given plant. The purpose of this experiment was to describe the
chronology of shoot growth flushes over the growing season and to determine the distribution of
flower buds between growth flushes.

Materials and Methods:
Study 1
Study was conducted in 2012 and 2013 using one-year-old ‘Liberty’ plants at commercial
planting near Coopersville, MI (43º 06’ N, 85º 93’ W ) and three-year-old ‘Draper’ and ‘Duke’
plants at a planting at Michigan State University Horticultural Teaching and Research Center
(HTRC) in Holt, MI (42º 67’ N, 84º 48’ W). Eight plants of similar size from each cultivar were
randomly selected in the spring prior to bud break. Plants at HTRC were managed under organic
practices, with manual removal of weeds, and solid set irrigation, while plants in Coopersville
were managed under conventional practices and irrigated with drip lines. Each new shoot was
tagged as it began growing, and shoot length was measured at approximately two week intervals
thereafter. Flush number was also determined for each axillary meristem by observing apical
meristem abortion evidence of a blackened shoot apex and subtending leaf and stem tissue and
subsequent shoot growth. The first flush of shoots originated from axillary buds on the previous
year’s shoots. Shoots in the second flush originated from axillary buds on the current-season
first shoots. The third flush of shoots originated from axillary buds on the current-season second
flush of shoots. Petal fall was recorded in the spring after full bloom, once half the flowers
abscised. The number of floral meristems on each shoot was recorded in late fall, once the

54

enlarged, rounded floral meristems could be distinguished from the smaller, pointed vegetative
buds.
Study 2
Fifteen three-year-old plants of ‘Bluecrop’ and ‘Elliott’ were randomly selected at the
HTRC. One major branch was randomly selected from each plant, brought indoors and
separated into individual growth flushes based on visual evidence of apical meristem abortion
and subsequent shoot growth. Evidence of apical meristem abortion consisted of circular rings
on the shoot, which was the abscission layer from the black tip event. The total shoot length and
number of floral meristems from each flush was recorded as previously described.
Growing degree days and temperature data for 2012 and 2013 was obtained from
automated weather stations (http://www.agweather.geo.msu.edu/mawn/) located nearest to study
locations at West Olive, MI (42º 97’ N, 86º 07’ W ) or at Holt, MI (42º 67’ N, 84º 48’ W). The
Baskerville-Emin growing degree model with a base temperature of 50° F was used to calculate
the number of growing degree days for each month from January to October. For 2012 and 2013,
the average maximum air temperate was determined from May to October, by using the mean
function feature.
Data from each study was statistically analyzed with SAS 9.3 (SAS Institute, Cary, NC)
with flush number as the main factor. PROC Glimmix was used to determine statistical
significance for the number of floral meristems, while PROC Mixed was used to determine
statistical significance for shoot length. When significant interactions (P<0.05) were found,
means separation was performed with PDIFF in LSMEANS statement.

Results:

55

The growth pattern observed was episodic and sympodial (Figure 4.1). In 2012, the first
growth flush started in April and concluded by June (Figure 4.2). The second growth flush
occurred from May to July, while the third flush grew from July to September (Figure 4.2).
About 80% of the primary shoots in ‘Draper’ and Liberty’ exhibited black tip formation by 22,
May. Primary shoots of Duke reached 80% dieback by 4, June. Black tip formation was
observed on 80% of all secondary shoots in ‘Draper’ and ‘Duke’ by 9, July and in ‘Liberty’ by
24, July. A third growth flush was first observed on 25, July.
In 2013, the first flush of growth began in May and continued until July (Figure 4.3).
The second flush grew from July through August, while the third flush grew from August to
September (Figure 4.3). 80% of primary shoots in ‘Duke’ exhibited black tip on 19 June and in
‘Liberty’ by 6 July and in ‘Draper’ on 5 June. 80% black tip formation of secondary shoots
varied considerably by cultivar, and was observed on 4 July (Draper), 21 July (Duke) and 11
August (Liberty).
In 2012, there was significantly more shoot length in the first and second flushes than in
the third flush (Table 4.1). In 2013, the shoot length in the first flush was greatest, accounting
for 71 to 90% of all shoot growth (Table 4.2). For ‘Draper’ and ‘Duke’ in 2012, there were
significantly more floral buds in the second flush, than in the first or third flush, accounting for
61 to 72% of all floral buds (Table 4.1). In 2012, average shoot length per growth flush was not
affected in ‘Draper’ and ‘Liberty’, but the third flush was significantly shorter than the first or
second flush in ‘Duke’ (Table 4.1). Similar trends were found in 2013 as ‘Liberty’ and ‘Duke’
had significantly shorter average shoot lengths in the third growth flush than other growth
flushes (Table 4.2).

