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This is to certify that the
thesis entitled

EVALUATING THE INFLUENCE OF TEMPERATURE
ON REMONTANCY IN STRAWBERRY.

presented by

EMMA BRADFORD

has been accepted towards fulﬁllment
of the requirements for the

Master of degree in Plant Breeding, and Genetics -
Science Horticulture

 

 

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5/08 K;lProj/Acc&PrelelRC/DateDuathdd

EVALUATING THE INFLUENCE OF TEMPERATURE
ON REMONTANCY IN STRAWBERRY.

By

Emma Bradford

A THESIS

Submitted to
Michigan State University
In partial fulﬁllment of the requirements
For the degree of

MASTER OF SCIENCE
Plant Breeding, Genetics, and Biotechnology - Horticulture
2009

ABSTRACT

EVALUATING THE INFLUENCE OF TEMPERATURE ON
REMONTAN CY IN STRAWBERRY.

By

Emma Bradford

Fragaria Xananassa Duch. ex Rozier (strawberry) cultivars are traditionally classiﬁed as
short-day (Junebearing), day-neutral, or long-day (everbearing) plants based on when and
how often ﬂowering occurs during the growing season. We propose that the term
remontant replace ‘day-neutral’ to describe strawberry genotypes producing multiple
ﬂowering cycles in a season, and present evidence that differences in remontancy across
strawberry genotypes are primarily a function of differential temperature tolerance, and at
temperatures above the threshold tolerance photoperiod becomes regulating. In support
of this, the remontant ‘Tribute’ exhibited superior heat tolerance to the short-day cultivar
‘Honeoye’, while RH 30 exhibited intermediate heat tolerance. Individuals from the F1
population ‘Honeoye’ >< ‘Tribute’ were replicated through either crown division or runner
propagation, and grown at 17, and 23°C under a 16 h photoperiod. Results show that
temperature is the primary factor in determining photoperiod dependent ﬂowering, and
how an experimental unit is derived will have an effect on the number of runners and

inﬂorescences produced.

AKN OWLEDGMEN TS

I’d like to thank Dr. Ryan Warner for giving me the opportunity to study under him, for

his unfaltering support, guidance, and patience throughout and for generally being a great

guy.

Thanks go to Dr. Jim Hancock for introducing me to the wonderful world of strawberries.
I especially appreciated the opportunity to make crosses and evaluate progeny, a process
in which I not only learned a lot about making selections but also picked up some

uniquely descriptive vocabulary.

Dr. Dechun Wang made a potentially hazardous subject, QTL mapping, a user friendly
experience thanks to his unwavering availability to answer questions, and ease

frustrations.

To my partner in crime, Travis Stegmeir, I’d like to extend a hearty thank you for being

there and for sharing the most amazing office in PSSB with me.

Pete Callow, who is possibly the most entertaining and helpful lab and ﬁeld technician, I

thank for all his tips and advice on lab techniques, horticulture, and parenting.

TABLE OF CONTENTS

LIST OF TABLES ............................................................................................................... v
LIST OF FIGURES ............................................................................................................ vi

TEMPERATURE IS THE PRIMARY FACTOR CONTROLLING
PHOTOPERIOD REQUIREMENTS FOR REPEAT FLOWERING

(REMONT AN CY) IN STRAWBERRY .......................................................................... 1
ABSTRACT ......................................................................................................................... 2
INTRODUCTION ............................................................................................................... 3
MATERIAL AND METHODS ........................................................................................... 5
RESULTS .......................................................................................................................... 7
DISCUSSION ...................................................................................................................... 9
REFERENCES ................................................................................................................ 23

INFLUENCE OF TEMPERATURE ON REMONTANCY AND RUNNERING IN

THE STRAWBERRY F1 POPULATION ‘HONEOYE’ X ‘TRIBUTE’ .................... 26
ABSTRACT ....................................................................................................................... 27
INTRODUCTION ............................................................................................................. 28
MATERIALS AND METHODS ....................................................................................... 30
RESULTS .......................................................................................................................... 32
DISCUSSION .................................................................................................................... 34
REFERENCES .................................................................................................................. 41
APPENDIX ...................................................................................................................... 42
REFERENCES ................................................................................................................. 44

LIST OF TABLES

Table 1: Effect of temperature, remontancy class, and propagule type on inﬂorescence
and runner production of F1 strawberry ‘Honeoye’ x ‘Tn'bute’ individuals.....37

Table 2: Effect of temperature, remontancy class, and propagule type on the percentage
of F 1 strawberry ‘Honeoye’ X ‘Tribute’ individuals producing inﬂorescences or

runners ................................................................................................................ 38

Table A. l: Segregation of the AF LP marker, aggcat187T, in sixty-ﬁve individuals of
the mapping population ‘Honeoye’ X ‘Tribute’ ............................................ 43

LIST OF FIGURES

Figure 1.1: The effect of temperature on the number of ﬂowers produced for strawberry
genotypes (A) ‘Honeoye’, (B) RH 30 and (C) ‘Tribute’ under a 9-hr (ﬁlled
circles) or l6-hr (empty circles) photoperiod during a 16 week period. Error
bars represent standard error of the mean (n=6) .............................................. 18

Figure 1.2: Inﬂuence of temperature and photoperiod on cumulative mean inﬂorescence
production on short-day ‘Honeoye’, remontant ‘Tribute’, and wild clone of
Fragaria virginiana ssp.virginiana RH 30 ...................................................... 19

Figure 1.3: The effect of temperature on the number of inforescences produced for
strawberry genotypes (A) ‘Honeoye’, (B) RH 30 and (C) ‘Tribute’ under a 9-
hr (ﬁlled circles) or 16-hr (empty circles) photoperiod during a 16 week
period. Error bars represent standard error of the mean (n=6) ...................... 20

Figure 1.4: The effect of temperature on the number of ﬂowers per inﬂorescence for
strawberry genotypes (A) ‘Honeoye’, (B) RH 30 and (C) ‘Tribute’ under a 9-
hr (ﬁlled circles) or 16-hr (empty circles) photoperiod during a 16 week
period. Error bars represent standard error of the mean (n=6) ..................... .21

Figure 1.5: The effect of temperature on the number of runners per plant for strawberry
genotypes (A) ‘Honeoye’, (B) RH 30 and (C) ‘Tribute’ under a 9-hr (ﬁlled
circles) or 16-hr (empty circles) photoperiod during a 16 week period. Error
bars represent standard error of the mean (n=6) ............................................. 22

Figure 2.1: The effect of temperature, remontancy class, and propagule type on
cumulative percent of replicates ﬂowering in the segregating population
‘Honeoye’ X ‘Tribute’ under a 16 h photoperiod ............................................ 39

Figure 2.2: The effect of temperature, remontancy class, and propagule type on
cumulative percent of replicates runnering in the segregating population
‘Honeoye’ X ‘Tribute’ under a 16 h photoperiod ............................................ 40

vi

CHAPTER ONE

TEMPERATURE IS THE PRIMARY FACTOR CONTROLLING
PHOTOPERIOD REQUIREMENTS FOR REPEAT FLOWERIN G
(REMONTANCY) IN STRAWBERRY.

