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Influences of day and night temperatures on flowering of Fragaria x ananassa Duch., cvs. Korona and Elsanta, at different photoperiods
Scientia Horticulturae 112 (2007) 200–206 www.elsevier.com/locate/scihorti
Influences of day and night temperatures on flowering of Fragaria x ananassa Duch., cvs. Korona and Elsanta, at different photoperiods Miche`l J. Verheul, Anita Sønsteby, Svein O. Grimstad * Norwegian Institute for Agricultural and Environmental Research, Horticulture and Urban Greening Division, Bioforsk Vest Særheim, N-4353 Klepp st., Norway Received 29 March 2006; received in revised form 13 November 2006; accepted 6 December 2006
Abstract The effects of photoperiod (12, 13, 14, 15 or 16 h), day temperature (12, 15, 18, 24 or 27 8C) and night temperature (6, 9 or 12 8C) and their interactions on flower and inflorescence emergence were investigated by exposing 4 week old runner plants of strawberry cvs. Korona and Elsanta during a period of 3 weeks. A daily photoperiod of 12 or 13 h resulted in the highest number of plants with emerged flowers. A photoperiod of 14 h or more strongly reduced this number, while no flowers emerged at a photoperiod of 16 h. Plants exposed to photoperiods of 12 or 13 h flowered earlier and had longer flower trusses. A day temperature of 18 8C and/or a night temperature of 12 8C were optimal for plants to emerge flowers and resulted in the shortest time to flowering. A night temperature of 6 8C strongly reduced the number of plants that emerged flowers, especially when combined with lower day temperatures. Photoperiod and temperature had no effect on the number of inflorescences, all flowering plants produced on average one inflorescence. The number of flowers on the inflorescence increased with decreasing day temperature and when photoperiod was raised from 12 to 15 h. In general, ‘Korona’ was more sensitive to photoperiod and temperature as ‘Elsanta’, and had a lower optimal day temperature for flower emergence. Results of this experiment may be used to produce high quality plant material or to define optimal conditions when combining flower induction and fruit production. # 2006 Elsevier B.V. All rights reserved. Keywords: Day temperature; Flowering; Fragaria x ananassa; Greenhouse production; Night temperature; Photoperiod
1. Introduction Interest in strawberry cultivation under glass or in high tunnels is increasing rapidly. In Norway, various production systems for winter production using artificial light were proposed (Sønsteby et al., 2001; Verheul and Grimstad, 2002), all requiring a detailed understanding of flowering behaviour as related to environmental conditions. In a production system with six successive plantings per year, where each planting is harvested only once, plant quality is of overriding importance for economic reasons (Verheul and Grimstad, 2002). In a production system where flower induction and fruit production are combined using the same plants during winter (Sønsteby et al., 2001), optimal conditions for both processes should be ensured in order to obtain maximum yield.
* Corresponding author. Tel.: +47 46413200; fax: +47 51426744. E-mail address: [email protected] (S.O. Grimstad). 0304-4238/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.scienta.2006.12.022
Environmental factors controlling the transition from vegetative to floral growth are playing a key role in strawberry production (Durner and Poling, 1988). The June bearing strawberry (Fragaria x ananassa Duch.) is classified as a facultative short-day plant (Darrow, 1966). However, an interaction of photoperiod and temperature in the flower initiation of this plant has been demonstrated in many studies (Heide, 1977; Smeets, 1980; Le Mie`re et al., 1996). Different threshold photoperiods and temperatures have been reported for different strawberry cultivars (Ito and Saito, 1962; Heide, 1977; Sønsteby and Nes, 1998; Konsin et al., 2001). Ito and Saito (1962) studied flower initiation in cv. Robinson, and found a clear cut-off between photoperiods of 12 and 16 h. Konsin et al. (2001) found the critical photoperiod for flower induction in ‘Korona’ to be in the range 12 and 15 h. Sønsteby and Nes (1998) showed maximum flowering in ‘Korona’ and ‘Elsanta’ at 15 8C and 24 days with 8 h photoperiod. However, for successful greenhouse production, more detailed knowledge is required about the effects of photoperiod and temperature and their interactions on flower and inflorescence emergence (Verheul et al., 2006).