56

A study was conducted to determine if the number and length of shoots and flushes and
position of flower buds could be determined more easily by examining dormant branches
collected after the growing season. Similar trends in shoot growth were observed by examining
dormant branches of ‘Bluecrop’ and ‘Elliott’ (Table 4.3). The first growth flush accounted for a
significantly greater amount of shoot length (57 to 80% of total growth) than later flushes (Table
4.3). The third flush resulted in minimal amounts of shoot growth, representing only 5 and 1%
of total growth in ‘Bluecrop’ and ‘Elliott’, respectively (Table 4.3). In ‘Bluecrop’ the greatest
number of floral meristems was in the second flush, while in ‘Elliott’ the greatest number of
floral meristems was in the first growth flush.
The 2012 growing season was warmer and started earlier than in 2013. In March of
2012, 400 growing degree days accumulated, compared to March of 2013 when less than 5
growing degree days accumulated. (Table 4.4). Average maximum air temperature was higher in
2012 compared to 2013. The greatest monthly difference occurred during July, with 2012
averaging about 6° C higher temperatures than 2013 (Table 4.5). In 2012, petal fall was
observed on 12 May, whereas in 2013 it was observed on 5 June.
The event of multiple growth flushes affected the distribution of floral buds on primary
shoots. Primary shoots that underwent multiple flushes had significantly fewer flower buds
those that did not have subsequent growth flushes during 2012 (Table 4.6) and 2013 (Table 4.7).

Discussion:
Growth patterns were episodic and sympodial, as described by Gough et al., (1978). In
2012, 49, 41, and 10% of shoot growth was associated with the first, second, and third flushes,
respectively (Table 4.1) In 2013, the first, second and third flushes accounted for 79, 19 and 2%
of shoot growth (Table 4.2). These results are in agreement with Bañados (2006) who found
57

80% of shoot biomass within the primary flush, and about 96% of all shoots were primary or
secondary flushes. This is also in agreement with Gough et al. (1978) who found the greatest
amount of growth occurring from late spring to early summer and Mingeau et al. (2001) who
found the greatest shoot elongation rates occurred between early June and the middle of July.
Mingeau et al. (2001) was likely viewing the first and second growth flush during that time
period. A reduced amount of shoot growth occurring later in the growing season may be a result
of decreasing photoperiods. Photoperiods were over 14 h in July and declined to 12 h in
September. Highbush blueberries in continuous 8 h photoperiods produced fewer growth flushes
and 80% less shoot biomass than plants in 16 h photoperiods (Bañados and Strik; 2004).
Whether plants in current studies exhibited similar reductions in growth as a result of decreasing
photoperiods is not known. Another possible explanation for a reduction in late season growth
may be decreased available photosynthates. Highbush blueberries are fairly heat sensitive, and
increasing temperatures from 20°C to 30°C reduced carbon assimilation 22 to 51% (Hancock et
al., 1992). In both years, average maximum temperatures were highest in July, with recordings
above or near 30°C (Table 4.5). In 2012, there were 16 days with daily maximum temperatures
at or above 30°C, whereas in 2013 there were 7 days of similar conditions
(http://www.agweather.geo.msu.edu/mawn/). This could lead to decreases in available
photosynthates, resulting in reductions of later growth flushes. A final possibility could be the
effect of fruiting stress on shoot growth. However, ‘Liberty’ plants had all flowers removed in
the spring, while ‘Duke’ and ‘Draper’ plants had partial fruit removal, thereby reducing the
potential impact on shoot growth.
The weather conditions differed in 2012 compared to 2013. The most striking difference
was the much warmer weather in March of 2012 compared to 2013 (Table 4.4), which resulted in