ABSTRACT
F ragaria Xananassa Duch. ex Rozier (strawberry) cultivars are traditionally classiﬁed as
short-day (Junebearing), day-neutral, or long-day (everbearing) plants based on when and
how often ﬂowering occurs during the growing season. We propose that the term
remontant replace ‘day-neutral’ to describe strawberry genotypes producing multiple
ﬂowering cycles in a season, and present evidence that differences in remontancy across
strawberry genotypes are primarily a ftmction of differential temperature tolerance
leading to photoperiodic sensitivity. To more clearly deﬁne the roles of temperature and
day length in ﬂowering control of strawberry, the short-day cultivar ‘Honeoye’, and two
remontant genotypes, ‘Tribute’ and an elite clone of Fragaria virginiana Duch. ssp.
virginiana, RH 30, were grown at 14 ,17, 20, 23, 26, or 29 °C, under a short (9 hr) or
long (16 hr) photoperiod. ‘Honeoye’ and RH 30 exhibited similar ﬂowering patterns in
response to temperature, with RH 30 producing ﬂowers at temperatures 3 °C warmer than
the threshold temperature for ﬂowering in ‘Honeoye’, regardless of photoperiod.
‘Tribute’ continued to produce ﬂowers under all treatments except 29 °C under short
days. Based on these results, we conclude that temperature is the primary factor in

determining photoperiod dependent ﬂowering.

Introduction
Strawberry cultivars have traditionally been classiﬁed into photoperiodic response groups
for ﬂowering. Junebearers are deﬁned as facultative short day (SD) plants (Darrow,
1936), everbearers are classiﬁed as long day plants (LD) (Darrow and Waldo, 1934), and
photoperiod insensitive varieties are deﬁned as day neutral (DN) (Bringhurst and Voth,
1978). However, early on, Darrow (1936) described the inﬂuence of temperature on
ﬂower initiation in strawberry, indicating the potential difﬁculty in photoperiodic
classiﬁcation of cultivars. Flower induction of SD types can occur under any
photoperiod if the temperature is cool enough, generally <15 °C (Guttridge, 1985). At
high temperatures, inhibition of ﬂowering in SD types is observed even under short
photoperiods (Ito and Saito, 1962), and ﬂowering of DN and LD types also decreases at
high temperatures (Dumer et al., 1984; Heide, 1977; Serce and Hancock, 2005a). In a
controlled environment study, Dumer et al. (1984) reported inconsistent effects of
photoperiod on ﬂowering of two SD cultivars grown under different temperature regimes,
further suggesting the inadequacy of classifying cultivars into photoperiodic response
groups to predict multiple ﬂowering cycles. Sonsteby and Heide ( 2007a) concluded that
seedlings of the F1 hybrid ‘Elan’ were quantitative long-day plants, as plants produced
more ﬂowers under continuous light than under short-day conditions. However,
ﬂowering under long photoperiods was inhibited at 27 °C. Nishiyama and Kanahama
(2002) also used a 24 h long-day treatment, but used two different temperature regimes, a
20 °C day temperature with a 15 °C night temperature and a 30 °C day temperature with
a 25 °C night temperature, to determine that ‘Summerberry’ and ‘Hecker’ were long-day

plants. However, plants were maintained at a temperature of 30 °C and an 8 h

photoperiod for 16 weeks to inhibit ﬂowering prior to commencing the experiment.

Maintaining the plants in a stressful environment may have inﬂuenced their results.

Strawberry cultivars exhibit considerable variation in the degree of repeat blooming, or
remontancy. The same genotype may be remontant in some parts of the USA, but not in
others (Dumer et al. , 1984). Since cultivars that produce multiple crops during the
summer can extend the growing season for farmers, understanding the genetic basis of
repeat ﬂowering has been an active area of investigation (Ahmadi et al. , 1990; Powers,
1954; Serce and Hancock, 2005a; Shaw, 2003; Sugimoto et al, 2005; Weebadde et al.,
2008). Unfortunately, the genetic basis of remontancy remains poorly understood, with
inconsistent inheritance ratios reported for different populations using the same genetic
source of remontancy (Hancock et al., 2002; Shaw, 2003; Shaw and F amula, 2005; Serce
and Hancock, 2005a) or in different trialing locations using the same clonally propagated
segregating populations (Hancock et a1, 2002; Weebadde et al., 2008). For example, a
clonally propagated mapping population derived from the DN ‘Tribute’ and SD
‘Honeoye’ grown in California, Oregon, Minnesota, Michigan, and Maryland exhibited
48-50% remontant individuals in MN, MD and MI, compared to 80% and 87% in OR
and CA, respectively (Weebadde et al. , 2008). The range of latitude and, therefore,
photoperiod covered by the three eastern sites was similar to the two western sites, while
recorded temperatures were considerably higher in MN, MD, and MI compared to OR
and CA during the experimental period, suggesting a strong inﬂuence of temperature on

remontancy.

The objectives of the work presented here are to determine the effects of temperature and
photoperiod on remontancy of ‘Honeoye’, ‘Tribute’ and another genetic source of
remontancy, an elite clone of F ragaria virginiana Duch. ssp. virginiana, RH 30,
previously classiﬁed as day neutral (Hancock er al., 2002). Results of this work will
allow us to identify permissive and inhibitive temperatures for remontancy in each
genotype which we can subsequently use to evaluate remontancy in segregating
populations derived from these parental genotypes. We have chosen to use established
plants as our case study in order to determine the effects of temperature and photoperiod

on re-blooming and not initial ﬂower induction.

Materials and Methods

During the summer of 2007, runners of Fragaria Xananassa (Duch. ex Rozier) ‘Tribute’
and ‘Honeoye’, and Fragaria virginiana ssp. virginiana elite clone RH 30 (Hancock et
al., 2001) were rooted in 10-cm square pots. After ca. 4 weeks, the connecting stolons
were severed, and the plants were transplanted into 3.79 L pots ﬁlled with soilless media
containing (v/v) 70% peat moss, 21% perlite and 9% vermiculite (Sure-Mix, Michigan
Grower Products, Galesburg, MI). On 25 October 2007, 72 plants consisting of a single
crown of each genotype were selected, and any ﬂowers or runners present were removed.
On 2 November 2007, twelve plants of each type were placed in one of six glass glazed
greenhouses set to a constant temperature of 14, 17, 20, 23, 26, or 29 °C. Air
temperature in each treatment was measured by a Type E thermocouple (TT-E-40;
Omega Engineering, Stamford, CT.) placed in an aspirated tube. Thermocouples were

connected to a data logger (CR1 0; Campbell Scientiﬁc, Logan, UT) and data were

recorded every 10 3. Weekly temperature averages for each temperature were within i
1.0 °C of the setpoint each week during the experimental period. Vapor pressure deﬁcit
was maintained between 0.7 and 1.0 kPa at each temperature by steam injection.

Within each temperature treatment, half of the plants were maintained under a 9-hr
photoperiod (plants were covered with an opaque cloth from 1700 to 0800 HR daily) and
half were maintained under a 16-hr photoperiod (ambient daylight supplemented with 50
umol m'2 s'1 photosynthetically-active radiation supplied by high-pressure sodium lamps
from 0600 to 2200 HR daily; lamps were programmed to turn off when ambient irradiance
outside the greenhouse exceeded 400 umol rn'2 s"). Plants were overhead irrigated with
reverse osmosis water supplemented with a water-soluble fertilizer to provide the
following (mg L"): 125 N, 13 P, 125 K, 15 Ca, 1 Fe, 0.1 B, 0.1 M0, 0.5 Mn, 0.5 Zn and

0.5 Cu (MSU Special; Greencare Fertilizers, Chicago, IL).

Data collection and analysis

Every 10 to 15 days during the experiment, the number of inﬂorescences per plant,
ﬂowers per inﬂorescence, and runners were determined. Inﬂorescences were counted
once all ﬂower buds within the inﬂorescence were clearly distinguishable. Aﬁer each
data collection, inﬂorescences, runners and any dead leaves were removed. Data
collected during the ﬁrst 49 days of the experiment were not included in the data analysis,
as it was assumed that any ﬂowers or runners produced during this period were induced
in conditions prior to initiation of the experiment. Therefore, only data collected

between days 63 and 178 were included for statistical analysis. Analyses of covariance

with temperature as the covariate were conducted using PROC GLM in the SAS software

package (SAS v. 8.2; SAS Institute, Cary, NC).

