Long-day rather than autonomous control of flowering in the diploid everbearing strawberry Fragaria vesca ssp. semperflorens

Journal of Horticultural Science & iotechnology (28) 83 (3) 36 366 Long-day rather than autonomous control of flowering in the diploid everbearing strawberry Fragaria vesca ssp. semperflorens y. SØNSTEY 1 * and O. M. HEIDE 2 1 rable Crops Division, Norwegian Institute for gricultural and Environmental Research, NO- 235 Nes Hedmark, Norway 2 Department of Ecology and Natural Resource Management, Norwegian University of Life Sciences, P.O. ox 53, NO-1432 Ås, Norway (e-mail: anita.sonsteby@bioforsk.no) (ccepted 9 January 28) SUMMRY The environmental control of flowering in the perpetual-flowering (everbearing) diploid strawberry Fragaria vesca ssp. semperflorens cultivars Rügen and aron Solemacher has been studied in controlled environments. Seedpropagated plants were exposed to 1-h short-day (SD) and 24-h long-day (LD) conditions at temperatures ranging from 9 27 C. The results revealed a quantitative LD response of flowering that increased in strength with increasing temperature, to become almost obligatory at 27 C. Occasional runner formation was observed in SD at high temperature, conditions which were inhibitory to flowering, demonstrating that runnering ability is not completely lost in these genotypes. comparison with the perpetual-flowering octoploid F. ananassa Elan, in one experiment, demonstrated an identical LD temperature interaction in the two species. The results are discussed in relation to available information on the genetics of flowering habits in the two species. Since seasonal flowering types of F. vesca and F. ananassa have also been shown to share a principally identical flowering response, controlled by SD and low temperature, it is concluded that a remarkably similar flowering control system is present in the diploid F. vesca and the octoploid F. ananassa. Despite the large genetic differences between the two species, and regardless of the origin of the cultivars, the seasonal flowering types are all SD plants, while their perpetual-flowering counterparts all appear to be LD plants. In both cases, there is a pronounced interaction with temperature; the photoperiodic responses increasing with increasing temperature, in both cases. This raises the question whether a common genetic flowering control system is present in both species. The genus Fragaria Duch. comprises at least 15 species with ploidy levels ranging from diploid (2n = 2x = 14) in most European and sian species, to octoploid (2n = 8x = 56) in the cultivated strawberry F. ananassa Duch. and its ancestors F. virginiana Duch. and F. chiloensis (L) Duch., of merican origin (Hancock, 199). The most widely distributed natural species is the diploid European wood strawberry F. vesca L. which inhabits large areas of the northern hemisphere. The physiology of flowering in cultivated strawberry has been studied extensively because of the economic importance of the species. Two distinct flowering habits are known to occur: seasonal flowering (June-bearing cultivars), and perpetual flowering (everbearing cultivars; Guttridge, 1985). s with cultivated strawberry, the wood strawberry F. vesca consists of two types, with contrasting flowering behaviour: F. vesca ssp. vesca which flowers only in early Summer (seasonal flowering), and F. vesca ssp. semperflorens, often referred to as lpine strawberry, which flowers throughout the Summer (perpetual flowering). The latter produces no runners, whereas wild-type F. vesca is free-runnering. ccording to Darrow (1966), an everbearing, non-runnering type of F. vesca was first found in the French lps near Grenoble approx. 35 years ago. It is believed that all ssp. semperflorens cultivars trace back to this source. *uthor for correspondence. Taxonomically, the perpetual-flowering form has, variously, been given the rank of a sub-species (F. vesca ssp. semperflorens) or a botanical variety (F. vesca var. semperflorens). However, since the everbearing forms are mainly known in cultivation, they are sometimes simply considered as cultivars of F. vesca (ailey Hortorium, 1976). In a classical genetic analysis, rown and Wareing (1965) crossed wild-type F. vesca with the everbearing cultivars aron Solemacher and ush White and showed that the dominant allele of a single Mendelian gene conferred seasonality of flowering, while a dominant allele of an independently segregating gene allowed runner formation in the wild-type F. vesca. They also