M.J. Verheul et al. / Scientia Horticulturae 112 (2007) 200–206
Interactions between photoperiod, day temperature, duration of short day treatment and plant age on flowering in strawberry cv. Korona were studied by Verheul et al. (2006). Young plants grown at 10–12 h photoperiod during 21 days at day temperatures between 12 and 18 8C achieved complete flower emergence, while no flowers emerged at a photoperiod of 16 h. More detailed knowledge is required on the reaction of strawberry plants to photoperiods between 12 and 16 h. Greenhouse production provides the opportunity to optimise both day and night temperatures for flower production. There is evidence from previous research that fluctuating temperatures has no effect on plant growth (Kumakura and Shishido, 1994). However, little is known about the effect of day/night temperatures on flowering. The aim of this study was to examine more accurately the effects of photoperiod and different day and night temperatures and their interactions on flower and inflorescence emergence. Also, the effect of photoperiod and temperature on the rate of progress to flowering, an important characteristic for production planning, was registered. Since considerable variation might be expected between cultivars in their response to photoperiod and temperature, ‘Korona’ was in this study compared with the main strawberry variety used for greenhouse production in NW-Europe, cv. Elsanta. 2. Material and methods Runner plants of Fragaria x ananassa Duch. cvs. Korona and Elsanta were collected from stock plants and grown in a greenhouse under conditions, as described earlier by Verheul et al. (2006). Four-week-old uniform plants of each cultivar, with three to four leaves and an average fresh weight of 6.4 g, were taken randomly into 75 treatments in growth chambers, 10 plants per treatment. The experimental design was fully factorial, factors being the length of the daily photoperiod, the day and the night temperature. Plants were exposed during 21 days to photoperiods of 12, 13, 14, 15 or 16 h at daytime temperatures of 12, 15, 18, 24 or 27 8C and night temperatures of 6, 9 or 12 8C. Control plants were kept at constant day and night temperatures of 12, 15, 18, 24 and 27 8C. All plants were exposed to lighting with fluorescent lamps (Osram L58W/21) providing a photon flux density (PPFD) of 160 mmol m 2 s 1 PAR and supplementary lighting from incandescent lamps with a PPFD of 7 mmol m 2 s 1 PAR at plant height. Temperature fluctuations were kept within 0.5 8C. A water vapour pressure deficit of 3.5–1.0 g m 3 and a CO2 level of 700 mmol mol 1 (10 mmol mol 1) were maintained at all temperatures. Plants were irrigated when needed, using a complete fertiliser solution (Verheul et al., 2006) at an EC level of 1.5 mS cm 1. After finishing 3 weeks of short-day and temperature treatments, plants were returned to the greenhouse with longday conditions, i.e. 20 h day 1 provided by high pressure sodium lamps (Philips SON/T) with a PPFD of 125 mmol m 2 s 1 PAR at plant height, and at temperature set points of 18 8C by day, 15 8C by night with venting at 20 8C (Verheul et al., 2006). The flowering response of the plants was evaluated 12 weeks after returning the plants to the greenhouse by recording the
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number of plants with emerged flowers, the number of inflorescence per plant, the number of flowers in each inflorescence and the length of the flower truss. The number of days until the appearance of the first open flower was assessed twice a week from the end of the treatment period onwards. The length of the flower truss was measured from the stipule site to the first branch on the main truss. Data were subjected to analysis of variance using the GLM procedure of the SAS statistical computer program (Version 6.10). Parallel plants within each treatment were considered as replications. The SNK-test was used to determine significant differences among treatments at a level of significance of P < 0.05. 3. Results 3.1. Flower emergence Flower emergence in strawberry plants was optimal at a photoperiod of 12 h, a day temperature of 18 8C and a night temperature of 12 8C (Table 1). A photoperiod longer than 13 h strongly reduced the proportion of plants with emerged flowers. At day temperatures of 12 or 27 8C or at a night temperature of 6 8C, an increasing number of plants remained vegetative. More plants of the cv. Elsanta than of cv. Korona emerged flowers (Table 1). The optimal day temperature at favourable photoperiod was higher for ‘Elsanta’ when compared to ‘Korona’ (Fig. 1). The effect of photoperiod was more pronounced at a night temperature of 9 8C when compared to 6 or 12 8C (Fig. 2a). Increase in night temperature had most effect on flower emergence at low day temperatures (Fig. 2b). 3.2. Number of flowers per plant The total number of flowers on flowering plants increased when photoperiod was raised from 12 to 15 h and with increasing night temperature, but declined with increasing day temperature (Table 1). On average, ‘Korona’ developed more flowers per plant than ‘Elsanta’ (Table 1). ‘Korona’ was also more sensitive to photoperiod (Fig. 3a) and day temperature (Fig. 3b). The decrease in the number of flowers per plant with increasing day temperature was more pronounced at higher night temperatures (Fig. 4). Results show that 4-week-old plants subjected to a short-day treatment for 3 weeks developed one inflorescence per plant (Table 1). Since the number of inflorescences per plant was not affected by any of the treatments, effects of photoperiod and temperature on the number of flowers of ‘Korona’ and ‘Elsanta’ plants were mainly caused by their effect on the number of flowers per inflorescence. 3.3. Number of days to flowering On average, plants flowered after 49 days counted from the end of the short-day treatment. Photoperiod, day temperature, night temperature and cultivar all affected the time to anthesis (Table 1). The number of days to flowering decreased with
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Table 1 Main effects of photoperiod (P, h), day temperature (DT, 8C), night temperature (NT, 8C), and cultivar (C) on the percentage of plants with flowers initiated, the number of flowers per plant, the number of inflorescences per plant, the number of days to flowering and the length of flower trusses Treatment
% Plants with flowers initiated
No. of flowers per flowering plant
No. of infloresc. per flowering plant
No. of days to flowering
Length of flower truss (cm)
Photoperiod (P) 12 h 13 h 14 h 15 h 16 h
73 72 41 13 0
(a) (a) (b) (c)
28.5 29.6 33.1 37.1 –
(d) (c) (b) (a)
1.11 1.05 1.00 1.11 –
(a) (a) (a) (a)
48.8 47.8 50.5 50.7 –
(b) (c) (a) (a)
10.9 (a) 9.4 (b) 9.0 (b) 9.3 (b) –
Day temperature (DT) 12 8C 15 8C 18 8C 24 8C 27 8C
45 55 60 51 38
(bc) (ab) (a) (ab) (c)
40.2 35.8 34.3 21.4 16.9
(a) (b) (c) (d) (e)
1.06 1.14 1.07 1.02 1.02
(a) (a) (a) (a) (a)
49.7 47.7 47.1 50.1 51.2
(b) (c) (d) (b) (a)
11.1 (a) 10.5 (a) 10.5 (a) 8.9 (b) 7.8 (c)
Night temperature (NT) 6 8C 9 8C 12 8C
35 (b) 54 (a) 61 (a)
28.8 (b) 30.7 (a) 31.0 (a)
1.03 (a) 1.06 (a) 1.08 (a)
50.1 (a) 49.4 (b) 47.8 (c)
7.3 (c) 10.9 (a) 10.4 (b)
43 (b) 57 (a) n.s.
