ABSTRACT
The U.S. Department of Agriculture, National Clonal Germplasm Repository (NCGR), Corvallis, Oregon, maintains a national genebank for strawberry, Fragaria L. This collection includes diverse species maintained in screenhouses. The plants are exposed to an annual temperature regime that allows flowering. The objective of this project was to examine the flowering tendencies of octoploid strawberry taxa to determine if the blooms were seasonal (once per year) or successive blooming (remontant). For each of 68 clones of nine taxa, flower presence was recorded on the first day of each month from June through Oct. 2002 and in 2004. Complete flower trusses were removed after scoring. Plants flowering only in June and/or July (or earlier) were considered “seasonal”; those with later flowers were considered “successive blooming.” For both years, North American F. chiloensis ssp. lucida and ssp. pacifica flowered seasonally, significantly different than clones of ssp. chiloensis f. patagonica, half of which were successive blooming. A clone of ssp. pacifica from Hartney Bay, Alaska, was successive blooming. The flowering in two clones of South American ssp. chiloensis f. chiloensis and one of the Hawaiian ssp. sandwicensis were seasonal. Fragaria virginiana ssp. grayana clones and half of the ssp. virginiana Mill. flowered seasonally; the rest of the clones of those subspecies were successive blooming. Flowers of a clone of F. virginiana ssp. platypetala from California were seasonal while those from a second one from Eastern Oregon were successive blooming. This clonal phenotypic information is of specific use for geneticists to determine parental crosses. This study will be continued and broadened to include diverse global species.
Introduction
The U.S. Department of Agriculture, National Clonal Germplasm Repository (NCGR) maintains a national genebank for strawberry. This collection includes 43 strawberry species and subspecies, and 1847 accessions. Some of the species are represented by seedlots. Others are represented by plants that are maintained in containers in screenhouses in Corvallis, Oregon. These plants are exposed to an annual temperature regime with winter minimum no less than –4.4 °C (24 °F). Chilling is sufficient so that the plants flower during spring and summer.
Many strawberry breeders and geneticists are interested in whether strawberries flower once during the year, leading to a single crop per season, or have successive or repeat flowering, which could result in multiple crops per growing season. The inflorescence in the octoploid strawberry occurs as a cymose compound dichasium (Staudt, Citation1999). Primary, secondary, and tertiary flowers of an inflorescence are differentiated in the fall for seasonal blooming (Lee, Citation1966). Repeat blooming could occur where latent buds develop either through genetic tendencies or through removal of inflorescences during the growing season.
Numerous terms have been used to describe these flowering tendencies in strawberries: June-bearing (Hancock, Citation1999), short day (Hancock, Citation1999), seasonal flowering (Mouhu et al., Citation2009), or once flowering (Gaston et al., Citation2013); in contrast with recurrent (Iwata et al., Citation2012), perpetual (Gaston et al., Citation2013), remontant (Hancock, Citation1999), continuous flowering (Kurokura et al., Citation2013), everbearing, (Hancock, Citation1999), day neutral (Hancock, Citation1999), or successive blooming (Sugiyama et al., Citation2004). Recent publications by molecular geneticists and biologists use “seasonal” versus “perpetual” flowering to describe these different flowering habits because of the genome synteny across many crops in the Rosaceae (Kurokura et al., Citation2013). However, in this study we follow the Sugiyama et al. (Citation2004) terminology because we cannot be sure that the removal of flowers did not promote new inflorescence development.
As Staudt (Citation1999) pointed out, some strawberry plants, such as the F. chiloensis ssp. chiloensis f. chiloensis ‘Ambato’ from Ecuador are “short day” plants though they have multiple bloom periods during the course of one year when grown at high elevations near the Equator. However, when transplanted to a high latitude location, such as in a German greenhouse or a field in Corvallis, Oregon, they bloomed once per year. The stresses on the plant, the environment, and geographical location can influence flowering (Darrow, Citation1966; Lee, Citation1966).
This study observed strawberry plants under one location. The objective of this study was to examine 68 representatives of 9 octoploid strawberry taxa in the NCGR screenhouse collection at Corvallis, Oregon, to determine their tendency to flower seasonally or with successive blooming.
Materials and methods
For this study, nine strawberry taxa in the Corvallis genebank were observed (). Flower presence was recorded in 2002 and 2004 on the first day of the month from June through October. Inflorescences and complete trusses were removed after scoring. Plants flowering only in June and/or July (or earlier) were considered “seasonal”; those with flowering in August, September, or October were considered as “successive blooming.” These observations () were made on a germplasm collection with unequal numbers of clones per subspecies, however, observations of each clone were replicated in both years. Because the response pattern was a binomial distribution and other basic assumptions of normality and homogeneity of variance were not met, one-way analysis of variance was inappropriate. The non-parametric Kruskal–Wallis test was applied. This statistic compared the ranks of the composite data from both years for the nine taxa. The test statistic H (adjusted for ties) was 38.893, d.f. = 8, P = 5.14E-6. The low probability indicated highly significant differences.
Table 1. Blooming tendency for nine American strawberry (Fragaria L.) taxa.
Results
Flower initiation in strawberries depends on environment, so this study described flowering tendencies only under Corvallis screenhouse conditions.