concluded that the recessive perpetual-flowering allele represented a loss of photoperiodic and thermal control of flowering. These results have since been confirmed by attey et al. (1998), who found that all F 1 generation plants showed seasonal flowering, and were free runnering, consistent with these being dominant characters. s to the physiology of flowering, rown and Wareing (1965) found that flowering of F. vesca was induced only within a narrow temperature range of 1 15 C. Within this temperature range, flowering was induced under both short-day (SD) and long-day (LD) conditions, while no flowering occurred at 7 C, or above 15 C, in either daylength. Similarly, attey et al. (1998) found that

. SØNSTEY and O. M. HEIDE 361 flowering in wild-type F. vesca was induced at 1 C in both SD and LD, while no induction took place at 19 C in either SD or LD. However, in a recent investigation using a range of Norwegian F. vesca populations, Heide and Sønsteby (27) demonstrated that flowering was controlled by a pronounced interaction of temperature and daylength. Thus, at 9 C, flowers were initiated in both 1-h SD and 24-h LD conditions, at 15 C in SD only (critical photoperiod about 16 h), while at 21 C, no initiation took place regardless of the daylength conditions. These responses are analogous to the well-documented interaction of temperature and photoperiod in non-everbearing cultivars of cultivated strawberry (Ito and Saito, 1962; Heide, 1977; Guttridge, 1985), although both the optimum temperature for SD induction, and the upper temperature limits for floral induction in both SD and LD were several C higher in F. ananassa than in F. vesca. While rown and Wareing (1965) concluded that the perpetual-flowering recessive allele of F. vesca ssp. semperflorens represents a loss of photoperiodic and thermal control of flowering, thus giving rise to spontaneous or autonomous control of flowering, detailed experiments by Sironval (1957) with another cultivar ( Fraisier des quatre-saisons ) showed that to be a strict LD plant. Thus, plants of this cultivar held in 8-h SD at 2 C for 3 years from sowing did not show any sign of flowering, while plants exposed to LD flowered rapidly and continued to flower for several months when transferred to SD conditions. Furthermore, Jonkers (1965) found that the cultivars aron Solemacher and Rügen were quantitative LD plants and produced flower buds after 118 and 17 short-days, compared with 64 and 63 long-days, respectively, for the two cultivars. On the other hand, attey et al. (1998) reported that their F. vesca ssp. semperflorens plants (cultivar not stated) flowered freely and independently of daylength conditions at both 1 C and 19 C, thus confirming the results of rown and Wareing (1965). Upon this background, and also because there are conflicting reports on the environmental control of flowering in the cultivated octoploid everbearing cultivars of strawberry (see Guttridge, 1985; Nicoll and Galletta, 1987; Sønsteby and Heide, 27a, b), we decided to study the photoperiodic control of flowering in F. vesca ssp. semperflorens in more detail using a wider range of well-controlled temperatures. The results of these investigations are presented here. MTERILS ND METHODS Plant material and cultivation Seeds of the everbearing Fragaria vesca ssp. semperflorens cultivars aron Solemacher and Rügen were obtained from a commercial seed company (http://www.b-and-t-world-seed.com) and sown at 21 C. Germination took place in 1 week. Two experiments were carried out in the Ås phytotron. ecause of the identical responses by the two cultivars in Experiment 1 (Expt. 1), only aron Solemacher was used in Expt. 2. In addition, the perpetual-flowering octoploid Elan was included in Experiment 2 to compare the responses of diploid and octoploid everbearing types. In Expt. 1, plants were raised in 1-h SD at 21 C for 29 d from germination, then exposed to photoperiods of 1 h (SD) or 24 h (LD), at temperatures of 9 C, 15 C, 21 C or 27 C for 5 weeks. ll plants were then transferred to 21 C for a further 6 weeks, with the photoperiods remaining unchanged. In Expt. 2, plants were raised in 12-h SD at 27 C for 23 d from germination, then exposed to photoperiods of 1-h SD or 24-h LD at temperatures of 15 C, 21 C or 27 C for 12 weeks, and the experiment terminated. In both cases, the plants were transplanted into 1 cm plastic pots at the stage when the first trifoliate leaves were starting to form, after 26 d and 21 d in the two Experiments, respectively. Plants were grown, throughout, in daylight phytotron compartments. ll plants received 1 h of Spring or Summer daylight from 8. 