31.6 (a) 29.5 (b) n.s. n.s.
1.04 (a) 1.08 (a) n.s. n.s. n.s. n.s. n.s. n.s. n.s. n.s. n.s. n.s.
47.3 (b) 50.1 (a) n.s. n.s. n.s.
9.4 (b) 10.2 (a)
***
***
***
***
**
***
n.s. n.s. n.s.
n.s.
**
***
Cultivar (C) Korona Elsanta P DT P NT PC DT NT DT C NT C P DT NT P DT C P NT C DT NT C
*
n.s.
**
***
***
***
**
n.s. n.s.
n.s. n.s. n.s. n.s. n.s.
*
n.s. n.s.
*** *** ***
*
n.s.
Values followed by different letters are significantly different according to SNK-test at P < 0.05 level. Significance of interaction between P, DT, NT and C is given: n.s: not significant. * P < 0.05. ** P < 0.01. *** P < 0.001.
Fig. 1. Flowering of strawberry cvs. Korona and Elsanta at different photoperiods and day temperatures (means of three night temperatures). LSD (P < 0.05) = 11.5 (Korona) or 11.3 (Elsanta).
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Fig. 2. Flowering of strawberry plants at (a) different photoperiods and night temperatures and (b) different day temperatures and night temperatures (means of two cultivars). LSD (P < 0.05) = 6.9 (a), 7.4 (b).
Fig. 3. Effects of (a) photoperiod and (b) day temperature on the number of flowers per flowering plant of strawberry cvs. Korona and Elsanta. LSD (P < 0.05) = 1.2 (a), 1.3 (b).
decreasing photoperiod and increasing night temperature. A day temperature of 18 8C resulted in the earliest flowering. The number of days to flowering increased at both higher and lower day temperatures (Table 1). Optimal day temperature had most effect on the time to flowering at a night temperature of 12 8C (Fig. 5). On average, ‘Korona’ flowered 3 days earlier than ‘Elsanta’ (Table 1). ‘Korona’ was more sensitive to day and night temperatures than ‘Elsanta’. Differences in the time to flowering between these to cultivars were most pronounced at low day temperature and high night temperature (Fig. 5).
3.4. Length of flower trusses Photoperiod, day and night temperature all affected the length of flower trusses. The longest flower trusses were observed at a photoperiod of 12 h, day temperatures of 12, 15 or 18 8C and a night temperature of 9 8C (Table 1). The cv. Elsanta developed longer flower trusses than ‘Korona’ (Table 1). Effects of photoperiod and day temperature on flower truss length were most pronounced in ‘Korona’ (Fig. 6). The effect of night temperature was most pronounced at a photoperiod of 14 h (Fig. 7). Day temperature had most effect on flower truss length of ‘Korona’, while night temperature had most effect on ‘Elsanta’ (Fig. 8). 4. Discussion
Fig. 4. Effects of day and night temperature on the number of flowers of flowering strawberry plants (means of two cultivars). LSD (P < 0.05) = 1.4.
The critical photoperiod for flower emergence in the greenhouse cultivars Korona and Elsanta is in this study specified to be 15 h. To achieve flower emergence in more than 90% of the plants, a photoperiod of 12 or 13 h is required. A longer photoperiod resulted in a sharp decrease in the number of plants with emerged flowers, even at low temperature. According to Heide (1977), flowers may initiate even in continuous light at temperatures below 16 8C in some cultivars. However, the cvs. Korona and Elsanta seemed to be more sensitive to photoperiod when compared to the cultivars studied
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Fig. 5. Effects of day and night temperature on the number of days to flowering of strawberry plants of cv. Korona and Elsanta. LSD (P < 0.05) = 2.2 (Korona), 1.6 (Elsanta).
Fig. 6. Length of flower trusses of strawberry cvs. Korona and Elsanta at different photoperiods and day temperatures. LSD (P < 0.05) = 1.2 (Korona), 1.6 (Elsanta).
earlier by Heide. The present experiment showed that maximal flower emergence not only occurs at an optimal photoperiod, but also at an optimal temperature. The number of plants that emerged flowers in ‘Korona’ decreased drastically at a day temperature lower than 15 8C or higher
Fig. 7. Effects of photoperiod and night temperature on the length of the flower trusses of strawberry plants (means of day temperature and cultivar). LSD (P < 0.05) = 0.8.
than 18 8C, while ‘Elsanta’ was less sensitive to temperature and showed a broader optimal day temperature range, between 15 and 27 8C. Results show that both day and night temperature had an effect on flower emergence. A night temperature of 6 8C strongly reduced the number of plants with emerged flowers, especially when combined with low day temperature. Also a combination of high night temperature and high day temperature reduced the number of plants with emerged flowers. There is reason to expect that flower emergence is related to the average daily temperature rather than to different day and night temperatures. Indeed, a good relationship between average daily temperature and the number of plants with emerged flowers was found (Fig. 9). Results of the present experiment showed that photoperiod and temperature did not have an effect on the number of inflorescences per flowering plant. Earlier experiments (Verheul et al., 2006) showed that the number of inflorescences per plant was mainly influenced by plant age and the duration of short-day treatment. The effect of photoperiod and temperature is therefore mainly an effect on the number of plants that emerge flowers.