Results

Flowering

Genotype, temperature and photoperiod interacted to impact the number of ﬂowers
(P<0.001) and inﬂorescences (P=0.003) produced. Under long days, ‘Honeoye’ ﬂower
and inﬂorescence number were greatest at 14 and 17 °C, decreased as temperature
increased to 20 °C, and ﬂowering was completely inhibited at 23, 26 and 29 °C (Figs.
HA and 1.2). Under short days, ‘Honeoye’ ﬂower number increased as temperature
increased from 14 to 20 °C (Fig. 1.1A). Flower number decreased as temperature further
increased to 23 or 26 °C, and ﬂowering was inhibited at 29 °C. Under 14 and 17 °C,
‘Honeoye’ produced more ﬂowers under LD than SD, while under temperatures of 20 to
26 °C ﬂower production was greater under SD. Inﬂorescence production in ‘Honeoye’
lagged about 84 days behind RH 30 and ‘Tribute’ (Fig. 1.2), and was highest under long
days at 14 and 17°C. At 20 and 23 °C, ‘Honeoye’ produced more inﬂorescences under
short days than long days and at temperatures of 26°C or more, inﬂorescence production
in ‘Honeoye’ is inhibited under both photoperiods. The genotype RH 30 displayed a
similar trend for ﬂower production, producing more ﬂowers per plant under long days
than short days at 14 °C and 17 °C, similar numbers under both photoperiods at 20°C,
and more ﬂowers under short days than long days at 23 to 29 °C (Figs. 13 and 2). RH 30
was the only genotype to produce ﬂowers under short days at 29 °C. In contrast to both

‘Honeoye’ and RH 30, ‘Tribute’ produced more ﬂowers under long days than short days,

regardless of temperature (Figs. 1C and 2). ‘Tribute’ was also the only genotype to
produce ﬂowers under long days at 29 °C. Flower production in ‘Tribute’ was similar as
temperature increased from 14 to 23 °C under long days, but decreased ca. 30% as

temperature further increased to 26 or 29 °C.

The observed differences in ﬂower production could be due to variations in inﬂorescence
production, the number of ﬂowers produced per inﬂorescence, or both. In general, the
differences in ﬂower production were more strongly associated with variation in
inﬂorescence number than ﬂowers per inﬂorescence, as patterns of inﬂorescence number
and ﬂower number were very similar across temperature for each genotype and
photoperiod (Figs. 1.1 and 1.3). This was particularly true at temperatures of 20 °C or
lower. Flower number per inﬂorescence was similar for ‘Honeoye’ at temperatures of
20 °C or less, regardless of photoperiod (Fig. 1.4A). As temperature increased from 23 to
26 °C, ‘Honeoye’ ﬂower number per inﬂorescence decreased from 8 to 3 ﬂowers under
short days. RH 30 ﬂower number per inﬂorescence was similar between 14 and 20 °C
under long days, but decreased as temperature further increased to 23 or 26 °C (Fig.
1.4B). Under short days, RH 30 ﬂower number per inﬂorescence was similar between 14
and 23 °C, decreasing as temperature increased to 26 or 29 °C. At 14 and 17 °C, RH 30
produced more ﬂowers per inﬂorescence under long days than short days, and more
ﬂowers per inﬂorescence under short days than long days at temperatures of 23 to 29°C.
Under long days, ﬂower number per inﬂorescence for ‘Tribute’ was relatively constant
across the entire temperature range (Fig. 1.4C). Under short days, ‘Tribute’ produced

fewer ﬂowers per inﬂorescence at 26 or 29 °C than under cooler temperatures.

Runner production

Temperature, photoperiod and genotype interacted to inﬂuence the number of nmners
produced (P<0.001). ‘Honeoye’ did not produce runners at 14°C, or 17°C, regardless of
photoperiod (Fig. 1.5A). Under short days, ‘Honeoye’ did not produce runners, regardless
of temperature. The number of runners produced increased as temperature increased
from 20 to 26 °C, but declined as temperature increased from 26 to 29 °C (Fig. 1.5A). In
contrast to ‘Honeoye’, RH 30 did produce runners under short days, but only at
temperatures of 23 to 29 °C (Fig. 1.5B). Runner production for RH 30 increased with
temperature from 17°C to 29°C under long days. RH 30 produced far more runners than
either cultivar under long days and temperatures _>_ 20 °C. The cultivar ‘Tribute’
produced very few runners under short days, and only if the temperature was 23 °C or
greater (Fig. 1.5C). Runner production for ‘Tribute’ under long days was lower than
either ‘Honeoye’ or RH 30 at temperatures 2 23 °C, with a maximum of seven runners

per plant produced at 23 °C.

Discussion

Repeat ﬂowering in strawberry is often referred to as ‘day-neutrality’ (Hancock, 1999).
We propose that the term remontancy more accurately describes the repeat ﬂowering
pattern of strawberry, and should be employed instead of day-neutrality. The genetics of
remontancy in strawberry have proven difﬁcult to dissect, with contrasting reports of
whether this trait is controlled by a single dominant gene (Bringhurst and Voth, 1978;
Ahmadi et al., 1990), complimentary dominant genes (Ourecky and Slate, 1967) or

quantitative inheritance (Powers, 1954; Hancock et al. , 2002; Serce and Hancock, 2005a;

Shaw, 2003; Weebadde et al., 2008). To identify the genetic basis of remontancy,
understanding the inﬂuence of environmental variables on this trait is critical to identify
the appropriate screening environment. Utilizing genotypes previously deﬁned as short
day (‘Honeoye’) and day-neutral (‘Tribute’ and RH 30) (Serce and Hancock, 2005a) , we
have shown that temperature is a primary factor determining whether these genotypes
exhibit remontancy, with each genotype possessing an unique threshold temperature
above which photoperiod becomes regulating. We have determined permissive and
inhibitive temperatures for remontancy in these genotypes under both long and short day

conditions.

Growing plants under short days compared to long days improved ‘Honeoye’ heat
tolerance by ca. 3 °C. That is, plants grown under long days at 20 °C produced similar
numbers of ﬂowers and inﬂorescences as plants grown under short days at 23 °C (Figs.
HA and 1.2). Plants produced few or no ﬂowers and inﬂorescences at temperatures of
26 and 29 °C, regardless of photoperiod. Deﬁning ‘Honeoye’ as a short-day plant is
inconsistent with the ﬂowering pattern observed in this study. RH 30 was previously
deﬁned as ‘day-neutral’ (Serce and Hancock, 2005a). However, in our study the
ﬂowering pattern in response to temperature and photoperiod was very similar to
‘Honeoye’, previously deﬁned as a short-day cultivar, but shifted up by 3 °C. ‘Honeoye’
ﬂower production under long days decreased as temperature increased above 17 °C, while
RH 30 ﬂower production under long days continued to increase up to 20 °C before
declining with a further temperature increase (Fig. HA and B). Similarly, under short

days, ‘Honeoye’ ﬂower production increased up to 20 °C, then declined, while RH 30

10

ﬂower production increased up to 23 °C before declining with ﬁtrther temperature
increase (Fig. 1.1A and B). Dumer et al. (1984) cautioned that the ﬂowering
classiﬁcation of strawberry genotypes must be regional due to inconsistent performance

across locations.

