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dissertation entitled

DETERMINATION OF DAMAGE THRESHOLD LEVELS OF
STRAWBERRIES (Fragan‘a x ananassa)

presented by

A. Zafer Makaraci

«has been accepted towards fulfillment
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Ph.D. degree in Horticulture

 

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6/01 cJClRC/DatoDuepGS-sz

DETERMINATION OF DAMAGE THRESHOLD LEVELS OF STRAWBERRIES
(Fragan'a x ananassa)

By

A. Zafer Makaraci

A DISSERTATION
Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of
DOCTOR OF PHILOSOPHY

Department of Horticulture

2003

ABSTRACT

DETERMINATION OF DAMAGE THRESHOLD LEVELS OF STRAWBERRIES
(Fragan'a x ananassa)
by

A. Zafer Makaraci

Damage thresholds of strawberry plants (Fragan'a x ananassa) were
investigated by using two different methods. The first method was mechanical
damage by punching holes in leaves such that a predetermined leaf area was
removed from each leaf. The second method was terbacil application.
Mechanical damage was applied, such that 10%, 20% and 30% of the leaf area of
a fully expanded single leaf was removed. Terbacil was applied to field-grown
plants during 2001 and 2002. In 2001, terbacil was applied at concentrations of
12.5, 25, 50, 100, 200 and 400 ppm. In 2002, a previously untreated group of two
year old strawberry plants were sprayed with terbacil at concentrations of 50, 100
and 200 ppm at two growth stages (during fruit set and after harvest stages).

Strawberry leaves that were mechanically damaged did not recover their
photosynthetic capacity following the damage. Chlorophyll fluorescence (Fv/Fm)
values were not affected by the mechanical damage. Increasing damage levels
decreased the ability of the strawberry leaf to use light and carbon dioxide in

photosynthesis. Difference in stomatal conductance and transpiration rates were

insignificant. lntemal 002 (0.) levels were higher in damaged plants compared to
the control plants.

Strawberry plants that were treated with terbacil (12.5, 25, 50, 100 and
200 ppm) were able to recover at certain levels, except 400 ppm level during the
first year experiment. Recovery occurred between 4 and 10 days after the
terbacil treatment. Average fruit weight was adversely affected during the year
following the 400 ppm terbacil treatment. Other concentrations of terbacil did not
have any affect on fruit yield. Stage of development did not alter the response of
the plants to terbacil. Difference in stomatal conductance and transpiration rates
were insignificant. lntemal C02 (Ci) levels were higher in plants that were treated
with high terbacil concentrations. Chlorophyll a and total chlorophyll content
decreased following the terbacil treatment. However, chlorophyll a and total
chlorophyll increased 8 days after terbacil treatment. Plant dry matter and chl b

values were not affected by the terbacil treatments.

ACKNOWLEDGMENTS

I would like to express my sincere gratitude to Dr. James Flore, my major
professor, for the support and advice that he has given during my doctoral
program. He always encouraged me and provided motivation for completion of
my doctoral program.

I also would like to thank to the members of my guidance committee, Dr.
James Hancock, Dr. Stanley Howell, Dr. Kenneth Poff and Dr. Frank Telewski
for their interest and suggestions with the dissertation projects.

I also thank to the Turkish government for providing scholarship for my

Ph.D. education.

iv

TABLE OF CONTENTS

LIST OF TABLES ......................................................................... vii
LIST OF FIGURES ....................................................................... viii
LIST OF ABBREVIATIONS ............................................................ xi
INTRODUCTION ........................................................................... 1
LITERATURE REVIEW .................................................................. 6
LITERATURE CITED ..................................................................... 14
CHAPTER 1
PHYSICAL DAMAGE THRACEHOLD LEVELS OF
STRAWBERRIES (Fragan'a x ananasa cv. ‘Honeoye’) ................ 21
ABSTRACT ......................................................................... 22
INTRODUCTION ................................................................... 23
MATERIALS AND METHODS ................................................. 26
RESULTS ........................................................................... 32
DISCUSSION ...................................................................... 44
LITERATURE CITED ........................................................... 47
CHAPTER 2

THE USE OF TERBACIL AS A TOOL TO ESTABLISH A
PHOTOSYNTHETIC THRESHOLD IN STRAWBERRIES

(Fragan'a x ananasa cv. ‘Honeoye’) ........................................ 50
ABSTRACT ......................................................................... 51
INTRODUCTION ................................................................... 53
MATERIALS AND METHODS ................................................. 55
RESULTS ........................................................................... 61
DISCUSSION ...................................................................... 93

LITERATURE CITED ........................................................... 99
SUMMARY AND CONCLUSIONS .................................................... 102
APPENDIXES .............................................................................. 104

vi

LIST OF TABLES

Table 1. Is, CE and C02 compensation values from the A/C; curves .......... 35
Table 2. 0E. and calculated light compensation values from the

light response curves ............................................................. 36
Table 3. Effects of the level of foliar damage on stomatal

conductance (gs). ................................................................. 37

Table 4. Effects of the level of foliar damage on transpiration rate (E) ....... 37

vii

LIST OF FIGURES

Figure 1. Effects of the level of foliar damage on CO2 assimilation ............ 38

Figure 2. Effects of the level of foliar damage on CO2 assimilation

expressed as a percentage of control ...................................... 39
Figure 3. Effects of the level of foliar damage on internal CO2 levels ...... 40
Figure 4. Effects of the level of foliar damage on Fv/Fm ........................... 41
Figure 5. Effects of foliar damage on the A/C. relationship ....................... 42

Figure 6. Light response curves for CO2 assimilation rate in plants

experiencing various levels of foliar damage.................................. 43
Figure 7. The effect of different terbacil concentrations on assimilation

rate (A) in 2001 growing season ............................................. 64
Figure 8. Change of assimilation rate (A) in percentage of control in

2001 growing season ............................................................. 65
Figure 9. The effect of different terbacil concentrations on stomatal

conductance (gs) in 2001 growing season ................................. 66
Figure 10. The effect of different terbacil concentrations on internal C02

(0,) in 2001 growing season ..................................................... 67
Figure 11. The effect of different terbacil concentrations on transpiration

(E) in 2001 growing season .................................................... 68
Figure 12. The effect of different terbacil concentrations on F,,/Fm

in 2001 growing season .......................................................... 69

viii

Figure 13. Relationship between F,,/Fm and assimilation in 2001

growing season .................................................................

Figure 14. The effect of terbacil application during the fruiting stage

on 002 assimilation rate (A) in 2002 ......................................

Figure 15. The effect of terbacil on assimilation rate (A) in 2002

growing season (After the harvest stage) ...................................

Figure 16. Effect of terbacil application during the fruiting stage

on assimilation rate (A) expressed as percentage of control

in 2002 ............................................................................

Figure 17. Effect of terbacil application during the after harvest

stage on assimilation rate (A) expressed as percentage of

control in 2002 ..................................................................

Figure 18. The effect of terbacil application during the fruiting stage

on stomatal conductance (gs) in 2002 ...................................

Figure 19. The effect of terbacil application on stomatal conductance

(gs) in 2002 (After harvest stage) ..........................................

Figure 20. The effect of terbacil on lntemal C02 (Ci) in 2002

(During fruit set stage) ........................................................

Figure 21. The effect of terbacil on internal C02 (Ci) in 2002

(After harvest stage) ...........................................................

Figure 22. The effect of terbacil on transpiration (E) in 2002

(During fruit set stage) ..........................................................

Figure 23. The effect of terbacil on transpiration (E) in 2002

ix

.. 70

....75

..... 76

77

78

79

...80

81

82

83

(After harvest stage) ........................................................... 84
Figure 24. The effect of terbacil on FV/Fm value in 2002

(During fruit set stage) ........................................................ 85
Figure 25. The effect of terbacil on Fv/Fm value in 2002

(After harvest stage) .......................................................... 86
Figure 26. Relationship between FV/Fm and assimilation in 2002

growing season (during fruit set stage) .................................... 87
Figure 27. Relationship between Fv/Fm and assimilation in 2002

growing season (after harvest stage) ........................................ 87
Figure 28. Effect of terbacil on root, crown, leaf and total plant dry

weights ............................................................................. 88
Figure 29. Effect of terbacil on root, crown, leaf and total plant dry

weights dry (After harvest stage) ............................................. 88
Figure 30. Effect of terbacil on fruit yield (Plants treated in 2001) ............. 89
Figure 31. Effect of terbacil on total fruit yield (Plants treated in 2002).... 89
Figure 32. Effect of terbacil on average fruit weight

(Plants treated in 2001) ......................................................... 90
Figure 33. Effect of terbacil on average fmit weight

(Plants treated in 2002) ...................................................... 90
Figure 34. Effect of terbacil on total chl, chl a, chl b and P chl

(During Fruit Set Stage) .......................................................... 91
Figure 35. Effect of terbacil on total chl, chl a, chl b and P chl

(After Harvest Stage) .............................................................. 92

LIST OF ABBREVIATONS

C.E.
chl a
chl b

Ci

Fv/Fm

9s

P chl
PAR
R2
RuBP

Q.E.

net 002 assimilation (umol.m'2.s'1)
carboxylation efficiency

chlorophyll a

chlorophyll b

lntemal 002 concentration (ppm)
transpiration rate (mmol.m'2.s")
maximal fluorescence

instantaneous fluorescence

variable fluorescence

photochemical fluorescence

stomatal conductance rate (umol.m‘2.s'1)
relative stomatal limitation to A
protochlorophyll

photosynteticly active radiation (pmol.m‘2.s")
regression coefficient

ribulose-1 ,5-bisphosphate

quantum efficiency

xi

INTRODUCTION

The damage threshold when used in a pest management context is
defined as the level of pest damage above which there are negative effects on
the growth or health of the plant. For a perennial crop like strawberry the effect
could either be in the current year, or in the subsequent crop year.

Foliage damage threshold levels have been determined for several plants,
but have not been determined for strawberry. Such thresholds have been
observed in wheat and barley (Shaw, 1956), soybean (Wareing, 1968), luceme
(Hodgkinson, 1974), sour cherry (Layne, 1989), (Disegna, 1994), apple (Ferree,
1982), (Lakso 1996). Foliage damage threshold levels for other crop plants range
from 5% - 20% depending on the crop, and the crop load (Disegna, 1994).
Determination of such a value would be useful in IPM and pesticide application
programs, the assessment of environmental impacts, and on economical studies.

Damage to strawberry foliage can be biotic (insect, disease or weeds) or
abiotic (temperature, drought, anoxia, etc). Biotic damage is a major concern to
growers. Growers need to intervene at different stages of growth and this
intervention may differ depending on the level of damage (Gut, 2003). Plants
may have different photosynthetic recovery levels at different growth stages,
such development stage of the leaves. Thicker leaves are usually more resistant
to damage from herbicides and pathogens. Kinivood (1983), Unrath (1981) and
Bukovac (1979) found that cuticle of older leaves is less permeable and thicker.

