Temperature Limitations for Flowering in Strawberry and Raspberry A. Sønsteby Arable Crops Division Norwegian Institute for Agricultural and Environmental Research Kapp Norway O.M. Heide Department of Ecology and Natural Resource Management Norwegian University of Life Sciences Ås Norway Keywords: climate, flowering, fruit yield, night temperature, Rubus Abstract The effect of night temperature on short day (SD) floral induction in three June-bearing strawberry cultivars of different geographic origin was studied and compared with yield performance in the cool Nordic environment. At the optimum day temperature of 18 C, the SD flowering response of the cultivars Florence and Korona increased significantly with increasing night temperature from 9 to 18 C, while an optimum was reached at 15 C in Frida, a cultivar selected under coolenvironment conditions in Norway. Also, while saturated flowering response was obtained with three weeks of SD treatment at all temperatures in Frida, several plants of Florence and Korona failed to initiate flowers at 9 C night temperature even with five weeks of SD. The effect of extended SD period was particularly pronounced in Florence. The slow SD floral induction response of Florence was associated with a two-week delay of anthesis in subsequent long day (LD) conditions at 21 C. Performance studies of the same cultivars during two years under field conditions also demonstrated that the yield potential of Florence was not realized under the climatic conditions prevailing at these locations. The yields varied significantly among the cultivars, Frida having the highest yields, followed by Korona and Florence far below. It is concluded that, in the Nordic environment, autumn (September) night temperatures are obviously sub-optimal for yield performance of some June-bearing strawberry cultivars, and that this effect is mediated by autumn temperature effects on flower initiation responses. In the biennial-fruiting raspberry cultivar Glen Ample, flowering and dormancy induction are controlled by the interaction of low temperature and short photoperiods. As neither process takes place at temperatures above 15 C regardless of the photoperiodic conditions, low temperature is of crucial regulatory importance. Environmental control of annual-fruiting cultivars is also briefly discussed. INTRODUCTION The single-cropping or June-bearing type of strawberry is classified as a quantitative or facultative short day (SD) plant. However, because of a pronounced interaction of photoperiod and temperature, floral initiation also takes place in most cultivars under long day (LD) conditions if the temperature is low (Ito and Saito, 1962; Heide, 1977; Verheul et al., 27). Threshold temperatures for the low temperature/ld floral induction response vary considerably among cultivars (Heide, 1977; Guttridge, 1985), and cultivars like Korona and Elsanta do not initiate flowers at all in LD, even at temperatures as low as 9 C (Sønsteby and Nes, 1998; Sønsteby and Heide, 26) or 12 C (Verheul et al., 26). Because of the long days prevailing throughout the growing season at high latitudes, the LD pathway to flowering is of particular importance as it controls the ability of cultivars to initiate flower buds under LD and low temperature conditions that determines the earliness of flowering in the high latitude environment (Heide, 1977). Temperature is also important for floral initiation under SD conditions. The optimum temperature for SD floral initiation is 15-18 C, while below 1 C and above Proc. W on Berry Prod. and Cult. Systems Eds.: E. Krüger et al. Acta Hort. 838, ISHS 29 93
25 C SD induction is rather ineffective (Heide, 1977; Sønsteby and Heide, 26; Verheul et al., 27). Although both day and night temperatures are of importance, the optimum night temperature depends on the prevailing day temperature and vice versa. It is a common notion among strawberry growers that low temperature and SD govern flower bud formation in single-cropping cultivars. Because of this, many growers are concerned about the effect of the predicted and ongoing global warming on flower bud formation and yield of this crop. However, as we have seen, this low temperature is as high as 15-18 C. Therefore, in the northern environment, temperature condition during the month of September, when floral induction mainly takes place, is actually sub-optimal for SD induction in these plants. In Norway, the relatively late-maturing cultivar Florence is grown for extension of the marketing season. However, growers in the various parts of the country have experienced very different results with this cultivar. While the yields have been on the level with other cultivars in the southern and coastal areas, flowering and yield has been disappointingly poor in continental and northern locations (Myrstad, 28). We suspected that this could be related to the local autumn temperature conditions, especially to night temperatures. Therefore, we have studied the effect of night temperature on