Classifying strawberry genotypes in photoperiodic categories appears inadequate for
predicting reblooming behavior across a range of geographic regions. We were able to
accurately determine the threshold temperature for remontancy of ‘Honeoye’ and RH 30.
Flower production in ‘Tribute’, however, followed a different trend. ‘Tribute’, classiﬁed
as a day-neutral plant, preferentially produced ﬂowers under long days regardless of
temperature, and ﬂower production under short days declined as temperature increased.
Based on the same trends, Sonsteby and Heide (2007b) used the term quantitative long-
day plant to describe European everbearing cultivars which produced a higher number of
ﬂowers under long photoperiods than under short photoperiods at high temperatures.
Other authors have concluded that traditional Junebearing varieties should be classiﬁed
as facultative short-day plants (Darrow, 1966), or single cropping (Dumer et al. , 1984;
Nicoll and Galletta, 1987). Day-neutral plants (or everbearers) have been more
problematic to classify as they tend to differ in strength of rebloom and fruit quality.
Several attempts have been made to categorize cultivars based on different fruiting, and
ﬂowering trends (Darrow, 1966; Nicoll and Galletta, 1987). After evaluating remontancy
of eleven strawberry genotypes in a controlled environment, Nicoll and Galletta (1987)
proposed classifying genotypes as Junebearing, weak day-neutral, intermediate day-

neutral, and strong day-neutral. The temperature environment chosen for their study was

11

22 °C day/18 °C night (under a 9-hr photoperiod plus night interruption lighting), for a
24-hr average temperature of 19.5 °C. Employing this classiﬁcation system to the current
study, ‘Honeoye’ would be classiﬁed as Junebearing, while RH 30 would be classiﬁed as
a strong day-neutral. If, however, 23 °C were used as the screening environment, RH 30
would be classiﬁed as a Junebearer, highlighting the critical inﬂuence of temperature on
remontancy. Utilizing genotypes previously deﬁned as short day (‘Honeoye’) and day-
neutral (‘Tribute’ and RH 30) (Serce and Hancock, 2005a), we have shown that
temperature is a primary factor determining whether these genotypes exhibit remontancy.
According to our results, all genotypes are photoperiod insensitive at permissive
temperatures, however, we propose that each genotype possesses an unique temperature

threshold above which ﬂowering become daylength dependent.

Therefore, we suggest that multiple cropping of strawberry is primarily a function of
temperature, with each genotype possessing a unique temperature threshold above which
photoperiod becomes regulating, and genotypes should be classiﬁed based on relative
heat-tolerance for remontancy. A multiple cropping genotype in one region may act as a
single cropping genotype in a warmer region, as alluded to by Dumer et al. (1984).
Genotypes should be screened for remontancy in several regions, including warm

regions, to ensure consistent multiple cropping across locations and years.

‘Tribute’ was the most heat-tolerant genotype under long days, producing similar
numbers of ﬂowers across the entire temperature range. All genotypes reached a

temperature where ﬂowering was inhibited except ‘Tribute’ grown under long days.

12

‘Tribute’ produced more ﬂowers under long days than short days, regardless of
temperature. RH 30 exhibited intermediate heat-tolerance between ‘Honeoye’ and
‘Tribute’, with threshold temperatures of 20 °C under long days and 23 °C under short
days. This may explain the inconsistent ﬁeld performance in successive years of progeny
utilizing RH 30 as a genetic source of remontancy, performing as a strong remontant one

year, and more as a single cropper the next (J .F .H. unpublished data).

Newly produced lateral meristems can form either a lateral crown, from which an
inﬂorescence develops, or a runner. Our results suggest that once ﬂowering is initiated,
as long as the temperature is permissive, development of inﬂorescences remains the
default pathway. If, however, temperature is too high, ﬂowering is inhibited in
subsequent meristems and runners are produced. No genotype produced runners at 14 °C
(Fig. 5), and generally few or no runners were produced when temperature was 20 °C or
less, regardless of genotype or photoperiod, similar to the results of Hartmann (1947) and
Smeets (1980). This is in contrast to the results of Dumer et a1. (1984) who, using
different genotypes, observed runner production in everbearing and day-neutral varieties
across a range of temperatures, from 18/ 14 °C to 30/26 °C day/night under both short
days and night-interruption (N I) lighting treatments, and in Junebearing varieties at all

temperatures under N1 and warm temperatures under short days.

It is not known whether the reduction in ﬂowering at high temperatures is due to a lack of
ﬂoral initiation, or failed development of an initiated inﬂorescence prior to macroscopic

visibility. Taylor (2002) suggested that ﬂowering inhibition of mature strawberry plants

13

may be due to early developmental arrest of initiated ﬂowers rather than failed initiation,
and that microscopic dissection should be conducted in addition to counting
inﬂorescences to describe meristem fate. In support of this are the results of Downs and
Piringer (1955) who, in a summer experiment in Beltsville, MD using a greenhouse
lacking temperature control, reported the presence of ﬂower primordia following
dissection but no macroscopic ﬂower buds on two Junebearing genotypes, Howard 17
and Klondike, under an ll-hr photoperiod. These results suggest that high temperatures

prevented ﬂoral development even under short day conditions.

Early research on ﬂowering control in strawberry deﬁned the interaction of photoperiod
and temperature, as Darrow and Waldo (1934) reported that at temperatures above 15 °C,
a photoperiod of 10 h or less is required for ﬂower initiation, while photoperiod did not
inﬂuence ﬂower initiation at temperatures below 15 °C, results that were conﬁrmed by
numerous other groups (Darrow, 1936; Ito and Saito, 1962; Darrow, 1966; Dumer et al. ,
1984; Nicoll and Galletta, 1987). Our results show that further increases in temperature
inhibit ﬂowering under short day conditions as well. Breeding efforts over the past few
decades have aimed to incorporate novel gerrnplasm into commercial strawberry cultivars
to improve ‘day-neutrality’. Several genetic sources of remontancy have been described
and reviewed by Ahmadi et al. (1990). The most recent genetic source of remontancy to
be introduced into commercial strawberry cultivars is a native genotype of Fragaria
virginiana (Mill) ssp. glauca (S. Watson) from the Wasatch mountains of Utah
(Bringhurst and Voth, 1984), and ‘Tribute’ is thought to have received its genes for

remontancy from a breeding parent derived from this genotype (Draper et al. , 1981).

14

Performance of modern remontant varieties has been inconsistent across regions.
Controlled environment studies on additional remontant cultivars and the wild sources of
remontancy will aid in dissecting the relative contributions of temperature and

photoperiod on ﬂowering control.

Further support for the importance of temperature on remontancy in strawberry comes
from a quantitative trait locus analysis (QTL) analysis for repeat blooming utilizing
phenotypic data for a ‘Honeoye’ X ‘Tribute’ mapping population grown at ﬁve different
locations throughout the United States (Weebadde et al., 2008). Runner plants of the
same genotypes were grown in CA (Watsonville), MD (Beltsville), MI (Benton Harbor),
MN (Victoria), and OR (Corvallis) for phenotypic analysis. The percentage of remontant
individuals varied by location, with 48-50% of plants repeat ﬂowering in the MD, MI,
and MN locations, 80% in OR, and 87% in CA. The latitudinal range covered by the
three eastern US. locations is similar to that of the CA and OR sites, indicating that
differences in photoperiod alone cannot explain the observed differences in ﬂowering.
However, weather station data from each location indicated that the CA and OR sites
were considerably cooler (both day and night temperatures) than any of the eastern sites.
A QTL explaining 36% of the phenotypic variation for remontancy identiﬁed in the MD,
MI and MN populations, but not in CA or OR, was suggested as a potential locus for heat
tolerance. The identiﬁcation of permissive and inhibitive temperatures for repeat
ﬂowering of ‘Honeoye’ and ‘Tribute’ will allow us to test this hypothesis utilizing the

same ‘Tribute’ X ‘Honeoye’ population in controlled environments.