58

early plant development in 2012. Petal fall occurred on 12 May in 2012 and on 5 June in 2013.
Shoot growth also started earlier in 2012 compared to 2013. In addition, average maximum
temperatures appeared higher in 2012 compared to 2013 (Table 4.5). These factors could have
resulted in differences in the shoot growth pattern between 2012 and 2013. High temperatures in
2012 could have caused plant stress and led to faster apical abortion of primary shoots. Evidence
of this can be seen when 80% black tip formation of primary shoots occurred. In 2012, 80% of
primary shoots underwent apical abortion by 22 May, about two weeks after petal fall. However,
in 2013 80% of primary shoots underwent apical abortion by 4 July, about a month after petal
fall. Faster abortion of primary shoots would have resulted in a decrease in the percentage of
shoot growth in the first flush during 2012 compared to 2013, which was also observed (Tables
4.1 and 4.2). Another possibility is that by starting shoot growth earlier in 2012, there was a
greater amount of time for multiple growth flushes to occur. This seems unlikely as Bañados
(2006) observed mature plants in Oregon, which have a long growing season, to have only 5% of
shoots within the third flush. However, Bañados (2006) did not describe the timing of shoot
growth, so the time interval of growth flushes is not known.
The distribution of floral buds within growth flushes differed between 2012 and 2013. In
2012, the second flush had significantly more flower buds (51 to 72% of all flower buds) than
the first or the third. However, in 2013 there was significantly more flower buds in the first and
second flushes of ‘Duke’, ‘Draper’, and ‘Liberty’ than in the third. The third growth flush
generally had the fewest flower buds, with 9 to 43% in 2012, and 1 to 12% in 2013. These results
are supported by Bañados (2006), who hypothesized that later growth flushes would produce
fewer flower buds.

Cultivars exhibit similar trends in floral bud distribution, even though

varieties included early (Duke), midseason (Draper) and late (Liberty) ripening types.

59

The number of flower buds on primary shoots was affected by subsequent growth
flushes. Primary shoots that did not undergo additional growth flushes had greater numbers of
floral buds than primary shoots that did undergo subsequent growth flushes (Tables 4.6 and 4.7).
This is in agreement with Bañados (2006), who found flower buds located on the distal portions
of the last growth flush. However, Gough et al. (1976) observed flower buds interspersed on all
flushes and not only on the distal portion of shoots. The reason for this discrepancy is not clear,
but in the current studies flower buds on the second and subsequent third growth flush were
observed. Subsequent growth flushes appear to inhibit floral initiation on primary shoots, thereby
ensuring that flowers and fruit will be near the apex of the branch.
Floral initiation in highbush blueberries appears to occur over an extended time period,
based on the timing and number of growth flushes. Tamada (1997) observed flower bud
initiation on primary shoots, which did not undergo subsequent flushes, starting two weeks after
shoot growth ceased. Based on this model, plants in the current study would begin initiating
flower buds on primary shoots during June (2012) or July (2013). It is not known if later growth
flushes exhibit a similar floral initiation timeline. However, even if initiation began immediately
after a shoot ceased growth, flower bud initiation would not occur until the end of July on second
growth flushes and in August on the third growth flush. In addition, subsequent growth flushes
appear to inhibit flower bud formation on primary shoots, which also indicates that highbush
blueberries initiate flower buds at separate intervals. Southern highbush blueberries undergoing
floral initiation months apart, depending on whether flower buds were formed on the first or
second growth flush (Pescie et al., 2011). This may partially explain results of Retamales et al.
(2000) who found variable inhibition of floral initiation with GA application. Due to long floral

60

initiation interval, GA would likely have to be repeatedly applied from May through October to
observe more complete inhibition of floral initiation.
In summary, highbush blueberries shoot growth occurs in flushes from April to
September, with the majority of growth occurring early in the growing season. The timing of
growth flushes varied between seasons and was associated with growing degree accumulation in
the spring and maximum ambient temperature in the summer. Flower buds were found on the
first, second, and third flushes, however the number of flower buds on primary shoots was
inhibited when subsequent growth flushes occurred. Due to this growth pattern, floral initiation
in highbush blueberries appears to occur from June to September. From current studies, it
appears that shoots did not cease growth after a set shoot length or node number. Further studies
could be needed to determine if bio-regulators could delay the process of apical abortion and
thereby increase shoot growth and plant establishment.