This decreases herbicide absorption.

C02 assimilation rates are similar to those of many other fruit crops (Flore,
1989). In Fragaria x ananassa Duch. Photosynthesis rate range from 15 to 25
pmol.m‘2.s'1 (Hancock, 1989). High photosynthesis rates do not result in increase
in strawberry fruit yield (Strick 1986). Strawberry plants are known to have active
sinks. Roots, runners, fruits and leaves are the sinks for strawberry plants
(Alpert, 1986). In order to have high yield high portion of the fixed carbon has to
be allocated to fruit (Hancock, 1991 ).

Deblossoming causes new leaf formation and total photosynthetic rate
increases on per plant basis in strawberry plants (Fomey, 1985). Defoliation of in
excess of 66% leaf area result in higher CO2 assimilation rate per leaf area.
However, photosynthesis of the whole plant is not compensated completely
(Kerkhoff, 1988). Removal of the flowers during the first year caused increased
vegetative growth in both years and increased yield during the second year
(Daugaard, 1999). I

Fruit load may also affect photosynthetic recovery, since fruits are one of
the major sinks for carbohydrates. Fruit removal often result in decrease in CO2
assimilation rates on a per leaf area basis for at least a few weeks in strawberry
plants (Schaffer, 1986).

Leaf removal decreases the total dry weight of strawberry plants
(Chandler, 1988).

Gucci (1990) found different responses in C02 assimilation rates in plum
trees depending on the stage that fruits were removed from trees. Allocation of

carbohydrates to fruit may also affect the recovery process. Perennial crops may

also have a carry over effect into the next season. This carry over effect may be
in the form of a decrease in cold hardiness. It has been found that early leaf loss
caused a decrease in cold hardiness of the sour cherry buds and this effect was
carried over into the next season (Howell, 1973).

The amount of the leaf damage may also have an affect on photosynthetic
recovery. Damage may occur by different organisms such as insects, diseases,
nematodes and mammals. Some environmental factors can also damage the
leaves such as low temperature, wind, hail and fire. Cultural practices may also
cause damage such as mechanical harvest or herbicide toxicity. Different parts
(organs) of the plant can be damaged and each plant part may have a different
damage threshold. Root damage may occur by nematodes in strawberry plants
and different insects and diseases cause leaf damage. Some major diseases for
strawberry plants in Michigan as follows: Leaf spot, Leaf blight, Scorch, Stem end
rot, Angular leaf spot, Red stele, Powdery mildew, Anthracnose, Gray mold,
Leather rot. Major insects that cause damage in strawberry plants in Michigan as
follows: Strawberry sap beetle, Mites, Tarnished plant bug, Spittlebug,
Strawberry leafroller, Strawberry clipper, Slugs, Leafhoppers, Strawberry aphips,
Grubs (Gut, 2003).

In woody plants trunk damage may occur by cold damage, small
mammals or mechanical harvest. Trunk damage thresholds have been
investigated in some trees (Layne, 1989).

The hypothesis tested in this research was “Leaf photosynthetic capacity

will determine the damage threshold levels for strawberry productivity”.

For this purpose two different methods were used to determine the
threshold levels in strawberry. Simulation of insects damage (mechanical
damage) to the leaves by a leaf punch and use of terbacil a photosytem ll
inhibitor as a tool to reduce photosyntesis.

Hole punching has been used as a method to simulate insect damage to
leaves on other species (Kappel ,1986; Layne, 1989).

Terbacil is a uracil type herbicide that blocks both the Hill reaction and
photosytem II in the photosynthetic pathway (Ashton , 1973). Terbacil was used
on fruit trees as a method to limit photosyntesis and to cause thinning (DeIValIe,
1985). Others have used terbacil as tool to investigate the damage thresholds
(Byers, 1990; Disegna, 1994). In this research, terbacil is used as a tool to
investigate the photosynthetic threshold of strawberry plants and to investigate
other effects which may be related to photosynthesis (e.g. fruit yield, dry weight
of plant). It is commonly used to control the weeds in strawberry. It is usually
applied before planting, in early season and after harvest renovation (Mahr et. al,
2002)

The objective of the first research (first chapter) was to determine the leaf
damage threshold for strawberry (Fragan'a x ananassa cv. Honeoye) by
simulating leaf damage with hole punches (0.33 cm2) . Because of the difficulty to
calculate the dynamically changing canopy area of strawberry plants
observations were conducted on single leaf.

The objective of second research (second chapter) was to determine the

leaf damage threshold for strawberry (Fragan’a x ananassa cv. Honeoye) on

whole plants under field conditions at different times during the growing season.
Leaf damage was simulated by applying terbacil to the foliage at different
concentrations and at different critical stages in crop development. The degree
and duration of photosynthetic inhibition are dose dependant and crop-specific. It
was hypothesized that different levels of Pn reduction could reduce the
production and storage of carbohydrates needed for growth and that reduced
carbohydrate production may affect yield and runner production negatively and

the ability of the plant to resist environmental stress.

LITERATURE REVIEW

Photosynthetic compensation and damage thresholds

Photosynthetic compensation in response to leaf injury and leaf area loss
has been reported in several species of plants. Such compensation has been
observed in wheat and barley (Shaw, 1956), soybean (Wareing 1968) and
luceme (Hodgkinson, 1974), sour cherry (Layne, 1989), (Disegna, 1994), apple
(Ferree, 1982), (Lakso 1996).

The effect of insect injury on whole plant productivity has been evaluated
by simulation of the injury caused by an insect. To simulate the damage caused
by the spotted tentiforrn leaf miner (Phyllonorycter blancardella), Kappel (1986)
punched holes in leaves of apple trees. This treatment reduced trunk growth,
rootstock growth, fruit number and fruit yield. Kappel (1986) also demonstrated
that leaf injury reduced return bloom and fruit set in the following year. In poplar
trees, removing 75% to 80% of the leaf area reduced the growth of young poplar
trees by 20% in nursery conditions (Bassman, 1982). In tomato, removing 75% of
the plant’s leaf area reduced fruit yield by 40% Stacey, 1983). Fruit yield was
reduced by 80% when all the spur leaves of Golden Delicious apple trees were
removed (Ferree, 1982).

Layne (1989) demonstrated that, in non fruiting ‘Montmorency’ sour cherry
trees, the removal of 20% of leaf area caused no significant reduction in net

carbon dioxide assimilation and had no effect on tree dry weight. In these trees,

photosynthetic compensation following leaf injury was observed four days after

the leaf damage had occurred.

Photosynthesis

C02 assimilation rates in strawberry are similar to other fruit crops (Flore
and Lakso, 1989). Under field conditions, strawberry plants (Fragan'a x
ananassa) typically have C02 assimilation rates of 15-25 |.lmol.m'2.s'1 (Hancock,
1989).

The light saturation point for photosynthesis in field-grown strawberry
plants (Fragan‘a x ananassa.) is between 800 and 1000 pmol. m‘2.s'1 (Cameron,

1990).

Diseases

Viral, bacterial and fungal diseases that infect leaves can cause a
decrease in CO2 assimilation rates. When young peach leaves were infected with
Peach rosette virus and decline disease, decreases in C02 assimilation rate and
leaf growth were observed (Smith 1977).

Apple scab infection (Ventura inaequalis) decreases C02 assimilation
rates of apple leaves within 28 days of inoculation (Spotts, 1979). However, the
average decrease in CO2 assimilation rate was smaller than the decrease in leaf

area caused by the disease. This would indicate that the remaining healthy

leaves may have increased their CO2 assimilation rate to compensate for the leaf
loss (Spotts, 1979).

Apple powdery mildew (Podospharea Ieucotn‘cha) reduced the C02
assimilation and transpiration rates of all leaves but had the greatest effect on
CO2 assimilation rate in young apple leaves (Ellis, 1981). Leaves that are
infected during the early stages of growth, never regain their full photosynthetic
capacity (Ellis, 1981).

Diseases reduce the amount of light penetrating the leaf (Smith-1977).
Contrary, Lakso (1982) found that low levels of powdery mildew (Unincula

necator) increased C02 assimilation rate in grape vines (Lakso 1982).

Damage thresholds for insect and mite infestations

Spider mites (Tetranchus urticae Koch) can reduce fruit yield in
strawberries. Thirty cumulative mite days during any monthly period was found to
be threshold level for strawberry plants. Higher values decreased the yield of
strawberry plants. (Walsh, 1998).

Tamished plant bug (Lygus Iineolan's) damages the blossom clusters and
reduces the yield of strawberries. Economic injury level, as indicated by
strawberry weight, was approximately 0.95-0.99 tarnished plant bug nymphs per
blossom cluster. Action threshold for the ‘Redcoat’ strawberry cultivar was

estimated at 0.26 nymphs per blossom cluster (Mailloux, 1988).

Flower bud removal trials on 12 strawberry cultivars indicated that most
could compensate for a significant amount of flower bud loss caused by the
strawberry bud weevil (Anthonomus signatus), provided that the loss occurred
during early development of the inflorescence (Pritts, 1999). In most of these 12
cultivars injury remained below the damage threshold (Pritts, 1999).

In ‘Redchief’ strawberry, the economic threshold for yield reduction by the
nymphs of the cercopid Philaenus spuman’us was found to be 20 nymphs per rn'2
(Zajac, 1984).

The effect of two-spotted spider infestations was investigated on ‘Franklin’
apple trees. Mite infestation levels of 15, 30 and 60 mites per leaf reduced C02
assimilation by 26, 30 and 43 percent respectively, when compared to control
plants (Hall, 1976).

Proctor (1982) investigated the effects of leaf injury caused by the spotted
tentiforrn leaf miner (Phyllonorycter blancardella) on CO2 assimilation rate. The
lowest net C02 assimilation rate was measured in leaves that had 3 mines per
leaf. Leaves injured by 20 mines per leaf suffered a 32.9% reduction in leaf area.
However, the decrease in the photosynthetic rate of these leaves was only
23.2%, which indicated that photosynthetic compensation had occured (Proctor,

1982)

Effect of Fruit Load on Photosynthesis

It has been reported that fruiting and non-fruiting had no difference C02
assimilation rate on either seasonal or diurnal basis in sweet cherries. It has
been suggested that CO2 assimilation rates in sweet cherry were primarily affect
by ontogeny and environment. Strength of the sink did not have influence the
C02 assimilation rates (Roper, 1988).

Gucci (1990) investigated the effects time of the season that fruits were
removed on CO2 assimilation rate. Removing fruit at pit hardening stage
decreased the 002 assimilation rate by 25% within 24 hours. However, removing

mature fruits did not have any effect on CO2 assimilation rate.