flowering in Florence and two contrasting cultivars under controlled environment conditions, and compared the yields of the same cultivars in southeastern Norway, where September night temperatures are relatively low (Sønsteby and Heide, 28a). MATERIAL AND METHODS A night temperature experiment was carried out in daylight compartments of the Ås phytotron with three cultivars. The Dutch cultivar Korona which has been the standard cultivar for the fresh market in Norway for many years, the late-flowering and -maturing British cultivar Florence, and the abundant flowering Frida bred and selected under cool-environment conditions in Norway. Four night temperatures, 9, 12, 15, and 18 C, were combined with a day temperature of 18 C, which was shown to be optimal for the cultivars Korona and Elsanta (Verheul et al., 27). The diurnal temperature fluctuations were synchronized with a 12 h photoperiod in such a way that a 12 h daylight period at 18 C was combined with a 12 h dark period at the various night temperatures. During the 12-h dark period, the plants were moved into adjacent growth rooms maintained at the respective night temperatures. After exposure to these temperature x photoperiod treatments for three weeks or five weeks, the plants were moved into a glasshouse maintained at minimum 2 C and a photoperiod of 24 h, for recording of growth and flowering responses. A field experiment was carried out at Bioforsk, Nes Hedmark (6º4 N, 1 11 E). Rooted plug-plants were planted on 9 th September in the field on raised beds with black polyethylene mulch in double rows, at a spacing of 3 x 45 x 16 cm, corresponding to 5, plants ha -1. The field experiment was a randomized complete block with four replicates, each with 2 plants of each cultivar. Yield was recorded over two years. RESULTS At the optimum day temperature of 18 C, the SD flowering response of the cultivars Florence and Korona increased significantly with increasing night temperature from 9 to 18 C, while an optimum was reached at 15ºC in the cultivar Frida (Fig. 1). The various flowering parameters were also differentially affected by the treatments in the three cultivars (data not shown). The total number of flowers per plant was significantly higher in Frida than in the other two cultivars. Also, while the number of flowers was unaffected and actually tended to decrease when the SD treatment was extended from three to five weeks in Frida and Korona, such an extension increased the number of flowers by nearly 5% in the slow-responding Florence. Extended SD treatment also had a highly significant main effect on the number of inflorescences per plant (data not shown), while the main effect of night temperature was non-significant. 94
Inflorescence numbers also varied significantly among the cultivars, Korona having on average only half as many inflorescences as the other two cultivars. This was, however, compensated for by a much higher number of flowers in each inflorescence in the Korona plants, bringing the total number of flowers at level with Florence (mean numbers = 28. and 28.9 respectively). Yield performance of the same cultivars under field conditions at Nes Hedmark also demonstrated that the yield potential of Florence was not realized under the climatic conditions prevailing at these locations (Fig. 2). The yield varied significantly among the cultivars, Frida having the highest yields followed by Korona, with Florence far below in both years (Fig. 2). DISCUSSION The results demonstrate and underline the relatively high temperature optimum for SD induction of flowering in June-bearing strawberries. At the optimum day temperature of 18 C, the night temperature optimum was as high as 18 C for Korona and Florence, while 15 C night temperature was optimal for Frida. Such high temperature optima assures that, although there might be other reasons for worrying about climate warming, strawberry growers should not be concerned about adverse effects of increasing autumn temperatures for flower formation of their strawberry crops. Especially in the Nordic environment, where autumn night temperatures currently are sub-optimal for floral induction, an increase in autumn temperature would actually be beneficial for SD floral induction in strawberry. The fruit yields of the three cultivars also demonstrate differential cultivar effects that may be related to temperature effects on flowering. Thus, the comparatively low yields of Florence may at least in part be explained as an effect of sub-optimal September temperature for floral initiation at Nes Hedmark. Clearly, the yield potential of this cultivar was not realized in this environment. Furthermore, cultivar trials in farmers fields in various regions of Norway points to the same effects. Thus, in the far north, in the county of Troms at ~69ºN, Florence consistently had very low yields compared with Frida and Korona (Myrstad, 28), whereas in the southern-most part of the country (Grimstad, ~58 N), where the September temperatures are ~5 C higher, the