18.h, and were then moved into adjacent growth rooms with darkness or low-intensity incandescent light (approx. 7 µmol quanta m 2 s 1 ) for daylength manipulation. The additional LD light amounted to only 2-3% of the total daily light integral; thus, both daylengths provided nearly the same daily light energy. Whenever the quantum flux in the daylight compartments dropped below 15 µmol m 2 s 1, on cloudy days, an additional 125 µmol quanta m 2 s 1 were added automatically by Philips HPT-I 4 W lamps. Temperatures were controlled to ± 1. C, and a water vapour pressure deficit of 53 Pa was maintained at all temperatures. Otherwise the plants were grown as described by Sønsteby and Heide (27a). Experimental design, data collection and analysis oth Experiments were fully factorial, of the split-plot design, with temperature as the main plots, and photoperiod and cultivar as the sub-plots. ll Experiments were replicated in three randomised blocks, each containing eight plants (Expt. 1), or five plants (Expt. 2), making a total of 24 or 15 plants per treatment, respectively, for the two Experiments. The progress of flowering (i. e., time of the first anthesis in each plant) was recorded twice a week and the total number of inflorescences recorded at termination of the Experiments. In addition, the total number of flowers on each plant was recorded in Expt. 2 by counting and removing flowers each week. The height of the first inflorescence, measured from the base to the primary flower, was recorded in each plant at primary flower anthesis. ll experimental data were subjected to analysis of variance (NOV) by standard procedures using a MiniTab Statistical Software programme package (Release 14; Minitab Inc., State College, P, US). RESULTS Experiment 1 In both strawberry cultivars, flower bud initiation and flowering were significantly (P <.1) advanced by LD, the photoperiodic effect increasing with increasing temperature. Thus, at 9 C and 15 C, all or nearly all plants also flowered in SD with an average delay of 9 and 13 d respectively; while at higher temperatures, flowering was increasingly suppressed and delayed in SD. t 27 C, most plants had not flowered at all after 11 weeks of treatment in SD, while all plants of both cultivars flowered after 4 weeks under LD conditions (Figure 1).

362 Flowering of Fragaria vesca ssp. semperflorens Flowering plants (%) 1 9 8 7 6 5 4 3 2 1 1 2 3 4 5 6 7 8 9 1 11 FIG.1 Effects of temperature and photoperiod on progress to flowering in F. vesca ssp. semperflorens seedlings of Rügen (Panel ) and aron Solemacher (Panel ). Plants were raised in 1 h SD at 21 C for 29 d, then exposed to the conditions indicated for 5 weeks (time point indicated by the vertical line in each Panel). The plants were then returned to 21 C, with the photoperiods remaining unchanged. Flowering was recorded at the time of the first open flower (first anthesis). Data are for 24 plants in each treatment. Similarly, the number of inflorescences per plant was approx. the same in both daylengths, at 9 C; while, at higher temperatures, it was several-fold higher in LD than in SD (Figure 2). In SD, the number of inflorescences was more-or-less constant over the range from 9 21 C before dropping off sharply at 27 C; however, numbers increased linearly over the same temperature range in LD before levelling-off at 27 C. For the percentage flowering of plants, as well as the number of inflorescences per plant and days to anthesis, the effects of temperature and photoperiod, and their interaction, were all highly significant by NOV (P <.1). There was no significant cultivar effect for any of these parameters, while there was a highly significant interaction (P =.9) of cultivar photoperiod on the number of inflorescences per plant, mainly due to more Inflorescences per plant 16 14 12 1 8 6 4 2 SD (1 h) LD (24 h) SD 9 C SD 15 C SD 21 C SD 27 C LD 9 C LD 15 C LD 21 C LD 27 C 1 9 8 7 6 5 4 3 2 1 1 2 3 4 5 6 7 8 9 1 11 flowering in SD in Rügen than in aron Solemacher (Figure 2). Days to anthesis are shown in Figure 3. For both cultivars, flowering was significantly delayed by SD compared to LD conditions, and the difference increased with increasing temperature. Inflorescence length was consistently (significant at P <.1) stimulated by long photoperiods, while temperature had no significant effect on this parameter (data not shown). Vegetative growth was generally more