Photosynthetic inhibition

Among the most commonly used herbicides in agriculture are herbicides
that act as photosynthetic inhibitors (Trebst, 1981), which includes ureas,
triazines and bipyridiniums (Van Rensen, 1989). Fifty percent of commercially
used herbicides are inhibitors of photosynthesis (Trebst, 1981 ). Terbacil, which is
used to control weeds in strawberries, is classified as a uracil herbicide (Aston,
1977)

Terbacil controls many annual weeds and some perennial weeds. Terbacil
is absorbed primarily by roots and translocated apoplastically to the leaves, but

can also be taken up directly by the leaves with the aid of surfactants. Adjuvants

10

increase herbicidal activity by increasing retention, penetration, absorption and
translocation of the herbicide (Kirkwood, 1983). A general symptom of terbacil
toxicity is chlorosis, which is a consequence of the degeneration of chloroplasts
in the leaves of susceptible plants (lzawa, 1965).

Photosynthetic inhibitors interfere with the Hill reaction, which occurs in
chloroplasts. Hill reaction is defined as the evolution of oxygen by a suspension
of isolated chloroplasts when illuminated in the presence of an artificial electron
acceptor (Moreland, 1980). When the Hill reaction is interfered, ATP formation is
also inhibited. Thus, energy production stops in the chloroplast. Tresbst (1981),
indicated that most of the herbicides are inhibitors of electron flow at the
functional site between the primary and secondary electron acceptors of
photosystem II.

Van Rensen (1989) reported that the damage caused by many uracil
herbicides was reversible. lzawa (1965) concluded that diuron binds weakly to
the receptor molecule in the thylakoid membrane.

Designa (1994) found that damage caused by the terbacil was reversible
in apple trees.

Degradation of terbacil also varies in plants. Terbacil was degraded more
in beans which are susceptible to terbacil than citrus. Citrus is considered
tolerant to terbacil (Herholdt, 1968). However, Barrentine (1970) found that
terbacil was metabolized at a higher rate in tolerant peppermint plants than in

susceptible sweet potato plants

11

Use of Chlorophyll Fluorescence to Determine the Herbicide

inhibition

Measuring chlorophyll fluorescence has been used to determine
photosynthetic activity and this method gives detailed information about
photosystem integrity system (Krause, 1984). Chlorophyll fluorescence
measurement at 685 nm indicates the energy state of the P 680 reaction centre
of photosytem II and its associated pigments reflects the rate of electron
transport from photo system II to chemical acceptors and the coupling between
ATP and electron transport (Krause, 1984).

It has been reported an that there is an inverse relationship between
assimilation and photosynthesis after the application of herbicide which limits
electron transport (Panneels, 1987).

Leaf fluorescence changes were found from the inhibition of
photosynthetic electron by using herbicide simazine and diuron (Miles, 1973).

Voss (1984) reported that when analyzing the chlorophyll fluorescence
from the leaves of different species treated with photosynthesis inhibitors, the
Fv/Fm provides a good estimate about the changes in the photosynthetic capacity
of the leaves after the herbicide treatment.

Designa (1994) found that Fv/Fm values can be used to assess the

photosynthetic inhibition cause by terbacil.

12

Terbacil Tolerance in Strawberries

The tolerance of strawberry to terbacil has been shown to be at least
partially attributable to restricted translocation of root-absorbed herbicide to the
site action in mesophyll chloroplasts. Uptake by the root did not appear to be a
factor in tolerance to terbacil (Genez, 1983).

Honeoye, Guardian and Darrow strawberry cultivars are reported to
susceptible to terbacil. Recommended rate is 138-419 g/ha . 559 g/ha are found
to be toxic to the strawberry. Rate should be chosen depending on the soil type.
Lower rate suggested on coarse type soils. (Mahr et.al. , 2002)

'Chambly' strawberry a hybrid of Sparkle X Honeoye, is reported to be

tolerant to terbacil (Khanizadeh, 1990)

13

LITERATURE CITED

Alpert, P., H.A. Mooney. 1986 Resource sharing among ramets in the
clonal herb, Fragan'a chiloensis. 0ecologia 70:227-233.

Ashton, F.M. , 0.T. De Villiers, R.K. Glenn, W.B. Duke, 1977. Localization
of metabolic sites of action of herbicides. Pestic. Biochem. Physiol. 7:122-124.

Bassman, J., W. Myers, D, Dickman, L. Wilson. 1982. Effects of
simulated insect injury on eany growth of nursery grown hybrid poplars in
Wisconsin. Can. J. For. Res. 12:1-9.

Byers, R.E., J.A. Barden, R.F. Polomski, R.W. Young, D.H. Carbaugh.
1990. Apple thinning by photosynthetic inhibition. J. Amer. Soc. Hort. Sci.
115214-19.

Cameron, J.S., C.A. Hartley. 1990. Gas Exchange Characteristics of
Fragan’a chiloensis genotypes. Hort-Science 25(3): 327-329.

Chandler, C.K., D.D. Miller, D.C. Ferree DC. 1988. Influence of leaf
removal, root pruning, and soil addition on the growth of greenhouse-grown
strawberry plants. J. Amer. So. Hort. Sci. 113:4, 529-532.

DelValle, T.B.G., J.A. Barden, R.E. Byers. 1985. Thinning of peaches by
temporary inhibition of photosynthesis with terbacil. J. Amer. Soc. Hort. Sci.
110:804-807

Disegna, E.J. 1994. The use terbacil as a tool to establish a
photosynthetic threshold in apples. Michigan State Univ., East Lansing, Msc

thesis.

14

Daugaard, H. 1999. The effect of the flower removal on the yield and
vegetative growth of A+ frigo plants of strawberry (Fragaria x ananassa Duch).
Scientia-Horticulturae. 98:1 -2, 153-1 57.

Ellis, M.A., D.C. Ferree, D.E. Spring. 1981. Photosynthesis, transpiration
and carbohydrate content of apple leaves infected by Podosphaera Ieucotricha.
Phyopahology 71:329-395.

Ferree, DC. and J.W. Palmer. 1982. Effect of spur defoliation and ringing
during bloom on fruiting, fruit mineral level and net photosynthesis of ‘Golden
Delicious’ apple. J. Amer. So. Hort. Sci. 107(6): 1 182-1 186.

Flore, J.A., A. Lakso. 1989. Environmental and physiological regulation of
photosynthesis in fruit crops. Horticulture Reviews 11, 111-157.

Forney, C.F., P.J. Breen. 1985. Dry matter partitioning and assimilation in
fruiting and deblossomed strawberry. J. Amer. Hort. Soc. 110:181-185.

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15

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16

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18

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19

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20

Chapter 1

DETERMINATION OF PHYSICAL DAMAGE THRESHOLDS TO LEAVES OF

STRAWBERRY (Fragaria x ananassa cv. ‘Honeoye’)

21

ABSTRACT

Damage thresholds of strawberry plants (Fragaria x ananassa) were
investigated by mechanical damage. Mechanical damage was applied such that
10%, 20% and 30% of the leaf area of a fully expanded single leaf was removed
Strawberry leaves that were mechanically damaged did not recover their
photosynthetic capacity at any damage level. Chlorophyll fluorescence (Fv/Fm)
values were not affected from the mechanical damage. Increasing damage levels
decreased the ability of the strawberry leaf to use light and carbon dioxide in
photosynthesis. Difference in stomatal conductance and transpiration rates were
insignificant. Internal CO2 (Ci) levels were higher in damaged plants compared to

the control plants.

22

INTRODUCTION

Damage threshold is defined as the level of pest damage above which
plant growth or health affected negatively. Strawberry plants are perennial plants
and the effect of such damage could be in the current year or in the subsequent
crop.

Foliage damage threshold research studies have been performed in other
plants, but such damage threshold level has not been determined for strawberry.
Such threshold has been observed in wheat and barley (Shaw, 1956), soybean
(Wareing 1986), luceme (Hodgkinson, 1974), sour cherry (Layne, 1989),
(Disegna, 1994). Determined threshold levels for other plants range from 5% -
20% depending on the crop. Crop load also affects the damage threshold levels
(Disegna, 1994). Determination of such a value for strawberry plants would be
useful for in strawberry production. These threshold levels can be used in IPM
and pesticide application programs and in economical studies in strawberry
production.

Biotic and abiotic damage can occur in strawberry plants. Biotic damage
may be caused by insects, diseases and weeds. Abiotic damage may be caused
by temperature, drought, anoxia etc. Biotic damage is a major concern in plant
production. Growers need to intervene at different stages of growth and this
intervention may differ depending on the level of the damage. Photosynthetic
recovery levels may be different in different growth stages. Sink load can also be

affected by the age of the plant. On the other hand leaf thickness is an important

23

factor that affects the resistance of the plants against pathogens and herbicides.
Kirwood (1983) and Unrath (1981) found that cuticle of older leaves is less
permeable and thicker.

Fruit load may also affect the photosynthetic recovery metabolism, since
fruits are one of the major sinks for carbohydrates. Gucci and Flore (1990) found
different responses in CO2 assimilation rates in plum trees depending on the
stage that fruits were removed from trees. Allocation of carbohydrates to fruit
may affect the recovery process.

In perennial crops, damage can cause negative effects into the next
season. This carry over effect may be in different forms such yield loss or
decrease in cold hardiness. It has been found that early leaf loss caused a
decrease in cold hardiness of the sour cherry buds and this effect was carried
over into the next season (Howell, 1973).

The amount of the leaf damage may also have an affect on photosynthetic
recovery. Damage occurs by different factors such as insects, diseases,
nematodes and mammals. Some environmental factors also damage the leaves
chilling from low temperature, wind, hall and fire. Cultural practices may also
cause damage such as mechanical harvest and herbicide toxicity. Different parts
(organs) of the plant can be damaged and each plant part may have different
damage thresholds. Root damage may occur by nematodes in strawberry plants
and different insects and diseases cause leaf damage. In woody plants trunk
damage may occur by cold, small mammals or mechanical harvest. Trunk

damage thresholds have been investigated in some trees (Layne, 1989).

24

The hypothesis tested in this research was “Leaf photosynthetic capacity
will determine damage threshold levels for strawberry productivity”.

For this purpose hole punching was used to simulate damage of insects in
strawberry.

Hole punching is used to simulate insect damage in threshold studies
(Kappel ,1986; Layne, 1989). Common pests that affect the strawberries are
aphids, leaf rollers, mealybugs, leafhoppers, spittlebugs and spider mites
(Hancock, 1999). In this study a mechanical method (hole punching) was used to
simulate the injury to leaves.