61

LITERATURE CITED

62

LITERATURE CITED

Bañados, M.P. 2006. Dry weight and 15N-nitrogen and partitioning, growth, and development of
young and mature blueberry plants. Ph.D. thesis, Oregon State University, 172p.
Bañados, M.P. and B.C. Strik. 2006. Manipulation of the annual growth cycle of blueberry using
photoperiod. Acta Hort. 715:65-72.
Black, B. L. and M. K. Ehlenfeldt. 2007. Foliar applications of GA4+7 reduce flowering in
highbush blueberry. HortScience 42:555-558.
Bowman, J. L., D. R. Smyth, and E. M. Meyerowitz. 2012. The ABC model of flower
development: then and now. Development 139:4095-4098.
Bradley, M. V. and J. C. Crane. 1960. Gibberellin-induced inhibition of bud development in
some species of prunus. Science 131:825-826
Finn, C.E., J.F. Hancock, T. Mackey, and S. Serce. 2003. Genotype x environment interactions
in highbush blueberry (Vaccinium sp L.) families grown in Michigan and Oregon. J.
Amer. Soc. Hort. Sci. 128:196-200.
Gough R. E., V. G. Shutak and R. L. Hauke. 1978. Growth and development of highbush
blueberry II Reproductive Growth, Histological Studies. J. Amer. Soc. Hort. Sci.
103:476-479.
Gough, R. E., V. G. Shutak, :N. D. Windus, 1976. Observations on vegetative and reproductive
growth in blueberry. HortScience 11:260-261.
Hall, I. V. and R.A. Ludwig. 1961. The effects of photoperiod, temperature, and light intensity
on the growth of the lowbush blueberry (Vaccinium angustifolium alt). Can J. Bot. 39:
1733-1739.
Hancock, J. F., K. Haghighi, S. Krebs, J. Flore, A. Draper. 1992. Photosynthetic heat stability in
highbush blueberries and the possibility of genetic improvement. HortScience 27:11111112.
Lenahan, O.M., M.D. Whiting, and D.C. Elfving. 2006. Gibberellic acid inhibits floral bud
induction and improves bing sweet cherry fruit quality. HortScience 41:654-659.
Lord, E. M. and K. J. Eckard. 1987. Shoot development in citrus sinensis L. (Washington Navel
Orange). II. alteration of developmental fate of flowering shoots after GA3 treatment.
Bot. Gaz. 148:17-22.

63

Pescie, M., M. Lovisolo, A. Magistris, B. Strik, and C. Lopez. 2011. Flower bud initiation in
southern highbush blueberry cv. O’Neal occurs twice per year in temperate to warmtemperate conditions. J. Applied Hort. 13:8-12.
Retamales, J. B., E. J. Hanson, and M. J. Bukovac. 2000. GA3 as a Flowering Inhibitor in
Blueberries. Acta Hort. 527:147-151.
Sachs, R. M., A. M. Kofranek, and Shiow-Ying Shyr. 1967. Gibberellin-induced inhibition of
floral initiation in fuchsia. Amer. J. Bot. 54:921-929.
Samach, A. and H. M. Smith. 2013. Constraints to obtaining consistent annual yields in
perennials. II: environment and fruit load affect induction of flowering. Plant Sci. 207:
168-176.
Spann, T., M., J. G. Williamson, and R. L. Darnell, 2004. Photoperiod and temperature effects
on growth and carbohydrate storage in southern highbush blueberry interspecific hybrid.
Amer. Soc. Hort. Sci. 129:294-298.
Strik, B. C. 2007. Horticultural practices of growing highbush blueberries in the ever-expanding
U.S. and global scene. J. Amer. Pomol. Soc. 61:148-150.
Strik, B. and G. Buller. 2005. The impact of early cropping on subsequent growth and yield of
highbush blueberry in the establishment years at two planting densities is cultivar
dependent. HortScience 40:1998-2001.
Tamada T. 1997. Flower bud differentiation of highbush and rabbiteye blueberries. Acta Hort.
446:349-356.
Turnbull, C. G., K. L. Anderson, and E. C. Winston. 1996. Influence of gibberellins treatment
on flowering and fruiting patterns in mango. Aust J. Exp. Agric. 36:603-611.
Williamson, J. G. and D.S. NeSmith. 2007. Evaluation of flower bud removal treatments on
growth of young blueberry plants. HortScience 42:571-573.