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Fig. 8. Effects of day and night temperature on the length of flower trusses of strawberry plants of cvs. Korona and Elsanta. LSD (P < 0.05) = 0.7 (Korona), 0.9 (Elsanta).
Fig. 9. Relation between average daily temperature and the percentage of plants with flowers emerged of cv. Korona (*, r2 = 0.79) and Elsanta (^, r2 = 0.81).
Temperature had a marked effect on the number of flowers per inflorescence. Low day temperature clearly increased the number of flowers per inflorescence. This is in according to earlier findings in tomato (Atherton and Harris, 1986). Le Mie`re et al. (1996) stated that temperature had little effect on the final number of flowers and found an upper limit of approximately 16 flowers in the primary inflorescence in cv. Elsanta. However, these authors studied the influence of temperature and photoperiod after the initial induction process had occurred. Results of the present experiment show that the temperature during flower initiation influences the number of flowers per inflorescence and that the upper limit of flowers might be twice as high. This indicates that effects of temperature during flower initiation are different from those after flower initiation, and that they should be studied separately. In addition, effects of temperature on flowering of secondary and tertiary inflorescences as observed by Le Mie`re et al. might disturb effects of temperature on flowering of the primary inflorescence. The difference between day and night temperature (DIF) is known to have an effect on stem elongation in plants. According to Moe and Heins (2000), DIF might also affect flower initiation and the rate of development. In the present
experiment however, DIF and length of flower trusses or DIF and number of days to flowering were not related. In the present experiment, the number plants with emerged flowers was, to some extent, related to the length of flower trusses (r2 = 0.61) and to the number of days to flowering (r2 = 0.47). This may indicate that photoperiod and temperature conditions that result in more complete flower initiation also result in longer flower trusses and a shorter period from initiation to anthesis. In conclusion, controlling photoperiod and temperature under greenhouse conditions can accurately regulate flower emergence in ‘Korona’ and ‘Elsanta’. Flower induction in the investigated varieties was mainly determined by photoperiod and temperature, while plant age and the duration of short-day treatment determined the number of inflorescences per plant (Verheul et al., 2006). Temperature during flower initiation was most important in determining the number of flowers on the primary inflorescence. Environmental effects after flower initiation may be different from those during flower initiation, and should therefore be further clarified. Results of this experiment may be used to produce high quality plant material or to define optimal conditions when combining flower induction and fruit production. Acknowledgement ˚ sbø for excellent technical assistance and the We thank B. A Norwegian Agricultural Bank for financial support. References Atherton, J.G., Harris, G.P., 1986. Flowering. In: Atherton, J.G., Rudich, J. (Eds.), The Tomato Crop, A Scientific Basis for Improvement. University Press, Cambridge, pp. 167–200. Darrow, G.M., 1966. The Strawberry: History, Breeding and Physiology. Holt, Rhinehart and Winston, New York. Durner, E.F., Poling, E.B., 1988. Strawberry developmental responses to photoperiod and temperature: a review. Adv. Strawberry Prod. 7, 6–15. Heide, O.M., 1977. Photoperiod and temperature interactions in growth and flowering of strawberry. Physiol. Plant. 40, 21–26.
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Ito, H., Saito, T., 1962. Studies on the flower formation in the strawberry plants. I. Effects of temperature and photoperiod on the flower formation. Tohoku J. Agr. Res. 13, 191–203. Konsin, M., Voipio, I., Palonen, P., 2001. Influence of photoperiod and duration of short-day treatment on vegetative growth and flowering of strawberry (Fragaria x ananassa Duch.). J. Hort. Sci. Biot. 76, 77–82. Kumakura, H., Shishido, Y., 1994. Effect of daytime, night time and mean diurnal temperatures on the growth of ‘Morioka-16’ strawberry fruit and plants. J. Jpn. Soc. Hort. Sci. 62, 827–832. Le Mie`re, P., Hadley, P., Darby, J., Battey, N.H., 1996. The effect of temperature and photoperiod on the rate of flower initiation and the onset of dormancy in the strawberry (Fragaria x ananassa Duch.). J. Hort. Sci. Biot. 71, 361–371. Moe, R., Heins, R.D., 2000. Thermo- and Photomorphogenesis in plants. In: Strømme, E. (Ed.), Advances in Floricultural Research. Report no. 6/2000. Agricultural University of Norway.
Smeets, L., 1980. Effect of temperature and day length on flower initiation and runner formation in two everbearing strawberry cultivars. Sci. Hort. 12, 19– 26. Sønsteby, A., Nes, A., 1998. Short day and temperature effects on growth and flowering in strawberry (Fragaria x ananassa Duch.). J. Hort. Sci. Biot. 73, 730–736. Sønsteby, A., Heide, O.M., Nes, A., 2001. Produksjon av jordbær i den mørke a˚rstid, blomstring og fruktsetting, resultat og muligheter. In: Fløistad, E. (Ed.), Jordbærdyrking i veksthus. Planteforsk Grønn forskning. 12/2001. Verheul, M., Grimstad, S.O., 2002. An effective method for year-round production of strawberries in Norway. Acta Hort. 567, 577–579. Verheul, M.J., Sønsteby, A., Grimstad, S.O., 2006. Interactions of photoperiod, temperature, duration of short day and plant age on flowering of Fragaria x ananassa Duch. cv. Korona. Sci. Hort. 107, 164–170.