vigorous in SD than in LD, especially at high temperatures. Petiole elongation was governed by an interaction between temperature and daylength, being promoted by LD at low, and by SD at high temperatures, respectively, (Table I). ecause of this temperature photoperiod interaction (P <.1), the main effect of temperature was not significant. The main effect of photoperiod, FIG. 2 Effects of temperature and photoperiod on the number of inflorescences per plant in F. vesca ssp. semperflorens seedlings of Rügen (Panel ) and aron Solemacher (Panel ). Plants were raised in 1 h SD at 21 C for 29 d, then exposed to the conditions indicated for 5 weeks, whereupon they were returned to 21 C, with the photoperiods remaining unchanged. Data are for 24 plants in each treatment and were recorded after 11 weeks of treatment. Vertical bars denote ± standard errors of the means. 16 14 12 1 8 6 4 2

. SØNSTEY and O. M. HEIDE 363 8 7 8 7 Days to anthesis 6 5 4 3 2 1 SD (1 h) LD (24 h) 6 5 4 3 2 1 FIG.3 Effects of temperature and photoperiod on days to anthesis in F. vesca ssp. semperflorens seedlings of Rügen (Panel ) and aron Solemacher (Panel ). Plants were raised in 1 h SD at 21 C for 29 d, then exposed to the conditions indicated for 5 weeks. They were then returned to 21 C for a further 6 weeks, with the photoperiods remaining unchanged. Data are for 24 plants in each treatment. Vertical bars denote ± standard errors of the means. however, was highly significant (P <.1). Interestingly, some plants of aron Solemacher, and a few of Rügen (63% and 13%, respectively), formed stolons in SD at 27 C, demonstrating that runnering ability is not entirely lost in these genotypes (Figure 4). Experiment 2 For aron Solemacher, all the major responses were similar to those in Expt. 1, except that the promoting effects of LD were even more pronounced when plants were raised at 27 C (Figure 5). Under these conditions, both earliness of flowering and accumulated flower numbers were almost identical in LD at 21 C and at 27 C, with plants at 15 C following about 2 weeks behind. In SD, on the other hand, flowering was delayed for 3 6 weeks compared to LD at 21 C; while, at 27 C, only a few plants eventually produced a few flowers. lso at 15 C, flowering was delayed for several weeks in SD compared to LD conditions, even though all plants eventually flowered in both photoperiods (Figure 5). For all flowering parameters (flowering plants, number of flowers and inflorescences per plant, and days to anthesis) the main effects of temperature and photoperiod were highly significant by NOV (P <.1 and P <.1, respectively), and so also was their interaction (P <.1). When the vegetative plants of aron Solemacher at 27 C were transferred from SD to LD after 12 weeks of treatment, they immediately initiated flowers and reached anthesis after 21 23 d. s in the previous Experiment, petiole elongation was promoted by LD at low temperatures and by SD at high temperatures (interaction significant at P <.1). gain, a few plants produced stolons at 27 C in SD (data not shown). The results in Figure 5 also demonstrate the striking similarity of responses between the diploid aron Solemacher and the octoploid Elan. Due to greater branching (bushiness) of the diploid cultivar, the total number of flowers was, however, higher in this cultivar. Otherwise, the results were almost identical for the two cultivars. s in previous experiments (Sønsteby and Heide, 27a, b) runner formation in Elan was strongly enhanced by SD and high temperature (data not shown). DISCUSSION These results demonstrate that flowering of the everbearing diploid strawberry Fragaria vesca ssp. semperflorens cultivars Rügen and aron Solemacher is not spontaneous (rown and Wareing, 1965; attey TLE I Effects of temperature and photoperiod on petiole elongation and stolon formation in two cultivars of Fragaria vesca ssp. semperflorens Petiole length (cm) No. of stolons Daylength (h) Temperature ( C) Rügen aron Solemacher Rügen aron Solemacher 1 9 16. ±.2 15.9 ±.1. ±.. ±. 15 16.7 ±.1 17. ±.2. ±.. ±. 21 18.5 ±.2 18.3 ±.2. ±..1 ±.1 27 2.1 ±.2 19.8 ±.2.5 ±.3 2.2 ±.5 24 9 17.1 ±.2 17. ±.3. ±.. ±. 15 16.9 ±.2 16.6 ±.2. ±.. ±. 21 17. ±.2 16.2 ±.2. ±.. ±. 27 13.8 ±.2 14. ±.2. ±.. ±. Plants were raised from seed in 1 h SD at 21 C for 29 d, then exposed to the conditions indicated for 5 weeks. The plants were then returned to 21 C for a further 6 weeks, with the photoperiods remaining unchanged. Values are means ± SE of 24 plants as recorded after 9 weeks of treatment.