64

APPENDIX

65

Introduction:
Highbush blueberry plants in Michigan have a lengthy plant establishment time from
planting prior to reaching peak fruit production (Strik, 2007). Preventing fruiting on young
plants reduces this interval and increases vegetative growth (Strik and Buller, 2005). Manual
removal of flower buds is recommended, but labor intensive (Pritts and Hancock, 1992). More
efficient ways to prevent fruit production on young plants are needed.
Floral initiation, which is the transition from a vegetative meristem to an inflorescence
meristem, is a complex process (Samach and Smith, 2013). Some plants use photoperiod to
regulate the timing of floral initiation (Wilkie et al., 2008). Plants may be classified based on
their flowering response as either long day, short day, or day neutral. Under long nights, short
day plants flowering is promoted, while with long day plants flowering is promoted in short dark
periods; day neutral plants flower development is not affected by photoperiod (Mockler et al.,
1999).
Blueberries are thought to be a short day plant, with floral induction occurring under long
dark periods (Darnell, 1991). Lowbush blueberries placed under 16 h photoperiods produced no
floral meristems, while those in 14 h or less photoperiods did (Hall et al., 1961). In northern
highbush blueberries, floral buds formed in 8 h photoperiods, but not in 16 h (Bañados and Strik,
2006). Southern highbush blueberries produced floral meristems in 8 h photoperiods but not in
16 h or 8 h photoperiods with a night interruption (Spann et al., 2003). In rabbiteye blueberries
the effects of photoperiod on floral initiation was cultivar specific and quantitative, not
qualitative, showing that flower buds could form under non-inductive photoperiods (Darnell,
1991). Day lengths under field conditions in Michigan (latitude 43º N) range from 15 to 13 h in
July and August when floral initiation is thought to occur. Altering the photoperiodic conditions
66

of highbush blueberries could be a labor saving alternative to reduce fruiting in young highbush
blueberries. The purpose of this experiment was to determine if supplemental lighting could
inhibit floral initiation in field grown highbush blueberries by interrupting plants photoperiod
response.
Material and Methods:
A study was performed at Michigan State HTRC in Holt MI. Sixteen four-year-old
highbush blueberry ‘Bluecrop’ plants were randomly selected and assigned into one of two
treatments, either a non-treated control or a light interruption treatment. Light interruption was
accomplished by using two 200W halogen lights per plant, with lighting directed to the plant
canopy. Light intensities at canopy level were 2 uMol.m-2..sec-1. Each plant was separated by
three foot by three foot black plastic squares, which prevented light pollution between plants.
Lights were turned on between midnight and 4 am daily from July to October. The number of
flower buds was recorded per plant in the fall after leaf senesce. Data was analyzed with SAS
9.3.
Results and Discussion:
A study was performed to determine if floral initiation could be inhibited in field grown
highbush blueberries by altering the photoperiod response using night interruption lighting.
Based on ANOVA table, no significant differences in the number of flower buds per plant was
found between the non-treated control (135) or light interrupted plants (109). The use of light
interruption would have simulated 16 h photoperiods by disrupting the long dark period, but
floral initiation still occurred.

These results contradict previous work of Spann et al. (2004)

67

who found that floral initiation in southern highbush blueberries was inhibited with night
interruption lighting.
There are several possible reasons to explain the discrepancy between studies. One
possibility is that the light intensity of 2 uMol.m-2..sec-1 was not intense enough to activate the
phytochromes in highbush blueberries. Spann et al. (2003) who was able to inhibit floral
initiation with night interruption lighting had greater light intensities, of 75 uMol.m-2..sec-1.
Another potential issue is whether enough of the plant canopy was receiving night interruption
lighting in current study. In summary, the use of night interruption lighting was not able to
inhibit floral initiation in field grown highbush blueberries in Michigan. Further studies could be
performed to investigate if higher light intensities could inhibit floral initiation in field grown
highbush blueberries.