15

Runner formation was photoperiod and temperature sensitive, consistent with previous
studies indicating that runner formation is stimulated by high temperatures and long days
(Darrow, 1936; Durner et al., 1984; Heide, 1977). Runners were only formed under long
days in ‘Honeoye’ and ‘Tribute’ (Fig. 1.4). Similarly, Serce and Hancock (2005b) did
not observe runner development under 12 hr days, regardless of temperature. RH 30 did
produce runners under short days at 23 and 26 °C, however runner production was greatly
reduced compared to long days at similar temperatures. The inhibition of runner
formation under short photoperiods appears to be related to gibberellin metabolism.
Hytonen et al. (2009) determined that strawberry ‘Korona’ formed branch crowns but not
runners under a 10 or 14 hr photoperiod (18/15 °C day/night in all photoperiods), and
formed runners under an 18 hr photoperiod. This corresponded to reduced concentration
of the active gibberellin GA 1 in axillary buds of plants exposed to short photoperiods.
Branch crown formation was promoted under an 18 hr photoperiod following application
of the gibberellin synthesis inhibitor prohexadione-calcium. Subsequent GA3 application
restored the development of runners and inhibition of branch crown formation.
Application of GA3 marginally promoted runner formation under short days, though to a
lesser extent than long day exposure, indicating an additional role for photoperiod beyond
GA metabolism in vegetative meristem differentiation. Our results suggest that ﬂowering
inhibits formation of runners, and that ﬂowering is primarily controlled by temperature.
If temperature is inhibitive for ﬂowering, runner formation is then promoted in a

photoperiod-dependent manner.

16

In conclusion, we propose that the term remontant replace ‘day-neutral’ to describe
repeat ﬂowering of strawberry within a growing season. Our results indicate that
temperature plays a primary role in determining remontancy in strawberry, and that each
genotype possesses a unique temperature threshold above which photoperiod becomes a
regulating factor. Evaluating strawberry responses to temperature and identifying the
threshold temperature at which photoperiod affects remontancy will likely be a better

predictor of repeat ﬂowering performance in the ﬁeld across locations.

17

Figure 1.1 The effect of temperature on the number of ﬂowers produced for strawberry
genotypes (A) ‘Honeoye’, (B) RH 30 and (C) ‘Tribute’ under a 9-hr (ﬁlled circles) or 16-
hr (empty circles) photoperiod during a 16 week period. Error bars represent standard
error of the mean (n=6).

150

 

A 'Honeoye'
120 _ + 9 hr
-<}—'16hr

90 ~

60 ~

Flowers (no)

30 A

 

B RH-30

200 -
150 ~

100 —

Flowers (no.)

50 ‘

 

C 'Tribute'

200 ~

150 -

Flowers (no.)

100 —

 

50 -

 

 

 

 

O I I I T

14 17 20 23 26 29

Temperature (°C)

18

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3.63 as: 29.2 95F 3.63 92:. 3.63 me: 333 «e: 3.63 92:
9919.99.19 99 .9. 999.9939...9 99 a. 9 99.9.7919... 99 .9. 999.939.... om. a. 9.99.9.5...9 99 a. 999199.99... 99 a. 9

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g XINMRHQGQQ- I L , . y m

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10010010011300
MMNNv—w- (‘0

('oN) seouesaiogul ('oN) saouaosaiouul

('on) saouaosalogul

19

Figure 1.3 The effect of temperature on the number of inﬂorescences produced for
strawberry genotypes (A) ‘Honeoye’, (B) RH 30 and (C) ‘Tribute’ under a 9-hr (ﬁlled
circles) or 16-hr (empty circles) photoperiod during a 16 week period. Error bars
represent standard error of the mean (n=6).

20

 

A 'Honeoye'

+9hr
15— —o—16hr

10-

lnflorescences (no)

 

B RH-30
20 ~

15-1

10-

lnflorescences (no)

 

C 'Tribute'

T

32*

24-

V

16‘

 

lnﬂorescences (no)

 

 

 

 

14 17 20 23 26 29

Temperature (°C)

20

Figure 1.4 The effect of temperature on the number of ﬂowers per inﬂorescence for
strawberry genotypes (A) ‘Honeoye’, (B) RH 30 and (C) ‘Tribute’ under a 9-hr (ﬁlled
circles) or l6-hr (empty circles) photoperiod during a 16 week period. Error bars
represent standard error of the mean (n=6).

12

 

A 'Honeoye'
1o —

Flowers per Inﬂorescence
O)
l

2- +9hr
—o—16hr

 

B RH-30

Flowers per Inﬂorescence

 

 

C 'Tribute'

 

Flowers per Inﬂorescence

 

 

 

14 17 20 23 26 29

Temperature (°C)

21

Fig. 1.5 The effect of temperature on the number of runners per plant for strawberry
genotypes (A) ‘Honeoye’, (B) RH 30 and (C) ‘Tribute’ under a 9-hr (ﬁlled circles) or 16-
hr (empty circles) photoperiod during a 16 week period. Error bars represent standard
error of the mean (n=6).

 

24
A 'Honeoye'

+9hr

18~

12-

Runners (no)

 

 

509

40‘

Runners (no)

20‘

 

C 'Tribute'
12 -

Runners (no)

 

 

 

 

 

14 17 20 23 26 29

Temperature (°C)

22

REFERENCES

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photoperiodism in Fragaria . Journal of the American Society for Horticultural Science
115:146-152.

Bringhurst, RS. and V. Voth (1978). Origin and evolutionary potentiality of the day
neutral trait in octoploid Fragaria. Genetics 90: 510 (abstr.)

Bringhurst, R. S., and V. Voth (1984). Breeding octoploid strawberries. Iowa State
Journal ofResearch 58: 371-381.

Darrow, G. M. (1936). Interrelation of temperature and photoperiodism in the production
of fruit-buds and runners in the strawberry. Proceedings of the American Society for
Horticultural Science 34: 360 - 363.

Darrow, GM. (1966). The strawberry: history, breeding and physiology. Holt, Rinehart
and Winston, New York.

Darrow , GM. and GP. Waldo (1934). Responses of strawberry varieties and species to
duration of the daily light period. USDA Tech. Bull, 453.

Downs, RJ. and AA. Piringer (1955). Differences in photoperiodic responses of
everbearing and June-bearing strawberries. Proceedings of the American Society for
Horticultural Science 66: 234-236.

Draper, A. D., G. J. Galletta and H.J. Swartz (1981). 'Tribute' and 'Tristar' everbearing
strawberries. HortScience 16: 794-795.

Dumer, E. F., J. A. Barden, D. G. Himelrick, and E. B. Poling (1984). Photoperiod and
temperature effects on ﬂower and runner development in day-neutral, Junebearing, and
everbearing strawberries. Journal of the American Society for Horticultural Science 109:
396-400.

Guttridge, GO (1985). Fragaria X ananassa. In: Halevy, A.H. (ed). CRC Handbook of
Flowering Vol. 3. CRC Press, Boca Raton, FL, pp. 16-33

Hancock, J .F. 1999. Strawberries. CAB International, Wallingfer, UK.
Hancock, J. F., P. W. Callow, A. Dale, J. J. Luby, C. E. Finn, S. C. Hokanson, K. E.

Hummer (2001). From the Andes to the Rockies: native strawberry collection and
utilization. HortScience 36: 221-225.

Hancock, J. F., J .J . Luby, A. Dale, P.W. Callow, S. Serce, and A. El-Shiek (2002).
Utilizing wild F ragaria virginiana in strawberry cultivar development: inheritance of

23

photoperiod sensitivity, fruit size, gender, female fertility and disease resistance.
Euphytica 126: 177 - 184.

Hartmann, H. T. (1947). Some effects of temperature and photoperiod on ﬂower
formation and runner production in the strawberry. Plant Physiology 22: 407-420.

Heide, OM. (1977). Photoperiod and temperature interactions in growth and ﬂowering of
strawberry. Physiologia Plantarum 40:21-26.

Hytbnen, T., P. Elomaa, T. Moritz and O. Junttila (2009). Gibberellin mediates
daylength-controlled differentiation of vegetative meristems in strawberry (Fragaria X
ananassa Duch.). BMC Plant Biology 9:18 doi:10.1186/1471-2229-9-18

Nicoll, M. F., G. J. Galletta (1987). Variation in growth and ﬂowering habits of
Junebearing and everbearing strawberries. Journal of the American Society for
Horticultural Science 112: 872-880.