364 Flowering of Fragaria vesca ssp. semperflorens FIG.4 ppearance of F. vesca ssp. semperflorens aron Solemacher plants after 3 weeks treatment at 27 C in 1 h SD (left) or 24 h LD (right). Note the absence of flowering and the presence of runners under SD conditions (circled). Pot diameter = 1 cm et al., 1998), but rather is governed by an intimate interaction of photoperiod and temperature. Thus, across the 9 27 C temperature range, flowering was consistently promoted and advanced by long photoperiods, the photoperiodic effect being quantitative at low temperature (9 C) and increasing in strength with increasing temperature to become nearly obligatory at 27 C (Figures 1 5). Plants raised and grown throughout in SD at 27 C in Expt. 2 produced only marginal flowering in a few plants before the experiment was terminated after 1 d from germination. While flowering was increasingly suppressed with increasing temperature in SD, there was no indication of any inhibitory effect of high temperature under LD conditions, even though the increase in both earliness and abundance of flowering levelled-off above 21 C (Figure 5). These findings are at variance with those of rown and Wareing (1965) and attey et al. (1998), who concluded that flowering was spontaneous and autonomously controlled in this everbearing form of F. vesca. The main reason for these conflicting results is that while the ritish studies were carried out at low and intermediate temperatures only, we tested the photoperiodic effect over a wider range of temperatures. Furthermore, while we took care to raise the plants under SD conditions, the results of the other investigations strongly suggest that the plants must have been raised under LD conditions and had thus been induced at an early stage before the photoperiodic treatments were started (cf. Figure 1.15 in attey et al., 1998). Such results underline the importance of careful phenotyping methods in this type of genetic studies. On the other hand, our results are in full agreement with those of Jonkers (1965) who found that Rügen and aron Solemacher are quantitative LD plants at intermediate temperature. t an average temperature of 2 C, time to visible buds was almost doubled in continuous SD compared with LD, whereas seed vernalisation for up to 1 weeks had no effect. Furthermore, Sironval (1957) found that the F. vesca ssp. semperflorens cultivar he worked with ( Fraisier des quatre-saisons ) was an obligatory LD plant that remained vegetative in SD for 3 years at 2 C. This local elgian cultivar is described as free-runnering (also evident from pictures in the publication) and produces remarkably constant and homogenous progeny by seed propagation (Sironval, 1957). eing both free-runnering and having an obligatory LD requirement for perpetual flowering, means that this is obviously a divergent genotype whose genetic makeup appears to deviate from the simple inheritance scheme described by rown and Wareing (1965) and confirmed by attey et al. (1998). In spite of the fairly tight linkage between the nonrunnering and perpetual flowering genes, recombinants are not uncommon, and non-runnering F. vesca ssp.