Influences of day and night temperatures on flowering of Fragaria x ananassa Duch., cvs. Korona and Elsanta, at different photoperiods Miche`l J. Verheul, Anita Sønsteby, Svein O. Grimstad * Norwegian Institute for Agricultural and Environmental Research, Horticulture and Urban Greening Division, Bioforsk Vest Særheim, N-4353 Klepp st., Norway Received 29 March 2006; received in revised form 13 November 2006; accepted 6 December 2006
Abstract The effects of photoperiod (12, 13, 14, 15 or 16 h), day temperature (12, 15, 18, 24 or 27 8C) and night temperature (6, 9 or 12 8C) and their interactions on flower and inflorescence emergence were investigated by exposing 4 week old runner plants of strawberry cvs. Korona and Elsanta during a period of 3 weeks. A daily photoperiod of 12 or 13 h resulted in the highest number of plants with emerged flowers. A photoperiod of 14 h or more strongly reduced this number, while no flowers emerged at a photoperiod of 16 h. Plants exposed to photoperiods of 12 or 13 h flowered earlier and had longer flower trusses. A day temperature of 18 8C and/or a night temperature of 12 8C were optimal for plants to emerge flowers and resulted in the shortest time to flowering. A night temperature of 6 8C strongly reduced the number of plants that emerged flowers, especially when combined with lower day temperatures. Photoperiod and temperature had no effect on the number of inflorescences, all flowering plants produced on average one inflorescence. The number of flowers on the inflorescence increased with decreasing day temperature and when photoperiod was raised from 12 to 15 h. In general, ‘Korona’ was more sensitive to photoperiod and temperature as ‘Elsanta’, and had a lower optimal day temperature for flower emergence. Results of this experiment may be used to produce high quality plant material or to define optimal conditions when combining flower induction and fruit production. # 2006 Elsevier B.V. All rights reserved. Keywords: Day temperature; Flowering; Fragaria x ananassa; Greenhouse production; Night temperature; Photoperiod
1. Introduction Interest in strawberry cultivation under glass or in high tunnels is increasing rapidly. In Norway, various production systems for winter production using artificial light were proposed (Sønsteby et al., 2001; Verheul and Grimstad, 2002), all requiring a detailed understanding of flowering behaviour as related to environmental conditions. In a production system with six successive plantings per year, where each planting is harvested only once, plant quality is of overriding importance for economic reasons (Verheul and Grimstad, 2002). In a production system where flower induction and fruit production are combined using the same plants during winter (Sønsteby et al., 2001), optimal conditions for both processes should be ensured in order to obtain maximum yield.
* Corresponding author. Tel.: +47 46413200; fax: +47 51426744. E-mail address: [email protected] (S.O. Grimstad). 0304-4238/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.scienta.2006.12.022
Environmental factors controlling the transition from vegetative to floral growth are playing a key role in strawberry production (Durner and Poling, 1988). The June bearing strawberry (Fragaria x ananassa Duch.) is classified as a facultative short-day plant (Darrow, 1966). However, an interaction of photoperiod and temperature in the flower initiation of this plant has been demonstrated in many studies (Heide, 1977; Smeets, 1980; Le Mie`re et al., 1996). Different threshold photoperiods and temperatures have been reported for different strawberry cultivars (Ito and Saito, 1962; Heide, 1977; Sønsteby and Nes, 1998; Konsin et al., 2001). Ito and Saito (1962) studied flower initiation in cv. Robinson, and found a clear cut-off between photoperiods of 12 and 16 h. Konsin et al. (2001) found the critical photoperiod for flower induction in ‘Korona’ to be in the range 12 and 15 h. Sønsteby and Nes (1998) showed maximum flowering in ‘Korona’ and ‘Elsanta’ at 15 8C and 24 days with 8 h photoperiod. However, for successful greenhouse production, more detailed knowledge is required about the effects of photoperiod and temperature and their interactions on flower and inflorescence emergence (Verheul et al., 2006).