Nishiyama, M., and K. Kanahama (2002). "Effects of Temperature and Photoperiod on
Flower Bud Initiation of Day-Neutral and Everbearing Strawberries." Acta Horticulture;
567: 253-255.

Ourecky, D.K and D.L. Slate (1967). Behaviour of the everbearing characteristics in
strawberries. Journal of the American Society for Horticultural Science 91: 236-241.

Powers, L. (1954). Inheritance of period of blooming in progenies of strawberries.
Proceedings of the American Society for Horticultural Science 64: 293-298.

Serce, S., and J. F. Hancock (2005a). Inheritance of day-neutrality in octoploid species of
F ragaria. Journal of the American Society for Horticultural Science 130: 580-584.

Serge, S., and J. F. Hancock (2005b). The temperature and photoperiod regulation of
ﬂowering and runnering in the strawberries, Fragaria chiloensis, F. virginiana, and F. x
ananassa. Scientia Horticulturae 103: 167-177.

Shaw, D.V. (2003) Heterogeneity of segregation ratios from selfed progenies
Demonstrate polygenic inheritance for day neutrality in strawberry (F ragaria X ananassa
Duch.). Journal of the American Society for Horticultural Science 128: 504-507.

Shaw, D. V. and T. R. Famula (2005). Complex segregation analysis of day-neutrality in
domestic strawberry (F ragaria Xananassa Duch.). Euphytica 145: 33 1 -33 8.

Smeets, L. (1980). Effect of temperature and day-length on ﬂower initiation and runner
formation in two everbearing strawberry cultivars. Scientia Horticulturae 12: 19-26.

24

Sonsteby, A., and O. M. Heide (2007a). "Quantitative Long-Day Flowering Response in
the Perpetual-F lowering F 1 Strawberry Cultivar Elan." Journal of Horticultural Science
& Biotechonology 82(2): 266-274.

Sonsteby, A., and O. M. Heide (2007b). "Long-Day Control of Flowering in Everbearing
Strawberries." Journal of Horticultural Science & Biotechonology 82(6): 875-884.

Sugimoto, T., K. Tarnaki, Y. Matsumoto, K. Shiwaku and K. Watanabe (2005). Detection
of RAPD markers linked to the everbearing gene in Japanese cultivated strawberry. Plant
Breeding 124: 498-501.

Taylor, D. R. (2002). The physiology of ﬂowering strawberries. Acta Horticulturae 567:
245-251.

Vince-Prue.D, C. G. G. (1973). Floral initiation in strawberry: spectral evidence for the
regulation of ﬂowering by long-day inhibition. Planta 110: 165-172.

Weebadde, C. K., D. Wang, C. E. Finn, K. S. Lewers, J. J. Luby, J. Bushakra, T. M.

Sjulin and J. F. Hancock (2008). Using a linkage mapping approach to identify QTL for
day-neutrality in the octoploid strawberry. Plant Breeding 127: 94-101.

25

CHAPTER TWO

INFLUENCE OF TEMPERATURE ON REMONTAN CY AND
RUNNERING IN THE STRAWBERRY F1 POPULATION
‘HONEOYE’ X ‘TRIBUTE’

26

ABSTRACT

Previously, we identiﬁed the threshold temperatures for repeat ﬂowering
(remontancy) of ‘Honeoye’ and ‘Tribute’ under long photoperiods. To further explore the
role of temperature in controlling ﬂowering, individuals from the F1 population
‘Honeoye’ X ‘Tribute’ were replicated through either crown division or runner
propagation, and grown at 17, and 23°C under a 16 h photoperiod. Remontancy class for
each individual genotype was determined using ﬂowering data obtained from ﬁve
different ﬁeld trial locations MD, MI, MN, OR, and CA. Results indicate that remontant
types (RM) produced more total inﬂorescences than non-remontant (N RM) types
regardless of temperature or propagule type, and NRM types produced the most runners.
In addition, how an experimental unit is derived will have an effect on the number of
runners and inﬂorescences produced. Runner derived plants have an afﬁnity to produce

more runners, and plants obtained through crown division produce more inﬂorescences.

27

INTRODUCTION

The cultivated strawberry, F ragaria Xananassa Duch., arose from the accidental
hybridization of two American species F ragaria virginiana Duch. and F ragaria
chiloensis (L.) Duch.. It was discovered in 1766 by French botanist Antoine Nicholas
Duchesne, possibly in the royal garden at Versailles where Duchesne studied, but
systematic efforts to improve F ragaria Xananassa through plant breeding did not begin

until the early 1800’s (Hancock 1999).

Fragaria Xananassa cultivars are classiﬁed by when and how often they ﬂower during a
growing season. Historically, cultivars have been divided into two main ﬂowering types,
‘Junebearers’(JB; or short-day (SD)) which produce one ﬂush of ﬂowers per season
during the spring, and ‘Everbearers’ (EB; or long-day (LD)) which, in addition to a
spring crop, produce another crop of fruit during the long days of summer (Darrow
1934). Because ﬂoral induction is regulated by both genetic and environmental factors,
more recent classiﬁcation systems have been proposed. Bringhurst and Voth (1978)
described everbearing cultivars derived from a cross using a multiple cropping clone of
F. virginiana ssp. glauca as ‘day-neutral’ (DN). Others have described ﬂowering as a
quantitative rather than a qualitative trait with plants displaying a continuum of ﬂowering

responses ranging from strictly SD types to strongly DN (N icoll 1987).

Early investigations reported the inﬂuence of temperature on ﬂower initiation in
strawberry, indicating the potential difficulty in photoperiodic classiﬁcation of cultivars
(Darrow 1936). Guttridge (1985) found that SD types can be induced to ﬂower under

any photoperiod given that temperatures are 15 °C or less. At temperatures higher than

28

this, ﬂowering in SD types is inhibited even under short photoperiods (Ito 1962), and
ﬂower numbers decrease for DN and LD types (Dumer et al., 1984; Heide, 1977; Serce
and Hancock, 2005a). As photoperiod appears not to be the only factor controlling repeat
ﬂowering, we propose that the term ‘remontancy’ be used to describe cultivars classiﬁed
as DN. This terminology describes the ﬂowering behaviour only and not the putative
underlying mechanism responsible for repeat ﬂowering. Results from Weebadde et al.
(2008) further support a role for temperature in controlling repeat ﬂowering in
strawberry. In this study, an F1 population was produced by crossing the SD ‘Honeoye’
by the DN ‘Tribute’. The population was grown in ﬁve different locations: MD, MI,
MN, CA, and OR and evaluated weekly for ﬂowering. Genotypes were considered to be
DN if they produced ﬂowers after June 15th. A higher percentage of DN progeny were
recorded in CA and OR compared to MD, MN, and MI. Field-collected weather data
indicated that the CA and OR sites were considerably cooler than the eastern locations,
while covering a similar latitudinal (and thus, photoperiodic) range. Therefore, the
observed differences in ﬂowering patterns of the population across location suggest that
high temperatures, rather than long photoperiods, may be the factor inhibiting ﬂowering

in SD types.

To test this hypothesis, the SD parent ‘Honeoye’, the DN parent ‘Tribute’, and a putative
DN clone of Fragaria virginiana ssp.virginiana, RH 30, were grown in six different
temperatures, 14, 17, 20, 23, 26, and 29°C, under a 9 or 16 h photoperiod (Chapter 1).
Results from this study indicate that ﬂowering in ‘Honeoye’ is inhibited at temperatures
of 20°C or more under long days. At temperatures of 17°C or less, ‘Honeoye’ produced

ﬂowers regardless of daylength, whereas ‘Tribute’ produced ﬂowers in temperatures up

29

to 29°C under long days. RH 30 behaved much as ‘Honeoye’, but its ﬂowering was

inhibited at the higher temperature of 23°C.