The objective of this research was to determine the leaf damage threshold
for strawberry (Fragaria x ananassa cv. Honeoye) by simulating leaf damage by
hole punching. Because of the difficulty to calculate the dynamically changing

canopy area of strawberry plants observations were conducted on single leaf.

25

MATERIALS AND METHODS

Plant Material

Strawberry plants (Fragaria x ananassa cv. Honeoye) were grown in the
Michigan State University Plant Science Greenhouses, East Lansing, MI. Plants
were planted in pots (2.2 L) containing a 1:1 (v/v) mix of sterilized greenhouse
soil and BACTTO potting media (80% peat, 20% perlite). They were grown under
long days to prevent flowering and encourage vegetative growth. Daylength was
adjusted to 16 hour days and 8 hour nights using supplemental illumination
provided by high pressure halogen lights (minimum of 110 pmol.m'2.s'1 PAR).
Average greenhouse temperatures were 21°C during the day and 17°C during
the night. Runners were removed from the plants every ten days. Plants were
irrigated using a drip irrigation system that delivered water approximately 60 ml
per pot three times during a 24 hours period. A soluble fertilizer (Peter’s 20-20-
20 N,P,K) was applied bi-weekly at the rate of 5 grams per plant by using the drip
irrigation system. Pest management (AvidTM 0.49g/L and StrikeT'“ 0.129/L) was
provided as necessary.

Leaf area was estimated by developing a regression equation formula
based on measurements of the length and width of each of the leaflet triplet of
strawberry. Fifty fully expanded leaves were measured at their widest points.

Leaf area was measured by using the Ll-COR (Lincoln, NE) Ll-3000 leaf area

26

meter. Based on these measurements, the regression equation that was used to
calculate the leaf area in these experiments was:
= -22.24 + (3.29W)+(4.97L) (R2=0.946)
Where: W is width of the triplet leaf
L is length of the triplet leaf
A is area of each triplet in cm2
Leaf area removal treatments were applied by removing 10% , 20% and
30% of the leaf area of recently fully expanded leaves using a paper hole punch
(0.33 cmz). One leaf was selected per plant and leaf discs were punched
randomly throughout the lamina while avoiding the midrib of the leaflet. The
number of punches was equal on either side of the midrib.
The experiment was arranged in a completely randomized design with

seven plants in each treatment. Control plants had no damage (0%).

Gas exchange measurements

All measurements were conducted in the laboratory in a walk in growth
chamber (Model PGV36, Conviron, Canada) using the open system described by
Sams and Flore (1982) and Gucci (1988). The following modifications were made
to the measurement system: a) A leaf cuvette was constructed from VeroliteTM
(Matra Industries Inc, Ontario, Canada) material which had dimensions of 30 cm
(W) x 30 cm (L) x 21 cm (H) b) b) to construct the top of the cuvette Maylar®

(DuPont Chemicals, US) was used c) The ClRAS-1 portable photosynthesis

27

g. I

system (PP Systems, Hertfordshire, UK) was used to measure differential in CO2
concentration and partial water vapor pressure at the inlet and outlet of the leaf
chamber. d) The air flow into the chamber was measured by using two Cole
Parmer 10620 flow meters (each of them has 5 L/min maximum flow rate ) f) A
110 V AC 12 cm fan was placed in the cuvette to provide uniform air circulation.
g) A 15 cm x 20 cm cooling radiator was used which cools the leaf cuvette.

The pressurized air used in these experiments which was filtered and had
a CO2 concentration of 380:1:10 ppm. Fluorescent lights inside the growth
chamber provided light at an intensity of 850 pmol.m'2.s'1. Air temperature inside
the growth chamber was maintained at 22°C, while relative humidity was
maintained at 75%.

Plant material was brought from the greenhouse to the growth chamber at
8:30 am on the day of the experiment and allowed to acclimate to growth
chamber conditions for 30 minutes prior to initiation of measurements. A single
treated leaf per plant was enclosed in the leaf chamber while still attached to the
plant. Leaf temperature was monitored by a thermocouple that was in contact
with the lower surface of the leaf. The temperature of the enclosed leaf ranged
between 21.5 and 23°C. Air flow into the chamber was maintained at a rate of 8.6
Umin. Gas exchange measurements were made when 002 levels inside the
cuvette stabilized. Gas exchange parameters (A, gs, E and C.) were calculated by
using Photosyn Assistant Software, IRGA module, Version 1.1.2 (Dundee
Scientific, Dundee, UK) . Measurements were made one day before the leaf

damage treatments and 2, 4, 6, 8, 10, 12, 14 and 16 days after the treatments.

28

Chlorophyll fluorescence

Chlorophyll fluorescence was measured on the same leaves that were
used for gas exchange measurements. The Plant Efficiency analyzer (Hansatech
Instruments Ltd, Norfolk, UK) was used for these measurements. Leaves were
dark acclimated for 20 minutes using dark acclimation cuvetes. Leaves were then
irradiated with actinic light for 5 seconds and chlorophyll fluorescence kinetics
were recorded. Measurements were performed on the same leaves just before

the treatments and 2, 4, 6, 8, 10, 12, 14 and 16 days after the treatments.

NC. curves

The effect of CO2 concentration on assimilation rate was measured eight
days after the initiation of the leaf damage treatment. Measurements were made
as described in the gas exchange measurements section. CO2 levels were
adjusted by using an ADC GD600 (ADC Bioscientific Ltd, UK ) gas dilutor and
monitored by a CIRAS 1 unit (PP Systems, Hertfordshire, UK). 3000 ppm CO2
was provided to gas dilutor. CO2 scrubbing was performed using a column filled
with lime when lower CO2 levels were necessary. C, levels were calculated using
Photosyn Assistant Software, IRGA module, Version 1.1.2 (Dundee Scientific,
Dundee, UK). CO2 assimilation was calculated approximately at the following Ci

levels (120 ppm): 30, 40, 60, 70, 100, 130, 150,180, 200, 220, 240, 260, 330,

29

r.

400, 500, 580, 640, 680 and 700 ppm. The actual Ci values were used in

calculations.
Light response curves

Gas exchange in response to light response was determined eight days
after the leaf damage treatments. Measurements were made as described in the
gas exchange measurements section. Light intensity was adjusted raising of
lowering the light bank in Conviron growth chamber (Model PGV36, Conviron,
Canada). Light intensities used in this experiment were 0, 50, 100, 150, 350, 500,
650, 750, 850 and 1000 umol.m’2. s“. The highest light intensity 1000 umol.m2.s'
1 was obtained using supplemental portable high pressure sodium light. Light
intensities were measured using a LI-COR quantum sensor (Ll-COR, Lincoln,
NE). Quantum efficiency and light compensation points were calculated using
Photosyn Assistant Software, A0 Curve Analyis Module, Version 1.1.2 (Dundee

Scientific,Dundee,UK).
Plot Design and Statistical Calculations
Completely randomized design was used in this experiment. There were

seven plants in each treatment. Data were subjected to analysis of variance

(ANOVA). Means were compared by Duncan’s test or by standard deviation.

30

.‘J

Any data represented in percentage was transformed by arcsin conversion
before AN OVA.

Error bars in the figures represent standard deviation.

The SAS base statistical program (version 8.2, SAS institute, Cary, NC)

was used for ANOVA.

31

RESULTS

Effects of foliar damage on CO2 assimilation rate

Foliar damage caused a significant reduction in leaf CO2 assimilation rates
on leaf area basis (Figure 1). This reduction was apparent within two days of
treatment in all foliar damage treatment levels. Plants exposed to 30% leaf
damage suffered a statistically significant 44% reduction in CO2 assimilation rate
as compared to control plants (Figure 2). The CO2 assimilation rate of these
plants remained depressed, relative to the control, and fluctuated within a tight
range of 5 to 6 umol.m'2.s'1 over the two weeks following the treatment. In
comparison, the average CO2 assimilation rate in leaves of control plants ranged
between 9.98 umol.m'2.s'1 and 11.71 umol.m’2.s'1 over the same period. The
decrease in assimilation rate on day 16 of the experiment was observed in
treated as well as untreated plants. CO2 assimilation rates in plants exposed to
10% and 20% foliar damage exhibited patterns that were generally similar to the
one observed in plants in the 30% damage treatment. Leaves exposed to 20%
foliar damage suffered a significant reduction in CO2 assimilation rates. The CO2
assimilation rate in these plants ranged between 7.2 and 8 umol.m'2.s'1, which
were 28% to 35% lower than the rates measured in control plants. The 10% foliar
damage treatment caused a decrease in CO2 assimilation rate. However, the
difference in assimilation rate between the damaged plants and control plants

was significant only on days 2 and 14 following the treatment.

32

Gas exchange parameters

Foliar damage had no significant effect on stomatal conductance (gs).
Stomatal conductance in leaves of control and treated plants ranged between
129 and 220 pmol.m'2.s", with an exception that occurred on the fourth day after
treatment in the 10% foliar damage treatment where gs reached 329 umol.m'2.s'1
(Table 3). lntemal CO2 (Ci) levels tended to be higher in treated plants than in
control plants (Figure 3). Ci increased by up to 50% in leaves of damaged plants
over the first 4 days after treatment then decreased through day 8 after the
treatment. In comparison, C, levels in control plants increased through day 8 by
only 16% from initial levels of 220 ppm. lntemal CO2 levels in plants subjected to
30% foliar damage were significantly higher than C; levels in control plants only
on days 4, 12 and 14. Foliar damage had no significant effect on leaf
transpiration rate (E) (Table 4) Transpiration rates ranged between 2.38 and 7

mmol.m'2.s".

Chlorophyll fluorescence

Chlorophyll fluorescence was evaluated as the ratio of Fv over Fm values

(Fv/Fm).

33

The FV/Fm was not affected by the foliar damage treatments (Figure 4). No
significant differences in F,,/Fm values were found at any of the dates on which

chlorophyll fluorescence was measured.

AIC. curves

Foliar damage at the 20% and 30% levels altered the A/C, relationship in
the damaged leaves. At the 30% damage level, maximum assimilation rate was
approximately 6 pmol.m’2.s", significantly lower than that of control plants which
reached 17.5 pmol.m'2.s“. At the 20% damage level, plants suffered a 38%
reduction in maximum assimilation rate as compared to the control. Ten percent
leaf damage caused only a small decrease in maximum assimilation rate.

The CO2 compensation point was higher in damaged plants than in control
plants (Table 1). The CO2 compensation point in plants at the 30% damage level
was 27 ppm higher than that of control plants. At the 20% damage level, the CO2
compensation point increased by 14 ppm. The 10% leaf damage treatment had
little effect on the CO2 compensation point.