M.J. Verheul et al. / Scientia Horticulturae 112 (2007) 200–206
Interactions between photoperiod, day temperature, duration of short day treatment and plant age on flowering in strawberry cv. Korona were studied by Verheul et al. (2006). Young plants grown at 10–12 h photoperiod during 21 days at day temperatures between 12 and 18 8C achieved complete flower emergence, while no flowers emerged at a photoperiod of 16 h. More detailed knowledge is required on the reaction of strawberry plants to photoperiods between 12 and 16 h. Greenhouse production provides the opportunity to optimise both day and night temperatures for flower production. There is evidence from previous research that fluctuating temperatures has no effect on plant growth (Kumakura and Shishido, 1994). However, little is known about the effect of day/night temperatures on flowering. The aim of this study was to examine more accurately the effects of photoperiod and different day and night temperatures and their interactions on flower and inflorescence emergence. Also, the effect of photoperiod and temperature on the rate of progress to flowering, an important characteristic for production planning, was registered. Since considerable variation might be expected between cultivars in their response to photoperiod and temperature, ‘Korona’ was in this study compared with the main strawberry variety used for greenhouse production in NW-Europe, cv. Elsanta. 2. Material and methods Runner plants of Fragaria x ananassa Duch. cvs. Korona and Elsanta were collected from stock plants and grown in a greenhouse under conditions, as described earlier by Verheul et al. (2006). Four-week-old uniform plants of each cultivar, with three to four leaves and an average fresh weight of 6.4 g, were taken randomly into 75 treatments in growth chambers, 10 plants per treatment. The experimental design was fully factorial, factors being the length of the daily photoperiod, the day and the night temperature. Plants were exposed during 21 days to photoperiods of 12, 13, 14, 15 or 16 h at daytime temperatures of 12, 15, 18, 24 or 27 8C and night temperatures of 6, 9 or 12 8C. Control plants were kept at constant day and night temperatures of 12, 15, 18, 24 and 27 8C. All plants were exposed to lighting with fluorescent lamps (Osram L58W/21) providing a photon flux density (PPFD) of 160 mmol m 2 s 1 PAR and supplementary lighting from incandescent lamps with a PPFD of 7 mmol m 2 s 1 PAR at plant height. Temperature fluctuations were kept within 0.5 8C. A water vapour pressure deficit of 3.5–1.0 g m 3 and a CO2 level of 700 mmol mol 1 (10 mmol mol 1) were maintained at all temperatures. Plants were irrigated when needed, using a complete fertiliser solution (Verheul et al., 2006) at an EC level of 1.5 mS cm 1. After finishing 3 weeks of short-day and temperature treatments, plants were returned to the greenhouse with longday conditions, i.e. 20 h day 1 provided by high pressure sodium lamps (Philips SON/T) with a PPFD of 125 mmol m 2 s 1 PAR at plant height, and at temperature set points of 18 8C by day, 15 8C by night with venting at 20 8C (Verheul et al., 2006). The flowering response of the plants was evaluated 12 weeks after returning the plants to the greenhouse by recording the
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number of plants with emerged flowers, the number of inflorescence per plant, the number of flowers in each inflorescence and the length of the flower truss. The number of days until the appearance of the first open flower was assessed twice a week from the end of the treatment period onwards. The length of the flower truss was measured from the stipule site to the first branch on the main truss. Data were subjected to analysis of variance using the GLM procedure of the SAS statistical computer program (Version 6.10). Parallel plants within each treatment were considered as replications. The SNK-test was used to determine significant differences among treatments at a level of significance of P < 0.05. 3. Results 3.1. Flower emergence Flower emergence in strawberry plants was optimal at a photoperiod of 12 h, a day temperature of 18 8C and a night temperature of 12 8C (Table 1). A photoperiod longer than 13 h strongly reduced the proportion of plants with emerged flowers. At day temperatures of 12 or 27 8C or at a night temperature of 6 8C, an increasing number of plants remained vegetative. More plants of the cv. Elsanta than of cv. Korona emerged flowers (Table 1). The optimal day temperature at favourable photoperiod was higher for ‘Elsanta’ when compared to ‘Korona’ (Fig. 1). The effect of photoperiod was more pronounced at a night temperature of 9 8C when compared to 6 or 12 8C (Fig. 2a). Increase in night temperature had most effect on flower emergence at low day temperatures (Fig. 2b). 3.2. Number of flowers per plant The total number of flowers on flowering plants increased when photoperiod was raised from 12 to 15 h and with increasing night temperature, but declined with increasing day temperature (Table 1). On average, ‘Korona’ developed more flowers per plant than ‘Elsanta’ (Table 1). ‘Korona’ was also more sensitive to photoperiod (Fig. 3a) and day temperature (Fig. 3b). The decrease in the number of flowers per plant with increasing day temperature was more pronounced at higher night temperatures (Fig. 4). Results show that 4-week-old plants subjected to a short-day treatment for 3 weeks developed one inflorescence per plant (Table 1). Since the number of inflorescences per plant was not affected by any of the treatments, effects of photoperiod and temperature on the number of flowers of ‘Korona’ and ‘Elsanta’ plants were mainly caused by their effect on the number of flowers per inflorescence. 3.3. Number of days to flowering On average, plants flowered after 49 days counted from the end of the short-day treatment. Photoperiod, day temperature, night temperature and cultivar all affected the time to anthesis (Table 1). The number of days to flowering decreased with
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Table 1 Main effects of photoperiod (P, h), day temperature (DT, 8C), night temperature (NT, 8C), and cultivar (C) on the percentage of plants with flowers initiated, the number of flowers per plant, the number of inflorescences per plant, the number of days to flowering and the length of flower trusses Treatment
% Plants with flowers initiated
No. of flowers per flowering plant
No. of infloresc. per flowering plant
No. of days to flowering
Length of flower truss (cm)
Photoperiod (P) 12 h 13 h 14 h 15 h 16 h
73 72 41 13 0
(a) (a) (b) (c)
28.5 29.6 33.1 37.1 –
(d) (c) (b) (a)
1.11 1.05 1.00 1.11 –
(a) (a) (a) (a)
48.8 47.8 50.5 50.7 –
(b) (c) (a) (a)
10.9 (a) 9.4 (b) 9.0 (b) 9.3 (b) –
Day temperature (DT) 12 8C 15 8C 18 8C 24 8C 27 8C
45 55 60 51 38
(bc) (ab) (a) (ab) (c)
40.2 35.8 34.3 21.4 16.9
(a) (b) (c) (d) (e)
1.06 1.14 1.07 1.02 1.02
(a) (a) (a) (a) (a)
49.7 47.7 47.1 50.1 51.2
(b) (c) (d) (b) (a)
11.1 (a) 10.5 (a) 10.5 (a) 8.9 (b) 7.8 (c)
Night temperature (NT) 6 8C 9 8C 12 8C
35 (b) 54 (a) 61 (a)
28.8 (b) 30.7 (a) 31.0 (a)
1.03 (a) 1.06 (a) 1.08 (a)
50.1 (a) 49.4 (b) 47.8 (c)
7.3 (c) 10.9 (a) 10.4 (b)
43 (b) 57 (a) n.s.