68

Table 2.1 Effect of GA applied at 400 mg.L-1 during 2009 on floral meristem numbers
in 2010. Data are means of three cultivars (Aurora, Elliott, Liberty) and two GA forms
(GA3, GA4+7)
Application dates
Flower buds/plant
Control
69
az
10, 17 Aug.
54
b
26 Aug., 3 Sep.
39
c
13, 21 Sep.
54
b
z

Means followed by the same letter are not significantly different based on LS Means test
at P ≤ 0.05.

Table 2.2 ANOVA results for 2009 study based on differences in the number of flower
buds per plant in response to different effects
Effect
P value
Form
0.27
Variety x form
0.47
Timing
0.001
Variety x timing
0.104
Form x timing
0.45
Variety x form x timing
0.19

Table 2.3 Effect of GA4+7 application time and concentration (mg.L-1 a.i. as ProVide)
during 2010 on plant development in 2011z. Data are means of four cultivars (Elliott,
Draper, Aurora, Liberty)
GA application
Flower buds/
Flowers/ bud
Berries/
plant
Plant
Time
None
92 az
4.8
182 a
Early (21July, 4, 18 Aug., 1Sep.)
57 b
4.2
132 bc
Late (8, 22 Sep., 6, 20 Oct.)
76 a
4.5
162 ab
Early and late (all dates)
47 b
4.2
105 c
y
Concentration
0
92
4.8 a
182
200
54
4.7 ab
133
400
58
4.1 b
124
z

Means within a column group not followed by the same letter are significantly different at
P ≤0.05 based on LS Means test
69

Table 2.4 ANOVA results for 2010 study based on differences in the number of
flower buds per plant in response to different effects
Effect
P value
Concentration
0.30
Variety x concentration
0.36
Timing
0.01
Variety x timing
0.53
Concentration x timing
0.57

Table 2.5: Effect of auxin applications on floral induction in
‘Liberty’ and ‘Chandler’ highbush blueberries grown near
Coopersville, MI
Application
Flower buds/plant
Ratey
z
time
Cultivar
x
Chandler
0
21 de
Chandler
early
0.2
24 bcde
Chandler
early
2
40 abc
Chandler
early
20
24 bcde
Chandler
early
200
33 abcd
Chandler
late
0.2
27 abcde
Chandler
late
2
20 de
Chandler
late
20
27 abcde
Chandler
late
200
19 de
Liberty
0
40 abc
Liberty
early
0.2
40 ab
Liberty
early
2
28 abcde
Liberty
early
20
44 a
Liberty
early
200
30 abcde
Liberty
late
0.2
34 abcd
Liberty
late
2.0
41 ab
Liberty
late
20
24 cde
Liberty
late
200
10 e
z
Application made biweekly from 14 July to 11 Aug. (early) or
biweekly from 25 Aug. to 22 Sept. (late).
y
Rate is mg.L-1 of a.i. of Fruitone, NAA
x

Means within a column not followed by the same letter are
significantly different at P ≤ 0.05 based on LS Means test

70

Table 3.1: Effect of thinning treatments on ‘Bluecrop’ highbush blueberry fruit set and canopy
growth in Holt, MI during 2012
Treatment
Fruit set (%)
Canopy growth (cm3)
55 az
63 a

BAy
Hand thinning

485 b

0 b

carbarylx

1341 b

5 b

Control

4051 a

559 b

z

Means with a column followed by different letters are significantly different at P ≤ 0.05 based
on LS Means test.
y
BA (Maxcell) applied at 400 mg.L-1 a.i.
x

Carbaryl (Sevin SL) applied at 600 mg.L-1 a.i.