The carboxylation efficiency (C.E.), measured as the initial slope of the
NC, curve, was also affected by leaf damage (Table 1). A substantial decrease
in carboxylation efficiency, approximately 61%, was observed in the 30%
damage treatment relative to the control plants. At the 20% damage level, the
decrease in C.E. was approximately 32%, whereas there was little effect on CE.

in plants damaged at the 10% level.

34

Relative stomatal limitations (ls) were calculated with sensitivity analysis

method according to Jones (1998).

Damage level Is C.E. Internal CO2 levels for A
compensation (ppm)

 

0% %37 a 0.62 a 82 c
10% %34 a 0.59 b 83 c
20% %23 b 0.42 c 96 b
30% %18 c 0.38 d 109 a

 

Table 1. Is, CE and CO2 compensation values from the A/C, curves.
Means followed by different letters are significantly different by Duncan’s Multiple

Range Test (P5005).

Equations for the NCI curves were as follows

0% damage y=7.8766Ln(x)—33.62 R2: 0.96
10% damage y=7.5186Ln(x)-32.28 R2: 0.96
20% damage y=5.0791Ln(x)—22.03 R2: 0.96
30% damage y=3.6672Ln(x)-16.48 R2: 0.87

Light response curves

Leaf damage altered the light response relationship for photosynthesis.
Plants in the 30% leaf damage treatment had the lowest CO2 assimilation rates

at all light intensities tested (Figure 6). In these plants, the maximum 002

35

assimilation rate achieved was 5.5 umol.m'2.s’1, which was approximately 50% of
the maximum rate attained by control plants. Plants subjected to damage levels
of 10% and 20% also had diminished assimilation rates that were approximately
18% and 36% lower than that of the control. In all plants, CO2 assimilation rates
reached light saturation levels at a light intensity of approximately
850 umol.m’2.s'1.

Photosynthetic quantum efficiency (Q.E.) was lower in damaged plants
than in the control plants (Table 2). Calculated light compensation levels were

higher in damaged plants than in the control plants (Table 2).

Damage level Q.E. Calculated light compensation
levels for A (umol.m'2.s")

 

0% 0.0421 3 24.4 I)
10% 0.0378 a 23.9 b
20% 0.0205 b 31.3 b
30% 0.0136 C 53.8 a

 

Table 2. GE. and calculated light compensation values from the light
response curves. Means followed by different letters are significantly different by

Duncan’s Multiple Range Test (P5005).

36

Equations for the light response curves were as follows

0% damage = - 0.000015x2 + 0.025848x — 0.2936 R2=0.98
10% damage y= - 0.00001x2 + 0.01974x — 0.141 R2=0.98
20% damage y= - 0.000006x2 + 0.013966x - 0.313421 R2=0.98
30% damage y= - 0.000004x2 + 0.010321x — 0.551721 R2=0.97

Days After Foliar Damage

 

Damage Level 0 2 4 6 8 10 12 14 16
0% 195 185 164 183 166 135 129 148 121
10% 205 152 329 120 140 140 135 221 153
20% 153 174 220 149 164 138 130 155 120
30% 217 151 185 167 165 119 118 190 119

 

Table 3. Effects of the level of foliar damage on stomatal conductance (gs).

Means followed by different letters are significantly different by Duncan’s Multiple

 

Range Test (P5005).
Days After Foliar Damage
Damage Level 0 2 4 6 8 10 12 14 16
0% 2.48 2.48 2.80b 2.72 2.48 2.75 2.07 2.30b 4.57
10% 2.58 3.11 3.79b 3.07 2.75 2.22 2.14 2.89ab 3.43
20% 2.38 2.69 4.93ab 2.94 2.67 2.29 2.37 2.55ab 5.29
30% 2.40 2.56 7.09a 3.35 2.63 2.78 2.10 3.64a 3.44

 

Table 4. Effects of the level of foliar damage on transpiration rate (E). Means
followed by different letters are significantly different by Duncan’s Multiple Range

Test (P5005).

37

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Discussion

Foliar damage significantly reduced CO2 assimilation rates in all of the
treatments. Leaves exposed to 20% and 30% damage did not show any
compensation comparable to control plants. Layne (1989) observed that sour
cherry leaves were able to compensate with up to 20% foliar damage. In this
research strawberry leaves did not show any compensation at this or any other
damage level. Leaves exposed to 10% leaf damage could recover after two days
following the treatments. However, this recovery was not maintained and the CO2
assimilation rate was significantly lower after that day.

Compensation in CO2 assimilation rates is generally due to changes in
carboxylation efficiency and or RUBP regeneration rates (Farquhar, 1982; Jones
1985) . Based on data from the A/C; curves, it was found that as the damage
increased carboxylation efficiency decreased and CO2 was not a limiting factor
for the photosynthesis in damaged leaves. It is also found that as the damage
increases carboxylation efficiency decreases and increases the CO2 assimilation
compensation point. These data indicated that strawberry leaves do not
compensate photosyntheticly to leaf damage as in found in other plants.

Foliar damage also caused lower quantum efficiency (Table 2). Quantum
efficiency decreased as the damage increase. Increasing light levels did not
compensate for the foliar damage. Foliar damage also increased the light
compensation values and decreased the light saturation points for CO2

assimilation (Figure 5 and 6).

44

Chlorophyll fluorescence (Fv/Fm) was not affected from the foliar damage
through out the measurements (Figure 4). Bounfeour (2002) also found that Fme
values were affected by spider mites (Tetranychus urticae and Tetranychus
urticae) feeding after two weeks of infestation (25 mites per leaflet). However,
Iatrou (1995) found that chlorophyll fluorescence values were reduced in beans
infested with Tetranychus urticae. At similar mite-days (Sances, 1979) found total
chlorophyll content of strawberry leaves was not reduced by Tetranychus urticae.
Since, chlorophyll content is related to the chlorophyll fluorescence values, it can
be expected that chlorophyll fluorescence values would be similar.

Feeding habits of the pests may result in different results for chlorophyll
fluorescence values. If the damage is limited to the spongy mesophyll and
palisade layer is not damaged by the insects such results can be expected
(Sances 1979). However, longer feeding time may decrease the chlorophyll
fluorescence values as the damage increases proportional to the time.
Population of the pests may also affect these values. In this study, damage was
limited to the hole area so, the undamaged parts of the leaves were not affected
by the foliar the damage. This may explain the lack of relationship between
chlorophyll fluorescence and simulated foliar damage.

Internal CO2 (Ci) values were higher in damage leaves than control plants
and C: levels increased as the level of foliar damage increased. This indicates
that ability of leaf to use CO2 was inhibited by the foliar damage.

Stomatal conductance (gs) and transpiration (E) were not affected by the

foliar damage at any level. On the fourth day of the measurements an increase in

45

stomata conductance (gs) were observed in leaves which were damaged at 10%
damage level. This result may explain the increase in the CO2 assimilation rates
on the fourth day. However, as indicated before this increases did not result in
full compensation of the leaf photosynthesis. Layne (1989) and Proctor (1982)
also found that simulated leaf and the leaf injury by 20 mines per leaf by damage
by the Phyllonorycter blancardella did not affect the stomatal conductance and
the transpiration rates of the leaves.

This study showed that photosynthetic compensation did not occur when
damage occurred to single leaves. Since, in this study only single leaf was
considered for measurements, to understand if a photosynthetic recovery
metabolism exists in strawberry plants, whole plant photosynthesis
measurements should be considered. However, the canopy of strawberry plants
changes continuously as new leaves are formed from the crown and old leaves
die. These are obvious limitations to calculate the canopy area of strawberry
plants. Thus, it may be also difficult to apply simulated damage to strawberry

plants.

46

LITERATURE CITED

Bounfour, M., L.K. Tanisgoshi, C. Chen, J. Cameron, 8. Klauer. 2002.
Chlorophyll content and chlorophyll fluorescence in red raspberry leaves infested
with Tetranychus urticae and Eoteranychus carpini borealis (Acari:
Tetranychidae). Phsiological and Chemical Ecology. 31: 215-220.

Disegna, E.J. 1994. The use terbacil as a tool to establish a
photosynthetic threshold in apples. Michigan State Univ., East Lansing, Msc
thesis.

Farquhar, G.D., T.D. Sharkey. 1982. Stomatal conductance and
photosyntesis. Ann. Rev. Plant. Physiol. 33:317-345.

Gucci, R. 1988. The effect of fruit removel on leaf photosyntesis, water
relations and carbohydrate partitioning in sour cherry and plum. Ph.D. Thesis.
Michigan State University, East Lansing, MI.

Gucci, R., J.A. Flore. 1990. Net Photosynthesis and non-structural
carbohydrates in leaves of field-grown plum trees following fruit removal. In: 3rd
lntemational meeting on “Regulation of photosynthesis in fruit crops”. Perigia.
Italy. P.127.

Jones, H.G. 1985. Partitioning stomatal and non-stomatal limitations to
photosyntesis. Plant, Cell and Environment 8:95-104.

Jones, H.G. 1998. Stomatal control of photosyntesis and transpiration.
Journal of Experimental Botany. 49:387-398.

Hancock, JP. 1999. Strawberries. CABI publishing, Oxon, UK.

47

Hodgkinson, KC. 1974. Influence of partial defoliation on photosynthesis,
photorespiration and transpiration by luceme leaves of different ages. Aust. J.
Plant Physiology 1: 561-578.

Howell, 0.8., 8.8. Stackhouse. 1973. The effect of defoliation time on
acclimation and dehardining in tart cherry (Prunus cerasus L.) J. Amer. Soc.
Hort. Sci. 98: 132-136.

Kappel, F. 1986. Influence of spotted tentiform leaf miner on apple leaf
and tree physiology. Ph. D. Thesis. University of Guelph, Guelph, Ontario,
Canada.

Kirkwood, DA 1983. The mode of action of herbicides. In: Recent
advances in weed research. W.W. Fletcher (ed.) 227-257.

Layne, D. 1989. Damage thresholds for simulated trunk and leaf injury in
‘Montmorency’ sour cherry. Ms. Thesis. Michigan State University, East Lansing,
MI.

Proctor, J.T.A., J.M. Bodnar, W.J. Balckbum, and R.L. Watson. 1982.
Analysis of the effects of the spotted tentiform leaf miner (Phyllonorycter
blancardella) on the photosynthesis characteristics of apple leaves. Can. J. Bot.
60:2734-2740.

Sams, C.E., J.A. Flore. 1982. The influence of age, position and
environmental variables on net photosyntetic rate of sour cherries. J. Amer. Soc.
Hort. Sci. 107:339-344.

Shaw, M. 1956. The physiology of host parasite relations. I. The

accumulation of radioactive substances at infections of facultative and obligate

48

parasites including tobacco mosaic virus. Canadian Journal of Botany 34:(389-
405)

Unrath, CR, 1981. An overview of environmental factors affecting
orchard growth regulator response with special reference to apple. Acta Hort.
120:43-52.