31.6 (a) 29.5 (b) n.s. n.s.
1.04 (a) 1.08 (a) n.s. n.s. n.s. n.s. n.s. n.s. n.s. n.s. n.s. n.s.
47.3 (b) 50.1 (a) n.s. n.s. n.s.
9.4 (b) 10.2 (a)
***
***
***
***
**
***
n.s. n.s. n.s.
n.s.
**
***
Cultivar (C) Korona Elsanta P DT P NT PC DT NT DT C NT C P DT NT P DT C P NT C DT NT C
*
n.s.
**
***
***
***
**
n.s. n.s.
n.s. n.s. n.s. n.s. n.s.
*
n.s. n.s.
*** *** ***
*
n.s.
Values followed by different letters are significantly different according to SNK-test at P < 0.05 level. Significance of interaction between P, DT, NT and C is given: n.s: not significant. * P < 0.05. ** P < 0.01. *** P < 0.001.
Fig. 1. Flowering of strawberry cvs. Korona and Elsanta at different photoperiods and day temperatures (means of three night temperatures). LSD (P < 0.05) = 11.5 (Korona) or 11.3 (Elsanta).
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Fig. 2. Flowering of strawberry plants at (a) different photoperiods and night temperatures and (b) different day temperatures and night temperatures (means of two cultivars). LSD (P < 0.05) = 6.9 (a), 7.4 (b).
Fig. 3. Effects of (a) photoperiod and (b) day temperature on the number of flowers per flowering plant of strawberry cvs. Korona and Elsanta. LSD (P < 0.05) = 1.2 (a), 1.3 (b).
decreasing photoperiod and increasing night temperature. A day temperature of 18 8C resulted in the earliest flowering. The number of days to flowering increased at both higher and lower day temperatures (Table 1). Optimal day temperature had most effect on the time to flowering at a night temperature of 12 8C (Fig. 5). On average, ‘Korona’ flowered 3 days earlier than ‘Elsanta’ (Table 1). ‘Korona’ was more sensitive to day and night temperatures than ‘Elsanta’. Differences in the time to flowering between these to cultivars were most pronounced at low day temperature and high night temperature (Fig. 5).
3.4. Length of flower trusses Photoperiod, day and night temperature all affected the length of flower trusses. The longest flower trusses were observed at a photoperiod of 12 h, day temperatures of 12, 15 or 18 8C and a night temperature of 9 8C (Table 1). The cv. Elsanta developed longer flower trusses than ‘Korona’ (Table 1). Effects of photoperiod and day temperature on flower truss length were most pronounced in ‘Korona’ (Fig. 6). The effect of night temperature was most pronounced at a photoperiod of 14 h (Fig. 7). Day temperature had most effect on flower truss length of ‘Korona’, while night temperature had most effect on ‘Elsanta’ (Fig. 8). 4. Discussion
Fig. 4. Effects of day and night temperature on the number of flowers of flowering strawberry plants (means of two cultivars). LSD (P < 0.05) = 1.4.
The critical photoperiod for flower emergence in the greenhouse cultivars Korona and Elsanta is in this study specified to be 15 h. To achieve flower emergence in more than 90% of the plants, a photoperiod of 12 or 13 h is required. A longer photoperiod resulted in a sharp decrease in the number of plants with emerged flowers, even at low temperature. According to Heide (1977), flowers may initiate even in continuous light at temperatures below 16 8C in some cultivars. However, the cvs. Korona and Elsanta seemed to be more sensitive to photoperiod when compared to the cultivars studied
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Fig. 5. Effects of day and night temperature on the number of days to flowering of strawberry plants of cv. Korona and Elsanta. LSD (P < 0.05) = 2.2 (Korona), 1.6 (Elsanta).
Fig. 6. Length of flower trusses of strawberry cvs. Korona and Elsanta at different photoperiods and day temperatures. LSD (P < 0.05) = 1.2 (Korona), 1.6 (Elsanta).
earlier by Heide. The present experiment showed that maximal flower emergence not only occurs at an optimal photoperiod, but also at an optimal temperature. The number of plants that emerged flowers in ‘Korona’ decreased drastically at a day temperature lower than 15 8C or higher
Fig. 7. Effects of photoperiod and night temperature on the length of the flower trusses of strawberry plants (means of day temperature and cultivar). LSD (P < 0.05) = 0.8.
than 18 8C, while ‘Elsanta’ was less sensitive to temperature and showed a broader optimal day temperature range, between 15 and 27 8C. Results show that both day and night temperature had an effect on flower emergence. A night temperature of 6 8C strongly reduced the number of plants with emerged flowers, especially when combined with low day temperature. Also a combination of high night temperature and high day temperature reduced the number of plants with emerged flowers. There is reason to expect that flower emergence is related to the average daily temperature rather than to different day and night temperatures. Indeed, a good relationship between average daily temperature and the number of plants with emerged flowers was found (Fig. 9). Results of the present experiment showed that photoperiod and temperature did not have an effect on the number of inflorescences per flowering plant. Earlier experiments (Verheul et al., 2006) showed that the number of inflorescences per plant was mainly influenced by plant age and the duration of short-day treatment. The effect of photoperiod and temperature is therefore mainly an effect on the number of plants that emerge flowers.