Table 3.2: Effect of thinning treatments on number of vegetative shoots in ‘Bluecrop’
highbush blueberry grown in Holt, MI during 2012
Treatment
Shoots / plant
Shoots / plant
13 June
21 Aug.
Control
95
135
b
bz
carbarylx

94

b

136

b

BAy
hand thinning

59

c

134

b

163

a

260

a

z

Means within a column followed by the different letters are significantly different at P ≤0.05
based on LS Means test.
y
BA (Maxcell) applied at 400 mg.L-1 a.i.,
x

Carbaryl (Sevin SL) applied at 600 mg.L-1 a.i.

71

Table 3.3: Effect of BA and carbaryl foliar sprays to individual shoots in May to June
on fruit set in ‘Jersey’ highbush blueberries grown in Holt, MI during 2012
Treatmentz
Fruit Set (%)
Application datey
Control
.
63x
BA
17 May
45
Carbaryl
17 May
56
BA
24 May
58
Carbaryl
24 May
54
BA
30 May
58
Carbaryl
30 May
56
z
. -1
BA (Maxcell) applied at 400 mg L a.i., carbaryl (Sevin SL) applied at 600 mg.L-1 a.i.
y

Anthesis occurred on 14, May

x

No significant differences between treatments or application dates, based on ANOVA
at P ≤0.05

Table 3.4: Effect of manual removal of flowers at different rates on 1 year old ‘Duke’ highbush
blueberries grown in Holt, MI in 2013
Treatmenty
Control
50%
100%

Berries/ plant
10 az
6 a
0 b

Berry weight
(g)
1.6

Vegetative
shoots/plant
41

Shoot length
(cm/plant)
216

1.5
-

45
46

211
213

z

Means within a column followed by different letters are significantly different at P ≤0.05 based
on LS Means test.
y
Treatments are amounts of floral buds manually removed

72

Figure 4.1: Diagram of the shoot growth cycle of highbush blueberries exhibiting (a) first
growth flush (dashed line) during June, (b) second growth flush (dotted line) during July, and (c)
third growth flush (dash dot line) during August. Solid line is previous year’s growth. Circles
are approximate position and numbers of flower buds on the first, second, and third growth flush

a

c

b

73

Figure 4.2: Timing of 2012 shoot growth within the first, second, or third flush in three young
highbush blueberry cultivars grown in Holt, (Duke, Draper) or Coopersville, MI (Liberty)
800

Duke

First

Second

Third

Total

600

400

200

0
300

Draper

Shoot growth (cm/plant)

250
200

150
100
50
0

400

Liberty

350
300
250

200
150
100
50
0
May

June

74

July

Aug.

Sept.

Figure 4.3: Timing of 2013 shoot growth within the first, second, or third flush in three young
highbush blueberry cultivars grown in Holt (Duke, Draper) or Coopersville, MI (Liberty)
800

Duke

First

Second

Third

Total

600
400
200
May

June

July

Aug.

Sept.

0

Shoot growth (cm / plant)

700
Draper

600
500

400
300
200
100
0
1500
Liberty
1000

500

0
May

June

July

75

Aug.

Sept.

Table 4.1. Shoot growth and floral bud numbers associated with the first, second and third
growth flushes of highbush blueberry cultivars grown in Coopersville, MI. (‘Liberty’) or
Holt, MI. (‘Duke’, ‘Draper’) in 2012
Cultivar
Vegetative Shoot length per Average shoot
Floral buds per
z
plant (cm)
length (cm)
plant
flush
Draper
first
144 ay
5.5
20 b

Duke

Liberty

second
third
first
second
third
first
second
third

125
17
332
254
51
94
82
30

a
b
a
a
b
a
a
b

5.1
5.5
8.9 a
8.2 a
5.7 b
1.9
2.3
2.2

78
10
39
105
28
2
19
16

a
b
b
a
b
b
a
a

Z

First flush shoots originated from axillary buds on previous year’s shoots. Second flush
shoots originated from axillary buds on current-season first shoots. Third flush shoots
originated from axillary buds on current-season second shoots.
y
Means within the same column and cultivar followed different letters are significantly
different at P <0.05 based on LS Means test.