Wareing, P.F., M.M. Khalifa, K.J. Trehame. 1968. Rate limiting process in

photosynthesis at saturating light intensities. Nature 220:453-457.

49

Chapter 2

THE USE OF TERBACIL AS A TOOL TO ESTABLISH A PHOTOSYNTHETIC

THRESHOLD IN STRAWBERRIES (Fragaria x ananassa cv. ‘Honeoye’)

50

ABSTRACT

Damage thresholds of strawberry plants (Fragaria x ananassa) were
investigated by terbacil application. Terbacil was applied to the field-grown plants
during 2001 and 2002. In 2001, terbacil was applied at concentrations of 12.5,
25, 50, 100, 200 and 400 ppm levels. In 2002, a previously untreated group of
two years old strawberry plants were sprayed with terbacil at concentration of 50,
100 and 200 ppm at two different growth stages (during fruit set and after harvest
stages).

Strawberry plants which were treated with terbacil were able to recover at
certain levels, except 400 ppm level during the first year experiment. CO2
assimilation rate of the plants treated with 200 ppm were lower than the control
plants 22 days after terbacil application. All other concentrations recovered to the
level of control plants. Recovery occurred between 4 and 10 days after the
terbacil treatment. Average fruit weight was adversely affected during the year
following the 400 ppm terbacil treatment. Other concentrations of terbacil did not
alter the response of the plants to terbacil.

Stage of the development did not alter the response of the plants to
terbacil. Difference in stomatal conductance and transpiration rates were
insignificant. Internal C02 (0,) levels were higher in plants which were treated
with high terbacil concentrations. Chlorophyll a and total chlorophyll content

decreased following the terbacil treatment. However, chlorophyll a and total

51

chlorophyll increased 8 days after terbacil treatment. Plant dry matter values and

chl b values were not affected from the terbacil treatments.

52

INTRODUCTION

Determination of damage thresholds is an important issue in plant
science. Damage threshold is defined as the level of pest damage above which
there are negative effects on the growth or the health of the plant.

In order to determine the plant damage thresholds different approaches
are used. These methods are based on simulating damage in plants. Hole
punching is used to simulate insect damage in threshold studies (Kappel, 1986;
Layne, 1989). However, this method requires lots of time and labor. Herbicides
that inhibits photosynthesis can also be used for threshold studies. Terbacil is a
uracil type herbicide that blocks both the Hill reaction and photosytem II in the
photosynthetic pathway (Ashton, 1973). It has been used by other researchers to
simulate damage in other crops (Byers, 1990; Disegna, 1994).

Damage thresholds levels have been investigated for several plants. Such
determinations were performed in wheat and barley (Shaw, 1956), soybean
(Wareing 1968), Lucerne (Hodgkinson, 1974), sour cherry (Layne,1989) and
(Disegna, 1994). Damage threshold levels for these crops range from 5% - 20%
depending on the crop and the crop load (Disegna, 1994). However, damage
threshold levels for strawberry plants have not been determined. Determination
of such value would be useful in IPM and pesticide application programs, the
assessment of environmental impacts and on economics studies.

In this research, terbacil is used as a tool to investigate the photosynthetic
threshold of strawberry plants and to investigate other effects which may be

related to photosynthesis (e.g. fruit yield, dry weight of the plant). Terbacil is

53

commonly used to control the weeds in strawberry production. It is usually
applied before planting, in early season and after harvest renovation (Mahr et. AI,
2002)

The hypothesis tested in this research was “Leaf photosynthetic capacity
will determine damage threshold levels for strawberry productivity”.

For this purpose terbacil was used as a tool to establish a threshold in
strawberry.

The objective of this research was to determine the leaf damage threshold
for strawberry (Fragaria x ananassa cv. ‘Honeoye’) on whole plants under field
conditions at different times during the growing season. Leaf damage was
simulated by applying terbacil to the foliage at different concentrations and at
different critical stages in crop development. The degree and duration of
photosynthetic inhibition are dose dependant and crop specific. It was
hypothesized that different levels of P,- reduction could reduce the production and
storage of carbohydrates needed for growth and reduced carbohydrate
production may affect yield and runner production negatively and the ability of the

plant to resist environmental stress.

54

MATERIALS AND METHODS

2001 experiment

Strawberry plants (Fragaria ananassa cv. Honeoye) were planted in three
raised beds (20 cm height, 50 cm width) at Michigan State University Horticulture
Teaching and Research Center (HRTC), East Lansing, MI. Each bed had two
rows of plants, 20 cm apart, and the distance between the plants within a row
was 30 cm. The experiment was designed as a randomized complete block with
three blocks with one bed per block. There were six plants per treatment. The
treatments consisted of a single application of terbacil at concentrations of 12.5
ppm, 25 ppm, 50 ppm, 100 ppm, 200 ppm and 400 ppm. X-77 (90%) surfactant
(Alkylarylpolyoxyethlene, Alkylopolyoxyethylene, Fatty acids, Glycols and
Dimethhypoly siloxane) was added to the herbicide at a concentration of 1.25
ml/L. Control plants were sprayed with water plus surfactant at 1.25 mI/L. Leaves
were sprayed to drip point. Border plants were used to separate treatment plots.
Root pruning was performed as needed and old leaves were removed before
planting. Plants were drip irrigated as follows. One drip line placed per hill.
Capacitiy of dripper was 4 L/Ih. Irrigation applied for 40 minutes at 7:30 am
every day by a Torro irigation timer (Model 53331, Bloomington, MN). A 20-20-20
(N,P,K) fertilizer was applied three times during the growing season at a rate of 5
grams per plant. Straw mulch was used as the mulching material. Manual

weeding was performed as necessary. No pesticides were applied to the

55

strawberry plants during the experiment and no significiant incest or disease

damage was observed during the experiment.

Gas Exchange Measurements

Gas exchange measurements were made on three plants per treatment
plot. One fully expanded leaf was selected for gas exchange measurements. The
CIRAS-1 portable photosynthesis system (PP Systems, Hertfordshire, UK) was
used to measure the gas exchange parameters which included CO2 assimilation
rate (A), stomatal conductance (9,) and lntemal CO2 (Ci). Gas exchange
measurements were performed one day before terbacil treatments and 2, 4, 6,
10, 14, 18 and 22 days after the terbacil treatments. All gas exchange

measurements were made between 8:30 am and noon.

Chlorophyll Fluorescence

Chlorophyll Fluorescence was measured on six plants per treatment plot.
One fully expanded leaf was selected for gas chlorophyll fluorescence. The Plant
Efficiency analyzer (Hansatech Instruments Ltd, Norfolk, UK) was used for these
measurements. Leaves were dark acclimated for 20 minutes prior to
measurements using dark acclimation cuvettes. These leaves were then
irradiated with actinic light for 5 seconds and chlorophyll fluorescence kinetics

were recorded (Krause,1984). Chlorophyll fluorescence measurements were

56

performed one day before terbacil treatments and 2, 4, 6, 10, 14, 18 and 22 days

after the terbacil treatments.

2002 experiment

Two years old strawberry (Fragaria x ananassa cv. Honeoye) plants were
used in this experiment which were planted in 2001 at Michigan State University
Horticulture Teaching and Research Center (HRTC), East Lansing, MI. Cultural
practices and planting distances were the same as described for the 2001
experiment. Terbacil treatments were applied at two different times. The first
terbacil treatment was applied during fruit set and the second terbacil treatment
was made after harvest. Based on the 2001 rates, terbacil was applied at rates of
50 ppm, 100 ppm and 200 ppm. X-77 (90%) surfactant (Alkylarylpolyoxyethlene,
Alkylopolyoxyethylene, Fatty acids, Glycols and Dimethhypoly siloxane) was
added to the spray solution at a concentration of 1.25 ml/L. Control plants were
sprayed with an aqueous solution containing the surfactant only. Leaves were

sprayed to the point of drip.

Gas Exchange Measurements

Four plants were selected for gas exchange measurements from each

treatment plot with three replicates (blocks). Measurements were performed one

day before terbacil treatment and 2, 4, 6, 8, 10, 14, 18 and 22 days after the

57

terbacil treatments. The same method was used for gas exchange
measurements as described 2001 experiment. C02 assimilation rate, stomatal

conductance and lntemal CO2 parameters were recorded.

Chlorophyll Fluorescence

Measurements were conducted as described for the 2001 experiment.
Measurements were performed one day before terbacil treatments and 2, 4, 6, 8,

10, 14, 18, and 22 days after the terbacil treatments.

Fruit Yield

Strawberry plants that were used in the 2001 experiment were harvested
in 2002 in order to assess the effect of the previous seasons’s damage on the
following year’s yield.

Plants that were treated during fruit set stage in 2002 experiment
harvested. Fruit number and weight was collected on individual plants. Two

harvests were performed.

Chlorophyll Content

Three leaf discs (0.33 cmz) were removed from three different leaves on

each plant using a paper punchhole. Chlorophyll was extracted by placing the

58

leaf discs in 7 ml N,N-dimethylformide for 36 hours in the dark at a temperature
of 5°C. Absorbance of the extracts at wavelength of 664, 647 and 625 nm was
measured using a Hitachi U-3110 spectrophotometer (Hitachi Ltd, Tokyo, Japan).
The concentration of chlorophyll a, chlorophyll b and chlorophyll P was calculated
according to the methods proposed by Moran (1982).

Chlorophyll content was determined one day before the treatments and 4,

8,12, 16 days after the terbacil treatments.

Dry Weight

Strawberry plants on which CO2 assimilation rates were measured during
the season (four plants per treatment), were removed from the field at the end of
the growing season and separated into three parts (root, crown and leaves).
Roots, crowns and leaves were placed in a forced air oven at a temperature of

60°C for four days until dry.

Plot Design and Statistical Calculations

A randomized complete block design was used in this experiment. Data

were subjected to analysis of variance (ANOVA). Means were compared by

Duncan test or by standard deviation.

Error bars in the figures represents standard deviation.

59

The SAS base statistical program (version 8.2, SAS institute, Cary, NC)
was used for ANOVA.