The adjusted H statistic indicated that the ranked responses were highly significantly different between taxa. Seasonal flowering taxa included F. chiloensis ssp. lucida, F. c. ssp. pacifica, F. c. ssp. chiloensis f. chiloensis, F. c. ssp. sandwicensis, and F. virginiana ssp. grayana. Successive bloomers included F. v. ssp. virginiana, F. v. glauca, F. v. platypetala, and, surprisingly, clones of F. c. ssp. c. f. patagonica. One clone of F. chiloensis ssp. pacifica from Alaska demonstrated successive blooming ().
Flowering in the white-pink fruited South American F. chiloensis ssp. chiloensis f. chiloensis clones was seasonal and significantly different from that of the red fruited forma patagonica, from which it most likely was derived, where a number of successive blooming clones were observed ().
Discussion
Strawberry breeders are greatly interested in cultivated strawberries with tendencies for successive blooming in northern or higher latitudes. This trait, initially called “double cropping,” has allowed for increased production in many parts of the world (Bringhurst et al., Citation1990). Geneticists are searching for molecular markers that can predict perpetual bloom (Castro et al., Citation2013; Gaston et al., Citation2013) resulting in continuously produced fruit for extended harvest.
Because the cultivated strawberry, F. × ananassa, can be traced back to an original accidental cross of the white-pink-fruited beach strawberry (F. chiloensis ssp. chiloensis f. chiloensis) with the Virginia strawberry (F. virginiana. ssp. virginiana), founding effects of these two parents can be noted. The light fruited F. chiloensis ssp. c. forma c. contributed genes for the tendency for seasonal blooming in the northern hemisphere, leading to the traditional heritage “June-bearing” of commercial strawberry production. Conversely, the F. virginiana parent brought the successive blooming trait into the genome. The now classic successive flowering F. virginiana ssp. glauca from the Wasatch Mountains in Utah, was first discovered by Bringhurst et al. (Citation1989). This clone became a donor of the trait in many named clones. This wild plant was backcrossed with cultivated strawberries to produce a breeding selection number Cal. 65.65-601. California breeders backcrossed this selection to cultivated strawberries and obtained successive blooming strawberries, such as ‘Aptos’, ‘Hecker’, and ‘Brighton’ (Bringhurst et al., Citation1990). In Maryland, breeders used the same selection number to produce successive blooming, disease-resistant cultivars: Tribute and Tristar. ‘Albion’ is another popular more recent California introduction with this heritage. This trait has become economically important in strawberry production.
Breeders have broadened the strawberry gene pool by “reconstituting” the original F. × ananassa hybrid cross (Hancock et al., Citation2010). They used a more diverse gene pool of both F. chiloensis subspecies as mothers, and F. virginiana ssp. as paternal parents, to produce new crosses for further development of improved cultivated strawberries. Our study confirmed that many clones of F. virginiana ssp. glauca or ssp. virginiana contain the genetic tendency for successive blooming ().
Unexpectedly, we observed that some clones of F. chiloensis ssp. chiloensis f. patagonica from Coyhaique, Chile had successive flowering. This was unusual because most of the other beach strawberries were seasonal blooming (). Notably, Coyhaique is inland and not a coastal location. We understand that when planted in the field in Corvallis, clones of F. chiloensis ssp. chiloensis f. patagonica bloom slowly over several weeks (Finn, personal communication). The difference between that field observation and our study is that we removed complete trusses after scoring for bloom. The trusses of F. chiloensis ssp. chiloensis f. patagonica have long peduncles so removal of the complete truss is easy and direct. The removal of these trusses may have stimulated the growth and emergence of latent flower buds in September and October on the indicated clones in both years ().
Conclusion
The octoploid strawberry gene pool contains traits for seasonal and successive blooming. These traits seem to be in the gene pool in roughly equal amounts, considering a combination of subspecies of both the beach and the Virginia strawberries. The original parental species that contributed to the cultivated strawberry genome have different phenotypes for blooming tendencies.
Our study confirmed that most beach strawberries, and particularly the white-pink-fruited F. chiloensis ssp. chiloensis f. chiloensis, had a seasonal blooming tendency under Northern Hemisphere high latitude conditions. However, surprisingly, some clones of F. chiloensis ssp. chiloensis f. patagonica had successive blooming tendencies. Our study confirmed successive blooming tendencies in clones of F. virginiana ssp. glauca and ssp. virginiana.
Sugiyama et al. (Citation2004) concluded that different cultivars had different successive bloom, which might relate to an apical dominance effect. To expand on that finding, our study indicates that some species or subspecies have a greater tendency for successive bloom, and perhaps apical dominance, than do other species.
This study will continue with more years of bloom observations for these octoploid clones and for world strawberry species of other ploidy levels. Screening of wild germplasm of other species for apical dominance or successive blooming tendencies may direct breeders to usable clones for parental possibilities in their breeding programs. The successive blooming clones identified in this study () may be of use in this endeavor.
Acknowledgments
We gratefully acknowledge the help of Megan Oakes, Debra Hawkes, and the Corvallis High School Wings team for assistance in counting blooms and flower removal for this study. We greatly appreciate the comments from anonymous reviewers who greatly improved the manuscript analysis and terminology.
Funding
We appreciate the support of ARS CRIS # 2027-21000-044-00D for funding the maintenance and evaluation of Fragaria genetic resources.
Additional information
Funding
Literature cited
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