The previous study indicated that both ‘Honeoye’ and ‘Tribute’ were remontant at 17 °C
under long photoperiod, while only ‘Tribute’ was remontant under long days at 23 °C.
The objective of the current study is to determine if the ﬂowering patterns observed
across locations for ‘Honeoye’ X ‘Tribute’ population individuals by Weebadde et al.
(2008) resulted from variation in heat-tolerance among population individuals. That is, do
individuals classiﬁed as DN exhibit greater heat-tolerance than individuals classiﬁed as

SD?

Materials and Methods

During the summer of 2007, plants of the segregating population ‘Honeoye’ X ‘Tribute’
(Weebadde et al., 2008) were dug and transferred from the Southwest Research and
Extension Centre in Benton Harbor, M1, to the Horticulture Teaching and Research
Centre in Holt, MI. Plants were potted in one gallon round pots using soilless media
containing (v/v) 70% peat moss, 21% perlite and 9% vermiculite (Sure-Mix, Michigan
Grower Products, Galesburg, MI) and maintained in an unheated greenhouse. In October
2008, these plants were moved to a 26°C heated greenhouse and given supplemental light
to induce runner production (Chapter 1). On 27 January 2009, individuals which had not
yet produced runners were replicated through crown division to produce four replicates of
each genotype, and potted as above. For individuals that did produce runners, the runners

were removed and potted as above to produce four replicates of each genotype. Sixty-

30

ﬁve genotypes were replicated through crown divisions. Of these, twenty-three were
classiﬁed as non-remontant and forty-two were classiﬁed as remontant. Ninety-three
genotypes were replicated through runner plants: sixty-nine classiﬁed as non-remontant,
and twenty-four classiﬁed as remontant. Both types of propagule were placed in a misted

propagation house to root.

On 23 February 2009, two plants of each genotype were placed in one of two glass
glazed greenhouses set to a constant temperature of 17 and 23°C and a 16 h photoperiod.
Air temperature in each treatment was measured by a Type E thermocouple (TT-E-40;
Omega Engineering, Stamford, CT.) placed in an aspirated tube. Thermocouples were
connected to a data logger (CR10; Campbell Scientiﬁc, Logan, UT) and data were
recorded every 10 5. Weekly temperature averages for each temperature were within d:
1.0 °C of the setpoint each week during the experimental period. Vapor pressure deficit
was maintained between 0.7 and 1.0 kPa at each temperature by steam injection. To
maintain a 16 h daylength, ambient daylight was supplemented with 50 umol m-z s-1
photosynthetically-active radiation supplied by high-pressure sodium lamps from 0600 to
2200 HR daily; lamps were programmed to turn off when ambient irradiance outside the

greenhouse exceeded 400 pmol m-z s-l.

Two representatives of each genotype were arranged in a randomized complete block
design within each temperature. Plants were overhead irrigated with reverse osmosis
water supplemented with a water-soluble fertilizer to provide the following (mg L-l): 125

N, 13 P, 125 K, 15 Ca, 1 Fe, 0.1 B, 0.1 M0, 0.5 Mn, 0.5 Zn and 0.5 Cu (MSU Special;

Greencare Fertilizers, Chicago, IL).

31

Data collection and analysis

Every 7 d during the experiment, the number of inﬂorescences and runners per plant were
determined. Inﬂorescences were counted once all ﬂower buds within the inﬂorescence
were clearly distinguishable. After each data collection, inﬂorescences, runners and any
dead leaves were removed. Data collected during the ﬁrst 49 days of the experiment was
assumed to be the result of conditions prior to initiation of the experiment and therefore

was analyzed seperately (Chapter 1, (Serce 2005).

Analyses of variance were performed on the total number of inﬂorescences and runners
produced between days 56 and 77 by each genotype. Fixed factors used in the analysis
were temperature (17°C, 23°C), remontancy class (remontant, non-remontant), and
propagule (crown divisions, runner plants). The dependent variables were total
inﬂorescences, and total runner production between days 56 and 77. Remontancy class
was determined using ﬂowering data obtained from ﬁve different growing regions MD,
MI, MN, OR, and CA (Weebadde, 2008). As growing conditions in OR and CA were
considered to be permissive, a genotype was determined to be remontant (RM) if it
ﬂowered in at least two of the eastern regions MN, MD, and MI, and non-remontant

(NRM) if ﬂowering only occurred in CA and/or OR.

Results
Flowering
Remontancy class and propagule type (Crown division (CR) or runner plants (RU))

interacted to inﬂuence inﬂorescence production both during the early (days 1-49) and late

32

 

(days 50-77) periods of the experiment (Table 1). RM types produced more total
inﬂorescences than NRM types in both temperature treatments, regardless of propagule
type, from day 50-77 (Table 1). Among the RM genotypes, CR plants produced more
total inﬂorescences than RU plants, during both the early and late periods of the

experiment.

There was considerable variation in the percentage of individuals producing
inﬂorescences, both across remontancy class, and across propagule type within a
remontancy class (Table 2, Figs. 2.1, 2.2). At 17 °C, 79% of RM CR plants, and 36% of
RM RU plants ﬂowered as opposed to 18% of NRM CR plants, and 16% of NRM RU
plants (Table 2, Figs. 2.1, 2.2). At 23°C, 58% of RM CR plants while only 22% of RM
RU plants ﬂowered. Very few NRM plants ﬂowered at 23 °C, with only 11% of NRM
CR plants and no NRM RU plants producing ﬂowers. RM types derived from crown
divisions produced the most inﬂorescences, and had the highest ﬂowering percentages in

both temperatures.

Runnering

Temperature interacted with remontancy class (type), and propagule interacted with
remontancy class to inﬂuence runner production (Table 1). More runners were produced
at 23°C than at 17°C regardless of remontancy class or propagule (Table 1). A higher
percentage of NRM types produced runners than RM types (Table 2, Figs.2. 1, 2.2), and

runner derived plants produced more runners than plants derived from crown divisions

33

(Table 1). At 23°C NRM types produced more runners then RM types, and runner

derived plants at 23°C produced more runners overall.
Discussion

The objective of this experiment was to test whether observed differences in ﬂowering
behaviour of individuals across different ﬁeld trialling sites may be related to differences
in heat-tolerance. Based on previous results (Chapter 1) 17°C was chosen as a
permissive temperature at which both RM and NRM types would repeat ﬂower, while at

23°C NRM plants would be inhibited from ﬂowering.

NRM types grown at 23°C produced the most runners regardless of propagule type. As
lateral buds can either develop into an inﬂorescence (short shoot) or into a runner (long
shoot) (Hyttinen 2009) the more runners a plant produced, the fewer lateral buds could
potentially develop into inﬂorescences. Furthermore, all plants grown at 23°C,
regardless of remontancy class or propagule type, produced more runners than those
grown at 17°C. This is in agreement with the previous study (Chapter 1) which showed
an increase in runner production in all three varieties as temperature and daylength

increased.

RM types generally produced more inﬂorescences than NRM types, and NRM types
produced more runners than RM types, regardless of temperature. These ﬁndings,
however, may have been confounded by the different ratios of propagule type for each
remontancy class. Because NRM plants runner more easily than RM plants, 78% of the
NRM plants were derived from runners as opposed to 46% runner derived RM plants.

The most inﬂorescences were produced by crown derived RM types at 23°C.