60

RESULTS

2001 Experiment

Effects of Terbacil on CO2 assimilation rate

Under the conditions of this experiment, the average CO2 assimilation rate
in leaves of control plants ranged between 12.5 and 18 umol.m'2.s'1 (Figure 7)
Terbacil, applied at a rate of 400 ppm, caused complete inhibition of CO2
assimilation two days after treatment (Figure 7). At the 200 ppm rate, terbacil
decreased leaf photosynthetic rates by 40% as compared to the untreated
control plants. At rates of 12.5, 25 and 50 ppm, terbacil had no significant effect
on leaf photosynthesis as indicated by measurements made over a period of 22
days following the treatment. Four days after treatment, leaf photosynthetic rate
in the 200 ppm treatment decreased to 55% of the rate measured in control
plants (Figure 8). At the same time, terbacil at 400 ppm continued to cause
complete inhibition of photosynthesis. By the sixth day after treatment, 002
assimilation rates in the 200 ppm treatment had partially recovered to
approximately 72% of the photosynthetic rate of control plants. CO2 assimilation
rates in the 400 ppm treatment also showed some recovery but remained at
significantly lower levels than the control. Ten days after treatment, the recovery
of photosynthetic activity continued in plants treated with 200 ppm of terbacil,

whereas the recovery observed earlier in the 400 ppm treatment was not

61

apparent on this date. However, 14 days after treatment, CO2 assimilation rates
in plants treated with 400 ppm of terbacil recovered to about 50% of the rates
measured in control plants. Leaf photosynthetic rates on this date for all other
terbacil treatments were not significantly different from the control. On day 18,
the CO2 assimilation rate of plants in the 400 ppm treatment again decreased to
less than 50% of the control, while plants in the 200 ppm treatment showed a
smaller drop in assimilation rate to approximately 68% of the control level. The
decrease in CO2 assimilation rates became more severe by day 22, as leaf
photosynthetic rates in the 100 ppm and 200 ppm treatments decreased by
approximately 30% and 60%, respectively. In plants treated with terbacil at 400
ppm treatment photosynthetic activity appeared to have ceased completely by

day 22 as leaves showed severe chlorosis.

Gas Exchange Parameters

At all rates tested in this experiment, terbacil had no significant effect on
stomatal conductance (Figure 9). Stomatal conductance for the for all plants
ranged between 125 and 375 pmol.m'2.s". lntemal CO2 levels were affected by
the terbacil treatments (Figure 10). Plants treated with terbacil at 400 ppm
consistently had the highest levels of internal CO2 throughout the two weeks
immediately following the treatment. The highest levels of internal CO2,
approximately 360 ppm, were observed in leaves of the 400 ppm treatment on

days two and four after the treatment. Internal CO2 levels in the 200 ppm

62

treatment were generally higher than those of the control and the other terbacil
treatments; however, C; levels in all but the 400 ppm treatment were similar by
day 14.

Transpiration (E) was not affected by the terbacil treatments (Figure 11).
No significant differences in E were found at any of the dates on which leaf gas

exchange was measured.

Chlorophyll Fluorescence

Chlorophyll fluorescence was evaluated as the ratio of Fv over Fm values
(Fv/F m).

Fv/Fm value gradually decreased at the plants which were treated with
400 ppm. It was 56% of the control value after 2 days (Figure 12). At the end of
the experiment Fv/Fm value was near to zero. At the 100 ppm and 200 ppm
levels Fv/Fm values were lower than the control , 12.5, 25, 50 ppm levels after 2
days of the terbacil treatment Fv/Fm values were not significantly different
between 2nd and 22nd of the treatments than each other except the 400 ppm
level.

The relationship between Fv/Fm and assimilation (A) is given in figure 13

as a regression curve.

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70

2002 Experiment

Effects of Terbacil on CO2 assimilation rate

The average 002 assimilation rate of control plants throughout the
experiment ranged between 10.71 and 17.05 umol.m'2.s'1 (Figure 14 and 15).
Two days after treatment with terbacil at a rate of 200 ppm leaf photosynthetic
rates decreased by 25% and 30% as compared to the untreated control plants at
the “during fmit set” and “after harvest” stages, respectively. These values were
75% for 100 ppm two days after the treatments. Compared to the untreated
control, reduction in CO2 assimilation rate at the 100 ppm rate was 70% and 60%
four days after the treatments (Figure 16 and Figure 17). Plants recovered 8 days
after the treatments at both stages. A reduction in assimilation rate of control
plants occurred after fourteen days, which may be due to leaf aging. After
fourteen days, there were no significant differences in the assimilation rates of
treated and untreated plants. This was observed between 14 and 22 days
following the treatments at both stages of the plants.

The effect of application at different stages of development showed a
small difference at 50 ppm level during the fruit set stage, 2 days after treatment.
CO2 assimilation rates were 78% of the photosynthetic rate of control plants and

this value was significantly lower than the control value.

71

Gas Exchange Parameters

At all rates of terbacil tested at both growth stages, terbacil had no
significant effect on stomatal conductance (Figure 18 and Figure 19 ). Stomatal
conductance ranged between 108 and 209 pmol.m'2.s'1 (Figure 18 and Figure
19). lntemal CO2 levels were affected by the terbacil treatments as in the 2001
experiment. Plants treated with 200 ppm terbacil had the highest level of CI two
days after treatments. Six days after treatment, lntemal CO2 levels in plants
treated with 100 ppm terbacil were higher than those observed in control plants.
CI levels dropped to control levels after 8 days of the terbacil treatments. This
was observed at both stages of the application (Figure 20 and Figure 21).

Transpiration (E) was not affected by the terbacil treatments. This was
observed at the both application stages. No significant differences in E were
found at any dates on which leaf gas exchange was measured (Figure 22 and

Figure 23).

Chlorophyll Fluorescence

Chlorophyll fluorescence was evaluated as the ratio of Fv over Fm values
(Fv/Fm).

Terbacil caused a significant decrease in Fv/Fm values within 2 days of
treatment (Figure 24 and 25). The most severe decrease in F,,/Fm occurred in that

plants were treated with terbacil at a concentration of 200 ppm, where F,,/Fm

72

decreased to 50% of F,,/Fm in control plants. In these plants, F,,/Fm remained at
the depressed level until day 4, after which they increased significantly but
remained below control levels until day 14. In plants treated with terbacil at 50 or
100 ppm the decrease in F,,/Fm was proportionate. A 10 to 30% decrease was
observed in Fv/Fm, but still significantly below control levels. However, in these
plants F,,/Fm had recovered partially 4 days after treatment and had recovered
completely by day 6.

There was no apparent effect of plant growth stage on the response of
F,,/Fm to terbacil.

The relationships between Fv/Fm and assimilation (A) are given figure 26
and 27 as a regression curve for both growing stages. R2 values were found 0.52

and 0.53 respectively.

Plant Dry Weight

Plant dry weight expressed in total dry weight and dry weights of three
parts of the plants (leaf, crown, root). Leaf dry weight values ranged between
10.33 g and 12.97 g . Crown dry weight values ranged between 5.1 g and 6.58 g
and root dry weight value ranged between 3.88 g and 4.38 9. There was no
significance difference due to treatment in dry weight based on the separation
into different organs. (Figure 28 and 29)

Total dry weight ranged between 19.23 g and 23.71 g. Terbacil had no

significant effect on plant dry weight.

73

Fruit Yield

Plants that were treated with terbacil were harvest in 2001 were harvested
in 2002 to assess the carryover effect of terbacil on fruit yield. Terbacil did not
have a significant effect on fruit yield in the two years of this study (Figure 30 and
31). Total yield per plant ranged between 55 g and 75 g. Average fruit weight
was not affected by the terbacil treatments except at the 400 ppm level, which
was 31% less than that of the control (Figure 32 and 33). The average fruit

weight ranged between 8.8 and 9.6 g.

Chlorophyll Content

Terbacil reduced chl a and total chlorophyll content of the strawberry
leaves. Total chl and chl a content increased in plant treated with terbacil 4 days
after application and there was no significant difference after 12 days. Terbacil
did not affect chl b and P chl content (Figure 34 and 35).

There was no apparent effect of plant growth stage on the chlorophyll

content to terbacil.

74

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Root
88

Crown

Leaf

(After harvest stage). Means followed by different letters are significantly different

Figure 29. Effect of terbacil on root, crown, leaf and total plant dry weights dry
by Duncan’s Multiple Range Test (P5005).

 

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Figure 30. Effect of terbacil on fruit yield (Plants treated in 2001). Means
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89

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92

Discussion

Terbacil was effective to limit photosynthesis in strawberry plants. Higher
concentration of terbacil at 100, 200 and 400 ppm level, limited the
photosynthesis in strawberry plants. However, 400 ppm level was toxic and
leaves could not recover from the damage of 400 ppm terbacil. 400 ppm terbacil
not only damage the photosynthetic apparatus but also it was phytoxic and
damaged the leaves at the end of the experiment. However, since strawberry
plants continued to form new leaves from the apical meristem of crown, plants
survived from the 400 ppm terbacil experiment. This was a recovery mechanism
for strawberry plants. Plants treated with 100 and 200 ppm were able to recover
from the photosynthetic limitation. Photosynthetic recovery of the plants that were
treated with 200 ppm was maintained after 14 days of terbacil application.
Twelve and half, 25 and 50 ppm concentrations of terbacil did not limit the
photosynthesis in 2001 experiment. In some species concentrations as low as
12.5 ppm terbacil treatment affected the photosynthesis of the plants. Disegna
(1994) found that 12.5, 25, 50 and 63 ppm of terbacil may limit the
photosynthesis in apple trees. Byers (1990) also found that photosynthesis was
inhibited then recovered when terbacil was applied 50 ppm concentration.
Catania (1993) reported 100 ppm terbacil treatment limit the photosynthesis in
peach trees and photosynthesis recovered within 7-10 days. However, they did
not test other concentrations of terbacil. So, these value may not be the threshold

values that inhibit the photosynthesis. In this research 100 ppm of terbacil was

93

the lowest concentration that limits the photosynthesis. This may be due to the
different cuticle structure and the epicuticular wax composition as suggested by
Baker (1974).

Recovery of photosynthesis occurred between four and ten days after the
terbacil treatment in 2001 experiment in plants which were treated with 200 ppm
terbacil. Disegna (1994) reported that recovery time for the apple trees was 15
days in trees that treated with 12.5, 25 and 50 ppm terbacil. In peach trees
recovery time was 7-10 days when terbacil applied at 100 ppm level. However, in
this research treated plants could not sustain their C02 assimilation rates after 14
days of terbacil application.

Stages of the development of the plant which were tested in this research
did not affect the 002 assimilation rates of the plants in 2002 experiments. Plants
which were treated during fruit set stage did not show any effect of sink-source
competition and their assimilation rates were similar to the plants which were
treated after the harvest stage. This indicates that having sinks did not affect the
002 assimilation rates. This was consisted with results in cherries (Roper
1988)

Internal CO; levels (C) were higher in the plants that were treated with
400 ppm and 200 ppm terbacil in 2001 experiment. This indicates that the 002 in
the plant is not being utilized by photosynthesis. Since photosystem II is inhibited
by terbacil higher Ci values were expected as suggested by Moreland (1980).