. SØNSTEY and O. M. HEIDE 365 Flowering plants (%) ) Flowering plants (% 1 9 8 7 6 5 4 3 2 1 1 2 3 4 5 6 7 8 9 1 11 12 1 C 9 8 7 6 5 4 3 2 1 1 2 3 4 5 6 7 8 9 1 11 12 Flowers per plant Flowers per plant FIG.5 Effects of temperature and photoperiod on flowering of seedlings of F. vesca ssp. semperflorens aron Solemacher (Panels and ) and F. ananassa Elan (Panels C and D). Seedlings were raised in 12 h SD at 27 C for 23 d (from time = ), then exposed to the conditions indicated until the Experiment was terminated after 12 weeks. Data are for 15 plants in each treatment. 65 6 55 5 45 4 35 3 25 2 15 1 5 25 2 15 1 5 SD 15ºC SD 21ºC SD 27ºC LD 15ºC LD 21ºC LD 27ºC 35 1 2 3 4 5 6 7 8 9 1 11 12 D 3 SD 21ºC SD 27ºC LD 21ºC LD 27ºC 1 2 3 4 5 6 7 8 9 1 11 12 semperflorens cultivars have long been known in Europe (Darrow, 1966). The sporadic runnering presently observed in aron Solemacher and Rügen is therefore no great surprise, although it deviates from the clean-cut, all-or-none responses reported by rown and Wareing (1965) and attey et al. (1998). It should be noted, however, that we only observed runnering in SD at 27 C, under which conditions flowering was suppressed (Figure 4). The genetic analyses of rown and Wareing (1965) and attey et al. (1998) clearly demonstrated that the differences in flowering habit within F. vesca are controlled by a single major gene, seasonal flowering being dominant to perpetual flowering. In terms of flowering physiology, the present results, as well as those from Sironval (1957) and Jonkers (1965), imply that the SD induction mechanism in wild-type F. vesca is dominant to the LD mechanism in the semperflorens mutant. Keeping in mind the large genetic differences between the cultivated octoploid strawberry F. ananassa and the wild diploid F. vesca, the similarity of their flowering control is remarkable in both seasonal flowering and perpetual-flowering types (Figure 5). In both species the seasonal flowering types are quantitative SD plants with inhibition of flowering by LD at high temperature (Guttridge, 1985; Heide and Sønsteby, 27), while their perpetual-flowering counterparts are quantitative LD plants with inhibition of flowering by SD at high temperature (Sønsteby and Heide, 27a, b). While the difference in flowering habit is attributed to a single genetic locus in F. vesca (rown and Wareing, 1965; attey et al., 1998), the genetics appear to be more complex in F. ananassa. While hmadi et al. (199) presented compelling evidence that perpetual flowering ( day neutrality ) in cultivated strawberry was controlled by a single dominant allele of a nuclear Mendelian gene, other genetic segregation studies have indicated that several genes are involved (Ourecky and Slate, 1967; Dale et al., 22; Shaw, 23). Furthermore, while perpetual flowering is a recessive character in F. vesca (rown and Wareing, 1965; attey et al., 1998), this

366 Flowering of Fragaria vesca ssp. semperflorens trait is dominant in F. ananassa (Ourecky and Slate, 1967;hmadi et al., 199). If this is correct, it appears that the perpetual flowering trait has been established by independent and different mechanisms in the two species. While the perpetual-flowering F. vesca ssp. semperflorens is considered a loss mutation of the seasonal flowering wild-type F. vesca (rown and Wareing, 1965; Darrow, 1966), it is commonly thought that the perpetual trait was introduced into octoploid cultivars from three independent sources, none of them being F. vesca ssp. semperflorens (ringhurst et al., 1989; hmadi et al., 199; Sakin et al., 1997). If this is correct, one should expect to find fundamental differences in the flowering control mechanisms between the perpetualflowering cultivars of F. ananassa and cultivars of F. vesca ssp. semperflorens. However, all critical flowering physiology investigations so far, have demonstrated the same LD promotion of flowering (cf. Figure 5), with a corresponding strong inhibition of flowering by SD at high temperature in a range of everbearing cultivars with widely divergent pedigrees, whether of merican, European or Japanese origin (Nishiyama and Kanahama, 22; Sønsteby and Heide, 27a, b). In addition, the