yields of Florence were at level with those of Korona. However, although Korona, exhibited much the same temperature responses as Florence under controlled environment conditions, Korona produced fairly good yields, both in farmers fields in the north (Myrstad, 28) and in the present field experiment at Nes Hedmark, indicating that low September temperature is not the whole explanation for the low yields of Florence. However, extension of the SD period from three to five weeks markedly enhanced flowering in Florence, but not in the other cultivars. This suggests that, as an additional effect, a more rapid decline in autumn temperature in northern and continental environments is rendering the length of the natural SD period with favorable temperature conditions marginal for satisfactory flowering and yield in the slow-responding Florence. While the environmental control of flowering in single-cropping (June-bearing) strawberry cultivars is well established and documented, the situation is still rather confusing for cultivars with recurrent flowering. Results reported by Sønsteby and Heide (27a, b) and of Nishiyama and Kanahama (22), demonstrate that everbearing strawberry cultivars, in general, are qualitative LD plants at high temperature (27 C), and quantitative LD plants at intermediate temperatures (15 C and 21 C). This applies to modern Californian cultivars as well as to older cultivars, mainly of European origin, and there is, thus, no rational for the classification of the former as day-neutral plants. In raspberry, the biennial-fruiting cultivars have an absolutely low temperature requirement and do not flower at temperatures above 15ºC, regardless of day length conditions (Sønsteby and Heide, 28b). High autumn temperature may therefore be critical and reduce flower initiation in these cultivars. In contrast, the annual-fruiting cultivars have no such low temperature requirement and flower freely at temperatures as 95
high as 3ºC (Sønsteby and Heide, unpublished results). However, at low temperature a large proportion of the lateral buds will become dormant, and this results in tip-flowering plants with low yields. Literature Cited Guttridge, C.G. 1985. Fragaria x ananassa. p. 16-33. In: A.H. Halevy (ed.), Handbook of Flowering, Vol. 3. CRC Press, Boca Raton, FL. Heide, O.M. 1977. Photoperiod and temperature interactions in growth and flowering of strawberry. Physiol. Plant. 4:21-26. Ito, H. and Saito, T. 1962. Studies on the flower formation in the strawberry plant. I. Effects of temperature on flower formation. Tohoku J. Agric. Res. 13:191-23. Myrstad, I. 28. På tide med en annen dans i valg av jordbærsorter i nord. Norsk Frukt og Bær 11(1):2-22. (In Norwegian). Nishiyama, M. and Kanahama, K. 22. Effects of temperature and photoperiod on flower bud initiation of day-neutral and everbearing strawberries. Acta Hort. 567:253-255. Sønsteby, A. and Heide, O.M. 26. Dormancy relations and flowering of the strawberry cultivars Korona and Elsanta as influenced by photoperiod and temperature. Sci. Hort. 11:57-67. Sønsteby, A. and Heide, O.M. 27a. Quantitative long-day flowering response in the perpetual-flowering F 1 strawberry cultivar Elan. J. Hort. Sci. Biotech. 82:266-274. Sønsteby, A. and Heide, O.M. 27b. Long-day control of flowering in everbearing strawberries. J. Hort. Sci. Biotech. 82:875-884. Sønsteby, A. and Heide, O.M. 28a. Temperature responses, flowering and fruit yield of the June-bearing strawberry cultivars Florence, Frida and Korona. Sci. Hort. 119:49-54. Sønsteby, A. and Heide, O.M. 28b. Environmental control of growth and flowering of Rubus idaeus L. cv. Glen Ample. Sci. Hort. 117:249-256. Sønsteby, A. and Nes, A. 1998. Short days and temperature effects on growth and flowering in strawberry (Fragaria x ananassa Duch.). J. Hort. Sci. Biotech. 73:73-736. Verheul, M.J., Sønsteby, A. and Grimstad, S.O. 26. Interaction of photoperiod, temperature, duration of short-day treatment and plant age on flowering of Fragaria x ananassa Duch. cv. Korona. Sci. Hort. 17:164-17. Verheul, M.J., Sønsteby, A. and Grimstad, S.O. 27. Influences of day and night temperatures on flowering of Fragaria x ananassa Duch., cvs. Korona and Elsanta. Sci. Hort. 112:2-26. 96
Figures Flowering p lants ( % ) 9 8 7 6 5 4 3 2 1 'Frida' Day/night temp.: 18ºC/ 9ºC 18ºC/12ºC 18ºC/15ºC 18ºC/18ºC A 1234567891111213 Time (weeks from end of SD) 9 8 7 6 5 4 3 2 1 'Korona' 1 2 3 4 5 6 7 8 9 1111213 Time (weeks from end of SD) B 9 8 7 6 5 4 3 2 1 'Florence' 1234567891111213 Time (weeks from end of SD) C Fig. 1. Effects of night temperature as indicated on the time-course of flowering (percentage of flowering plants) in three strawberry cultivars. Values are means of duration of three and five weeks SD treatment. Yield (g/plant) 7 6 5 4 3 'Frida' 'Korona' 'Florence' 2 25 26 27 28 29 3 31 32 33 Time (week no.) Fig. 2. Cumulative fruit yields of three strawberry cultivars in the second fruiting year of the experiment. 97
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