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Fig. 8. Effects of day and night temperature on the length of flower trusses of strawberry plants of cvs. Korona and Elsanta. LSD (P < 0.05) = 0.7 (Korona), 0.9 (Elsanta).
Fig. 9. Relation between average daily temperature and the percentage of plants with flowers emerged of cv. Korona (*, r2 = 0.79) and Elsanta (^, r2 = 0.81).
Temperature had a marked effect on the number of flowers per inflorescence. Low day temperature clearly increased the number of flowers per inflorescence. This is in according to earlier findings in tomato (Atherton and Harris, 1986). Le Mie`re et al. (1996) stated that temperature had little effect on the final number of flowers and found an upper limit of approximately 16 flowers in the primary inflorescence in cv. Elsanta. However, these authors studied the influence of temperature and photoperiod after the initial induction process had occurred. Results of the present experiment show that the temperature during flower initiation influences the number of flowers per inflorescence and that the upper limit of flowers might be twice as high. This indicates that effects of temperature during flower initiation are different from those after flower initiation, and that they should be studied separately. In addition, effects of temperature on flowering of secondary and tertiary inflorescences as observed by Le Mie`re et al. might disturb effects of temperature on flowering of the primary inflorescence. The difference between day and night temperature (DIF) is known to have an effect on stem elongation in plants. According to Moe and Heins (2000), DIF might also affect flower initiation and the rate of development. In the present
experiment however, DIF and length of flower trusses or DIF and number of days to flowering were not related. In the present experiment, the number plants with emerged flowers was, to some extent, related to the length of flower trusses (r2 = 0.61) and to the number of days to flowering (r2 = 0.47). This may indicate that photoperiod and temperature conditions that result in more complete flower initiation also result in longer flower trusses and a shorter period from initiation to anthesis. In conclusion, controlling photoperiod and temperature under greenhouse conditions can accurately regulate flower emergence in ‘Korona’ and ‘Elsanta’. Flower induction in the investigated varieties was mainly determined by photoperiod and temperature, while plant age and the duration of short-day treatment determined the number of inflorescences per plant (Verheul et al., 2006). Temperature during flower initiation was most important in determining the number of flowers on the primary inflorescence. Environmental effects after flower initiation may be different from those during flower initiation, and should therefore be further clarified. Results of this experiment may be used to produce high quality plant material or to define optimal conditions when combining flower induction and fruit production. Acknowledgement ˚ sbø for excellent technical assistance and the We thank B. A Norwegian Agricultural Bank for financial support. References Atherton, J.G., Harris, G.P., 1986. Flowering. In: Atherton, J.G., Rudich, J. (Eds.), The Tomato Crop, A Scientific Basis for Improvement. University Press, Cambridge, pp. 167–200. Darrow, G.M., 1966. The Strawberry: History, Breeding and Physiology. Holt, Rhinehart and Winston, New York. Durner, E.F., Poling, E.B., 1988. Strawberry developmental responses to photoperiod and temperature: a review. Adv. Strawberry Prod. 7, 6–15. Heide, O.M., 1977. Photoperiod and temperature interactions in growth and flowering of strawberry. Physiol. Plant. 40, 21–26.
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Ito, H., Saito, T., 1962. Studies on the flower formation in the strawberry plants. I. Effects of temperature and photoperiod on the flower formation. Tohoku J. Agr. Res. 13, 191–203. Konsin, M., Voipio, I., Palonen, P., 2001. Influence of photoperiod and duration of short-day treatment on vegetative growth and flowering of strawberry (Fragaria x ananassa Duch.). J. Hort. Sci. Biot. 76, 77–82. Kumakura, H., Shishido, Y., 1994. Effect of daytime, night time and mean diurnal temperatures on the growth of ‘Morioka-16’ strawberry fruit and plants. J. Jpn. Soc. Hort. Sci. 62, 827–832. Le Mie`re, P., Hadley, P., Darby, J., Battey, N.H., 1996. The effect of temperature and photoperiod on the rate of flower initiation and the onset of dormancy in the strawberry (Fragaria x ananassa Duch.). J. Hort. Sci. Biot. 71, 361–371. Moe, R., Heins, R.D., 2000. Thermo- and Photomorphogenesis in plants. In: Strømme, E. (Ed.), Advances in Floricultural Research. Report no. 6/2000. Agricultural University of Norway.
Smeets, L., 1980. Effect of temperature and day length on flower initiation and runner formation in two everbearing strawberry cultivars. Sci. Hort. 12, 19– 26. Sønsteby, A., Nes, A., 1998. Short day and temperature effects on growth and flowering in strawberry (Fragaria x ananassa Duch.). J. Hort. Sci. Biot. 73, 730–736. Sønsteby, A., Heide, O.M., Nes, A., 2001. Produksjon av jordbær i den mørke a˚rstid, blomstring og fruktsetting, resultat og muligheter. In: Fløistad, E. (Ed.), Jordbærdyrking i veksthus. Planteforsk Grønn forskning. 12/2001. Verheul, M., Grimstad, S.O., 2002. An effective method for year-round production of strawberries in Norway. Acta Hort. 567, 577–579. Verheul, M.J., Sønsteby, A., Grimstad, S.O., 2006. Interactions of photoperiod, temperature, duration of short day and plant age on flowering of Fragaria x ananassa Duch. cv. Korona. Sci. Hort. 107, 164–170.