present results, and those of Jonkers (1965), also demonstrate the identical responses in everbearing diploid F. vesca ssp. semperflorens. This suggests that a common genetic flowering control system may be involved in both species. However, it should not be forgotten that all known strawberry species are considered to have evolved from a single ancestor, the everbearing F. vesca ssp. semperflorens (Darrow, 1966). We are indebted to the Research Council of Norway for financing this work through Project No. 161971/I1. REFERENCES HMDI, H., RINGHURST, R. S. and VOTH, V. (199). Modes of inheritance of photoperiodism in Fragaria. Journal of the merican Society for Horticultural Science, 115, 146 152. ILEY HORTORIUM (1976). Fragaria L. Strawberry. Hortus Third. MacMillan Publishing Company, New York, US. p. 484. TTEY, N. H., LE MIÈRE, P., TEHRNIFR,., CEKIC, C., TYLOR, S., SHRIVES, K. J., HDLEY, P., GREENLND,. J., DRY, J. and WILKINSON, M. J. (1998). Genetic and environmental control of flowering of strawberry. In: Genetic and Environmental Manipulation of Horticultural Crops. (Cockshull, K. E., Gray, D., Seymour, G.. and Thomas,., Eds.). C International Publications, Wallingford, Oxford, UK. 111 131. RINGHURST,R.S.,HMDI, H. and VOTH, H. (1989). Inheritance of the day-neutral trait in strawberries. cta Horticulturae, 265, 35 41. ROWN, T. and WREING, P. F. (1965). The genetical control of the everbearing habit and three other characters in varieties of Fragaria vesca. Euphytica, 14, 97 112. DLE,., HNCOCK, J. F. and LUY, J. J. (22). reeding dayneutral strawberries for Northern North merica. cta Horticulturae, 567, 133 136. DRROW, G. M. (1966). The Strawberry. History, reeding and Physiology. Holt, Rinehart and Winston, New York, US. 447 pp. GUTTRIDGE, C. G. (1985). Fragaria ananassa. In: CRC Handbook of Flowering. (Halevy,., Ed.). Volume III. CRC Press Inc., oca Raton, FL, US. 16 33. HNCOCK, J. F. (199). Ecological genetics of natural strawberry species. HortScience, 25, 869 871. HEIDE, O. M. (1977). Photoperiod and temperature interactions in growth and flowering of strawberry. Physiologia Plantarum, 4, 21 26. HEIDE, O. M. and SØNSTEY,. (27). Interactions of temperature and photoperiod in the control of flowering of latitudinal and altitudinal populations of wild strawberry (Fragaria vesca). Physiologia Plantarum, 13, 28 289. ITO, H. and SITO, T. (1962). Studies on the flower formation in the strawberry plants. Tohoku Journal of gricultural Research, 13, 191 23. JONKERS, H. (1965). On the flower formation, the dormancy and the early forcing of strawberries. Mededelingen van de Landbouwhogeschool Wageningen, 65 6, 1 59. NICOLL, M. F. and GLLETT, G. J. (1987). Variation in growth and flowering habits of Junebearing and everbearing strawberries. Journal of the merican Society for Horticultural Science, 112, 872 88. NISHIYM, M. and KNHM, K. (22). Effects of temperature and photoperiod on flower bud initiation of day-neutral and everbearing strawberries. cta Horticulturae, 567, 253 255. OURECKY, D. K. and SLTE, G. L. (1967). ehaviour of the everbearing characteristics in strawberries. Proceedings of the merican Society for Horticultural Science, 91, 236 241. SKIN, M., HNCOCK, J. F. and LUY, J. J. (1997). Identifying new sources of genes that determine cyclic flowering in Rocky Mountain populations of Fragaria virginiana ssp. glauca Staudt. Journal of the merican Society for Horticultural Science, 122, 25 21. SHW, D. W. (23). Heterogeneity of segregation ratios from selfed progenies demonstrate polygenic inheritance for day neutrality in strawberry (Fragaria ananassa Duch.). Journal of the merican Society for Horticultural Science, 128, 54 57. SIRONVL, C. (1957). La photopériode et la sexualisation du Fraisier des quatre-saisons à fruits rouges. Comptes Rendus de Recherches IRSI, 18, 1 229. SØNSTEY,. and HEIDE, O. M. (27a). Quantitative long-day flowering response in the perpetual-flowering F 1 strawberry cultivar Elan. Journal of Horticultural Science & iotechnology, 82, 266 274. SØNSTEY,. and HEIDE, O. M. (27b). Long-day control of flowering in everbearing strawberries. Journal of Horticultural Science & iotechnology, 82, 875 884.