34

Although the results do show NRM types being inhibited at 23°C, with only 11% of
NRM CR and no NRM RU plants ﬂowering, these percentages were not signiﬁcantly
different from those found at 17°C, where only 18% of NRM CR and 11% of NRM RU
plants ﬂowered. This lack of signiﬁcance may due to the fact that the plants grown at
17°C developed at a slower rate than those grown at 23 °C. In the previous study
(Chapter 1), ‘Honeoye’ took 112 days to repeat ﬂower. The current study had to be
terminated after only 77 days due to loss of temperature control in the greenhouses
because of rising outside temperatures. Had the experiment continued, it is possible that a
higher percentage of NRM types would have ﬂowered. Altemately, this discrepancy may
be because the NRM types in the population have a higher temperature threshold than
that of its NRM parent ‘Honeoye’. The progeny may have inherited some heat tolerance
from the RM parent ‘Tribute’. It is also possible that the results were confounded by the

disparaging ratios of propagule represented in each ﬂowering type.

The results show that the way an experimental unit is produced, whether by runners or
crown division, inﬂuences the number of inﬂorescences and runners produced. Because
plants derived from crown divisions had a greater afﬁnity for inﬂorescence production
than those obtained from runners, and runner derived plants preferentially produced
runners, the prior life history of a fully grown strawberry plant impacts how it performs
under experimental conditions. Another problem that strawberry researchers encounter
in trying to obtain replicates is the fact that RM types produce few to no runners (Dale
2002). This results in a discrepancy in the number of RM and NRM types represented in

experiments using runner derived plants.

35

Sonsteby and Heide (2007a) chose to use seedlings in an attempt to avoid these
complications, but as strawberries are octoploid heterozygotes, it is impossible to recover
the exact genetic proﬁle of the parent in the progeny, as each round of self-pollination
decreases heterozygosity by 50% (Fehr 1987). Thus, the alleles of interest may be lost

when selﬁng heterozygous octoploids.

To lessen the impact of genetic variation between sibs, it may be possible to use
immature runner plants obtained from one seedling plant so that all replicates are
identical clones. Also, because strawberry seedlings have the ability to runner prior to
ﬂowering (Lacey, 1973), it is possible that this phenomenon occurs regardless of seedling
remontancy class; therefore plants may be obtained without any one remontancy class
being preferentially represented. Future studies in this area would be helpful in answering
this question, and determining whether replicates obtained in this way are immature
enough to be placed in experimental conditions without their previous life history

confounding the data obtained.

36

 

 

 

 

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38

Figure 2.1: The effect of temperature, remontancy class, and propagule type on
cumulative percent of replicates ﬂowering in the segregating population
‘Honeoye’ >< ‘Tribute’ under a 16 h photoperiod.

 

 

 

 

—.-— NRMcrown O
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Time (days)

39

 

Figure 2.2: The effect of temperature, remontancy class, and propagule type on
cumulative percent of replicates runnering in the segregating population
‘Honeoye’ X ‘Tribute’ under a 16 h photoperiod.

 

—.— NRM crown

 

 

   

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40

REFERENCES

Bringhurst, RS. and V. Voth (1978). Origin and evolutionary potentiality of the day
neutral trait in octoploid Fragaria. Genetics 90: 510 (abstr.)

Dale, A., J .F . Hancock and J .J . Luby (2002). Breeding day-neutral strawberries for
Northern north America. Acta Horticulturae 567: 133-136.

Darrow, G. D. ( l 93 6). Interrelation of temperature and photoperiodism in the production
of fruit-buds and runners in the strawberry. Journal of The American Society For
Horticultural Science 34: 360 - 363.

Darrow, G. M. a. G. F. W. (1934). "Responses of strawberry varieties and species to the
duration of the daily light period" USDA Tech. Bul. 453.

F ehr, W. R. (1987). Principles of cultivar development. New York, Macmillan Publishing
Company.

Hancock, J. F. (1999). Strawberries. Wallingford, CABI Publishing.

Hytonen, T., P. Elomaa, T. Moritz, and O. Junttila (2009). Gibberellin mediates
daylength-controlled differentiation of vegetative meristems in strawberry (Fragaria
xananassa Duch.). BMC Plant Biology 9(18): doi:10.1186/1471-2229-9-18.

Ito, H., and T. Saito (1962). Studies on the ﬂower formation in the strawberry plants. I.
Effects of temperature and photoperiod on the ﬂower formation. Tohoku Journal of
Agricultural Research 13: 191-203.

Lacey, C.N.D. (1978). Phenotypic correlations between vegetative characters and yield
components in strawberry. Euphytica 22 (197): 546-554

Nicoll, M. F ., G. J. Galletta (1987). Variation in growth and ﬂowering habits of
junebearing and everbearing strawberries. Journal of the American Society for
Horticultural Science 112(5): 872-880.

Serce, S., and J. F. Hancock (2005). The temperature and photoperiod regulation of
ﬂowering and runnering in the strawberries, Fragaria chiloensis, F. virginiana, and F. x
ananassa." Scientia Horticulturae 103: 167-177.

Sonsteby, A., and O. M. Heide (2007a). Quantitative long-day ﬂowering response in the
perpetual-ﬂowering F1 strawberry cultivar Elan. Journal of Horticultural Science &
Biotechonology 82(2): 266-274.

Weebadde, C. K., D. Wang, et al. (2008). Using a linkage mapping approach to identify
QTL for day-neutrality in the octoploid strawberry. Plant Breeding. 127: 94-101.

41

APPENDIX

Weebadde et al. (2008) successfully identiﬁed a major QTL for remontancy in the
segregating population ‘Honeoye’ >< ‘Tribute’. The marker closest to this QTL is AF LP
band aggcatl 87T. As this marker was found to be present in the remontant parent,
‘Tribute’, but was absent in the non-remontant parent, ‘Honeoye’, it could provide a
useful marker to genetically screen for remontantcy. A re-investigation of the
segregation ratios of the marker in 65 individuals of the mapping population (29 RM, 36
NRM) revealed that of the 29 individuals classiﬁed as RM the marker was present in 9,
and of the 36 that were classiﬁed as NRM, the marker was absent in 20 individuals.
Therefore, it was concluded that the AF LP marker (aggcat187T) was not closely linked to

the major QTL for remontancy in this population.

42

 

Table A. l. Segregation of the AF LP marker, aggcat187T, in sixty-ﬁve individuals of

the mapping population ‘Honeoye’ >< ‘Tribute’.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Genotype Phenotype Marker
Honeoye NRM Absent
Tribute RM Present

1 RM Present
2 RM Absent
3 NRM Present
4 RM Absent
5 RM Present
6 NRM Present
7 NRM Present
8 RM Absent
9 NRM Absent
1O NRM Absent
11 NRM Absent
12 NRM Absent
1 3 RM Absent
14 RM Present
1 5 RM Present
16 NRM Absent
17 NRM Absent
18 RM Present
19 NRM Present
20 RM Absent
21 NRM Present
22 NRM Present
23 NRM Absent
24 NRM Present
25 NRM Present
26 RM Absent
27 RM Absent
28 RM Present
29 RM Absent
30 NRM Absent
31 NRM Absent
32 RM Absent

 

 

43

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Genotype Phenotype Marker
39 NRM Present
40 NRM Present
41 RM -

42 NRM Absent
43 RM Present
44 NRM Present
45 RM Absent
46 RM Absent
47 RM Absent
48 NRM Present
49 NRM Absent
50 RM Absent
51 NRM Present
52 NRM Present
33 NRM Absent
34 RM Absent
35 NRM Absent
36 RM Absent
37 NRM Absent
38 NRM Absent
53 NRM Absent
54 RM Present
55 RM Present
56 NRM Absent
57 RM Present
58 RM Present
59 NRM Absent
60 NRM Absent
61 RM Present
62 RM Absent
63 NRM Present
64 NRM Absent
65 RM Absent

 

 

 

 

REFERENCES

Weebadde, C. K., D. Wang, et al. (2008). Using a linkage mapping approach to identify
QTL for day-neutrality in the octoploid strawberry. Plant Breeding. 127: 94-101.

44

l

l

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