Stomatal conductance (9..) and transpiration (E) was not affected from

terbacil treatments and they did not show any trend related to the terbacil

94

treatments. This may indicated that terbacil did not cause any stomatal limitations
and any existing inhibition of photosynthesis was related to the inhibition of photo
system ll. Transpiration values were not affected by the terbacil treatment.
Transpiration is affected by temperature and humidity and the weather conditions
during measurements may have great effect on E. Even though, the CIRAS-1
has the ability to adjust the humidity and temperature during the measurements.
Since, the plants were already acclimated to out side conditions any slight
change in the humidity or temperature before or during the measurements may
affect actual transpiration rate.

In 2001 experiment, chlorophyll fluorescence (Fv/Fm) measurements
indicated the inhibition of photosynthesis in plants which had been treated with
400 ppm. Plants treated with 50, 100 and 200 ppm terbacil had lower values
Fv/Fm values than plants treated with 0, 12.5 and 25 ppm terbacil. However,
FV/Fm measurements did not fully reflect the reduction in photosynthesis in the
2001 experiment in which plants treated with 50, 100 and 200 ppm. Regression
analysis indicated that correlation between A and FV/Fm was low (R2=0.56).

In 2002 experiment Fv/Fm measurements indicated the inhibition of
photosynthesis and recovery of the photosynthesis in both growing stages.
Lower readings of Fv/Fm values in 2001 may be related with the growing stage of
the plants. Measurements in 2001 experiments were performed late in the
season August and September. However, measurements in 2001 experiment
were performed in June and July. This may be related to thickness of cuticle of

the end of the season. In this experiment cuticle thickness were not investigated.

95

If the cuticle the cuticle thickness is different in plants which were measured in
August and September 2001 than plants measured in June and July in 2002,
herbicide absorption may be different than each other. Kinlvood (1983) and
Unrath (1981) found that cuticle of older leaves is less permeable and thicker and
this decreases herbicide absorption.

FV/Fm measurements showed that plants treated with higher
concentrations of terbacil recovered later than the plants treated with lower
concentrations of terbacil. Plants treated with 200 ppm recovered after 6 days of
terbacil treatments. Plants treated with 100 ppm recovered 4 days after terbacil
treatments.

Disegna (1994) also found that late treatments of terbacil caused less
F.,/Fm reduction in Apple trees.

Byers (1990) and Disegna (1994) found that development stages may
affect the 002 assimilation response of the plants to the terbacil treatments in
Apple and Peach trees. In 2001, 002 assimilation rates of plants which were
treated with 100 and 200 ppm were higher compared to the plants treated in
2002 with the same concentrations. This may be related to different absorptions
of herbicide at different stages.

0n the other hand, unlike deciduous trees, strawberry plants continue to
form new leaves through out the season. Further research may be needed to
investigate the relationship between cuticle thickness and absorption of herbicide

and the cuticle thickness of strawberry leaves in different growing stages.

96

Plants that were treated with 400 ppm in 2001, had low fruit yields when
they are harvested in 2002. Since, photosynthesis was inhibited completely 22
days after the treatment. Carbohydrate storage for the following season may be
lower and this may affect the next years yield.

In 2002 experiment, yield was not affected from the terbacil treatments in
which plants treated were during fruit set stage. This may indicate that when
photosynthesis was inhibited there was enough carbohydrate supply to maintain
carbohydrate demand to sinks (fruits). Disegna (1994) found that cropping apple
trees could not maintain enough carbohydrate to sinks when treated with terbacil.
So, fruit yield was lower in apple tress depending on the fruit load. However,
since fruit formation time and photosynthetic recovery times were short in
strawberry plants, they may have enough carbohydrates stored to compensate
the demand for a short time.

Dry matter content was not affected from the terbacil treatments. This
was not expected, because photosynthesis was inhibited for a limited time.
Photosynthetic recovery occurred 14 days after the treatment for all of the
concentrations. Dry matter contents were not significant when crown, leaf and
roots were compared individually. This indicates that treatments did not cause
any alteration to allocation of carbohydrates due to the terbacil treatments.
Designa (1994) reported that 20 days of reduction of photosynthesis due to the
terbacil did not alter wood carbohydrate storage in apple trees. This was also the
case in strawberry considering the main carbohydrate storage organ are crowns,

there were significant difference in dry matter content of the crowns.

97

Total chlorophyll content was affected from the treatment and temporary
chl a reduction occurred. This was consisted with results of lzawa (1965) and
Disegna (1994). An incerase in chl 3 followed the reduction of chl a and total chl
indicating that chl a generated in the recovery process.

Terbacil was useful to study the damage thresholds in strawberry plants. It
has advantages to simulate effects of disease and insects. Using insects and
disease infestation have difficulties and limitations. Such research may require
lots of time, insect or disease material. This usually increases the cost. Terbacil
application is easy and cheap. So terbacil can be applied to simulated such
disease and insects. Damage can be regulated by using different concentrations
of terbacil.

Use of terbacil as a herbicide is common and this research was useful to
determine to figure out the threshold levels for strawberry. However, since
terbacil is applied on area basis to soil. This threshold levels may vary depending

on soil type and conditions. Genotype is also an important factor that may affect

the threshold levels.

98

LITERATURE CITED

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100

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101

SUMMARY AND CONCLUSIONS

Use of terbacil was more practical than mechanical damage determine the
threshold levels in strawberry plants. Use of terbacil was easy and it was
possible to test many different of damage levels. Simulated leaf injury is time
consuming to study the damage thresholds.

Strawberry leaves which were damaged mechanically could not recover
photosynthetically due to a lack of compensation by carboxylation. Increasing
damage limited the carboxylation efficiency. It may also be possible that even
the 10% damage level was too severe for strawberry leaves to recover. Under
severe damage strawberry plants may compensate to damage by simply forming
new leaves instead of increasing photosynthetic rate. However, there are certain
difficulties to calculate the changing area canopy. In order to calculate net
photosynthesis whole plant leaf area must be calculated precisely. 0n the other
hand aging of the older leaves should also be considered. Aging effect may hide
any photosynthetic compensation in younger leaves. Thus, further research is
needed to study whole plant photosynthesis of strawberry plants and how it
affects the yield and dry weight of the strawberry plants.

Since, the yield and dry weight values were not affected by terbacil 200
ppm concentration, we may consider that the threshold level for strawberry plants
were greater than that caused by this level of damage. Terbacil used between

100 and 200 ppm concentration found to be useful to study the threshold level of

102

strawberry plants. However, these concentrations may be different depending on

the cultivar used.

 

103

APPENDIXES

Appendix 1. Daily weather data for the period that measurements were
performed in year 2001.

 

Date Maximun Minimum Precipitation Solar

Temperature Temperature (mm) Flux
(C) (C) (KJ/mz)
August 28, 2001 26.2 13.4 0.76 22974
August 29, 2001 27.2 10.2 23009
August 30, 2001 29.2 13.2 19278
August 31, 2001 25.4 12.9 15641
September 1, 2001 22.1 6.4 23691
September 2, 2001 25.2 6.8 21957
September 3, 2001 28.2 12.8 20933
September 4, 2001 23 10.8 21925
September 5, 2001 25 7.5 22403
September 6, 2001 27.8 8.7 18923
September 7, 2001 30.6 18 3.56 15917
September 8, 2001 28.5 18.7 10.16 11465
September 9, 2001 25.4 15.4 17.53 10954
September 10, 2001 22.9 12.3 20136
September 1 1, 2001 23.9 9.4 21058
September 12, 2001 26.9 10.2 20877
September 13, 2001 18.7 8.1 2.03 9472.2
September 14, 2001 17.7 4.8 16639
September 15, 2001 18.8 5 15801
September 16, 2001 22 4.8 20273
September 17, 2001 22.1 7 11716
September 18, 2001 23.6 10.6 12956
September 19, 2001 19.8 14.5 23.11 3289.8
September 20, 2001 19.1 12.2 3.81 9552.6

 

104

Appendix 2. Daily weather data for the period that measurements were
performed in year 2002 (During fruit set stage).

 

Date Maximum Minimum Precipitation Solar

Temperature Temperature (mm) Flux
(C) (C ) (KJ/mz)
June 21, 2002 27.9 20.1 12375
June 22, 2002 32 20 22295
June 23, 2002 32.1 18.7 23130
June 24, 2002 32.6 18.2 24344
June 25, 2002 33.4 16.7 21726
June 26, 2002 29.1 20.8 0.76 20549
June 27, 2002 27.3 20.4 18550
June 28, 2002 29.1 13.4 27514
June 29, 2002 31.1 13.8 26475
June 30, 2002 32.2 16.8 22556
July 1, 2002 33.6 19.9 25224
July 2, 2002 32.9 19.4 27532
July 3, 2002 33.5 18.7 26180
July 4, 2002 32.6 21.2 25578
July 5, 2002 24.8 15 24839
July 6, 2002 27.8 10.7 24921
July 7, 2002 31.8 12.5 27891
July 8, 2002 32.7 15.2 5.08 23389
July 9, 2002 27.1 19.4 28.96 10679
July 10, 2002 24.2 15.5 27176
July 11, 2002 25.2 10.2 26343
July 12, 2002 26.7 8.4 29078
July 13, 2002 30.2 9.7 27989
July 14, 2002 29.5 11.7 28486
July 15, 2002 31.3 12.9 24021

105

Appendix 3. Daily weather data for the period that measurements were
performed in year 2002 (After harvest stage).

 

Date Maximum Minimum Precipitation Solar
Temperature Temperature (mm) Flux
(C) (C) (KJ/mz)
July 16, 2002 32.4 18.6 25810
July 17, 2002 30.4 17.9 24379
July 18, 2002 29.5 18.1 8.13 15258
July 19, 2002 28.5 17 19579
July 20, 2002 30 14.5 26749
July 21, 2002 32.9 18.9 0.25 15258
July 22, 2002 33.5 21.9 4.32 20035
July 23, 2002 23.5 13.9 28118
July 24, 2002 25.6 9.4 27153
July 25, 2002 26 12.6 20401
July 26, 2002 29.5 17.8 17.02 21276
July 27, 2002 27.6 18.1 1.52 12205
July 28, 2002 28.6 20.9 9.91 10968
July 29, 2002 30.2 20.8 19.81 13970
July 30, 2002 29.8 19.1 26690
July 31, 2002 32.2 19.1 26388
August 1, 2002 31.2 19 21983
August 2, 2002 27.5 16.2 1.52 25501
August 3, 2002 30.4 13.4 26328
August 4, 2002 31.1 19.7 0.25 15108
August 5, 2002 27.6 18.7 0.25 18739
August 6, 2002 21.8 11.7 25927
August 7, 2002 23.9 8 23930
August 8, 2002 25.5 8.1 25246

 

106

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