Table 4.2. Shoot growth and floral bud numbers associated with the first, second and
third growth flushes of highbush blueberry cultivars grown in Coopersville, MI.
(‘Liberty’) or Holt, MI. (‘Duke’, ‘Draper’) in 2013
Cultivar
Vegetative Shoot length
Average shoot
Floral buds per
z
per plant (cm)
length (cm)
plant
flush
Draper
first
502 ay
7.1
92 a
second
53 b
9.1
22 b
third
1 b
2.5
2 c
Duke
first
485 a
12.6 a
55 a
second
147 b
9.2 a
63 a
third
4 c
3.3 b
3 b
Liberty
first
1074 a
14.9 a
82 a
second
362 b
5.4 b
109 a
third
62 c
4.9 b
26 b
Z
First flush shoots originated from axillary buds on previous year’s shoots. Second flush
shoots originated from axillary buds on current-season first shoots. Third flush shoots
originated from axillary buds on current-season second shoots.
y
Means within the same column and cultivar followed different letters are significantly
different at P<0.05 based on LS Means test.

76

Table 4.3. Shoot growth and floral bud numbers associated with the primary, secondary
and tertiary growth flushes during 2012 of ‘Bluecrop and Elliott’ highbush blueberry
cultivars grown in Holt, MI. by inspecting dormant branches
Cultivar
Vegetative Shoot length
Average shoot
Floral buds per
z
per plant (cm)
length (cm)
plant
flush
Bluecrop
first
125
4.6
19 b
ay
second
82
b
4.5
33 a
third
13
c
4.2
8 c
Elliott
first
216
a
4.0
79 a
second
53
b
4.6
30 b
third
1
b
3.2
0.3 b
Z
first flush shoots originated from axillary buds on previous year’s shoots. Second flush
shoots originated from axillary buds on current-season first shoots. Third flush shoots
originated from axillary buds on current-season second shoots.
y
Means within the same column and cultivar followed different letters are significantly
different at P <0.05 based on LS Means test.

Table 4.4 Growing degree daysz for 2012 and 2013 at weather stations in West Olive
and Holt, MI
Month

West Olive

Holt

2012

2013

2012

2013

January

2

3

1

0

February

0

0

0

0

March

202

1

193

4

April

91

66

88

74

May

385

383

386

389

June

562

518

569

507

July

832

635

782

633

August

589

566

580

542

September

370

403

360

358

October

132

167

154

162

z

Basekerville-Emin growing degree model with base temperature of 50 °F, data from
Enviro-weather, (http://www.agweather.geo.msu.edu/mawn/)

77

Table 4.5 Average maximum air temperatures from May to October in 2012 and 2013
at West Olive, MI and Holt, MIz
Month
West Olive
Holt
May

2012
23.3

2013
22.3

2012
23.4

2013
22.8

June
July

27.8
32

25.2
26.8

27.2
31.2

24.8
26.8

August
September

27.5
23.2

26.2
23.1

27.3
22.9

25.8
22.4

October

16

17.1

16.9

17.4

z

Temperature measured in °C, data from the mean function at Enviro-weather
(http://www.agweather.geo.msu.edu/mawn/)

Table 4.6: Number of flower buds on primary shoots with or without subsequent growth flushes
in highbush blueberries grown at Coopersville, MI (Liberty) or Holt, MI (Duke, Draper) during
2012
Cultivar
Duke

Primary shoot without subsequent
growth flushes
2.1 az

Primary shoot with subsequent
growth flush
0.1 b

Draper

1.3 a

0.3 a

Liberty

2.4 a

0.8 b

z

Means within the same row followed different letters are significantly different at P <0.05 based
on LS Means test.

78

Table 4.7: Number of flower buds on primary shoots with or without subsequent growth flushes
in highbush blueberries grown at Coopersville, MI (Liberty) or HTRC (Duke, Draper) during
2013
Cultivar
Duke

Primary shoot without subsequent
growth flushes
2.4 az

Primary shoot with subsequent
growth flush
0.8 b

Draper

1.3 a

0.3 b

Liberty

2.1 a

0.06 b

z

Means within the same row followed different letters are significantly different at P <0.05 based
on LS Means test.

79

LITERATURE CITED

80

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81