This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and

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1 This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier s archiving and manuscript policies are encouraged to visit:

2 Scientia Horticulturae 123 (2009) Contents lists available at ScienceDirect Scientia Horticulturae journal homepage: Interaction of short day and timing of nitrogen fertilization on growth and flowering of Korona strawberry (Fragaria ananassa Duch.) Anita Sønsteby a, *, Nina Opstad a, Unni Myrheim a, Ola M. Heide b a Arable Crops Division, Norwegian Institute for Agricultural and Environmental Research, NO-2849 Kapp, Norway b Department of Ecology and Natural Resource Management, Norwegian University of Life Sciences, P. O. Box 5003, NO-1432 A s, Norway ARTICLE INFO ABSTRACT Article history: Received 20 February 2009 Received in revised form 8 July 2009 Accepted 14 August 2009 Keywords: Flowering Fragaria Growth Photoperiod Strawberry Timing of N fertilization The effects of timing of nitrogen (N) fertilization relative to the beginning of a 4-week floral-inducing short-day (SD) period have been studied in Korona strawberry plants under controlled environment conditions. Groups of low fertility plants were fertilized with 100 ml of calcium nitrate solution for 3 days a week for a period of 3 weeks starting at various times before and at the beginning of the SD period, as well as at different times during the SD period. All plants, including SD and long day (LD) control plants, received a weekly fertilization with a low concentration complete fertilizer solution throughout the experiment. Leaf area, fresh and dry matter increments of leaves, crowns and roots, as well as leaf chlorophyll concentration (SPAD values) were monitored during the experimental period. A general enhancement of growth took place at all times of N fertilization. This was paralleled by an increase in leaf chlorophyll concentration, indicating that the control plants were in a mild state of N deficiency. When N fertilization was started 2 weeks before beginning of the SD period, flowering was delayed by 7 days, and this was gradually changed to an advancement of 8 days when the same treatment was started 3 weeks after the first SD. The amount of flowering was generally increased by N fertilization although the effect varied greatly with the time of N application. The greatest flowering enhancement occurred when N fertilization started 1 week after the first SD when the number of flowering crowns and the number of inflorescences per plant were more than doubled compared with the SD control, while fertilization 2 weeks before SD had no significant effect on these parameters. Importantly, the total number of crowns per plant was not affected by N fertilization at any time, indicating that enhancement of flowering was not due to an increase in potential inflorescence sites. No flowering took place in the control plants in LD. Possible physiological mechanisms involved and practical applications of the findings are discussed. ß 2009 Elsevier B.V. All rights reserved. 1. Introduction The June-bearing strawberry is a quantitative or facultative short-day (SD) plant that initiates flowers under SD conditions at temperatures ranging from about 15 to 25 8C (Guttridge, 1985; Taylor, 2002). At higher temperatures flowering is increasingly inhibited also under SD conditions (Verheul et al., 2006, 2007). However, because of a pronounced interaction of photoperiod and temperature, floral initiation also takes place in many cultivars even in 24-h long days (LD) if the temperature is below about 15 8C (Ito and Saito, 1962; Heide, 1977). Flowering of strawberry may also be modified by the plant s water regime and nutrient status, especially nitrogen (N) status. However, as stated by Guttridge (1985), the nutritional effects on flowering in strawberry are complex. While growth-stimulating * Corresponding author. Tel.: ; fax: address: anita.sonsteby@bioforsk.no (A. Sønsteby). doses of mineral nutrients tend to inhibit floral initiation per se, the number of inflorescences may be increased indirectly if the dominant response to nutrition is to increase the number of crowns and thereby the number of potential inflorescence sites (Abbott, 1968; Breen and Martin, 1981). These opposing effects are problematic to resolve and are seldom analysed in the literature on strawberry nutrition. Increasing the nutrient supply from a low base will generally increase flowering (Breen and Martin, 1981) and fruit yields (Lineberry et al., 1944), but too much, especially of nitrogen, can inhibit flower formation and reduce fruit yield (Whitehouse, 1928; Lineberry et al., 1944). However, withholding nitrogen and phosphorus may not increase flowering (Abbott, 1968). Extra nitrogen has been reported to reduce summer flower initiation for the autumn crop of a double-cropping cultivar in England, but had little influence on spring flowering (Way and White, 1968). An important aspect of the fertilization/flowering complex is the question of timing of fertilization relative to the flowerinducing SD period. An early investigation by Long (1939) /$ see front matter ß 2009 Elsevier B.V. All rights reserved. doi: /j.scienta

3 A. Sønsteby et al. / Scientia Horticulturae 123 (2009) demonstrated that the fertility level during the period of flower bud initiation was important for rich flower formation. Fertilization in the following spring was not equally effective. Similar results were reported by Opstad and Sønsteby (2008). Lieten (2002) varied the fertility level of Elsanta strawberry during various periods of late summer and autumn, and observed large effects on crown size as well as flowering and fruit yield. Potted plants were raised outdoors until the first week of December, cold stored and cropped in a greenhouse. In 2 out of 3 years the highest flower numbers and fruit yields were obtained when nutrition was withheld until the end of August, followed by feeding with a complete fertilizer solution throughout September November. Withholding nutrition beyond mid-september significantly reduced flowering and yield, as did also fertilization during the month of August. By varying the time of N fertilization with ammonium nitrate in the ornamental SD plant Kalanchoë blossfeldiana, Rünger (1961) demonstrated that while application before the start of SD reduced and delayed flowering significantly, the same applications during the early part of the SD period had the opposite effect and increased flowering. The experiment was started with plants in a mild state of N deficiency. The earlier the application and the larger the amount of N applied, the stronger was the inhibitory effect on flowering. The strongest promotion of flowering resulted when N fertilization was started at the first day of the SD period. The strength of the inhibitory effect of N application prior to SD exposure was enhanced by high temperature during the same period. Protected cultivation of strawberry for extended marketing season has been increasing worldwide (Wagstaffe and Battey, 2007). This type of intensive production requires high input investments, and accordingly, high yields are therefore required to make the production profitable. Production of quality plants with rich flowering and high yield potential is of particular importance in such a production system. As discussed above, timing and rate of fertilization during the floral induction period can be important in this connection. In order to provide additional information on this issue, we have carried out a controlled environment experiment with the strawberry cultivar Korona in which additional nitrogen was supplied at different periods before and during a 4-week flower-inducing SD period. The results are reported here. 2. Materials and methods 2.1. Plant material and cultivation Runner plants were collected in the field in mid-july and rooted in 12 cm plastic pots filled with a peat-based potting compost. During rooting and early growth the plants were held in a greenhouse maintained at minimum 20 8C under natural LD summer conditions (18 21-h), until the 4-leaf stage when the experimental treatments were started on August 4. From this stage onwards and throughout the experiment, the plants were grown in daylight compartments of the Ås phytotron at a constant temperature of C and light conditions as described by Sønsteby and Heide (2008). During the same period all plants were fed weekly with 100 ml of a compound fertilizer solution (1.0 g l 1 of Superba TM Rød from Yara International (85 mg N l 1 )) applied to the pots at the first day of the week. From August 19 to September 16 the plants were exposed to 10-h SD for 4 weeks for floral induction. Starting at various times before and during this SD period, groups of plants were given an extra nitrogen supply for three consecutive weeks as shown in Table 1. Control groups in SD and LD received no extra nitrogen (Table 1). During the 3 weeks of N application the treated plants received a daily dose of 100 ml of calcium nitrate solution (7.0 g l 1 of Calcinit TM from Yara International (1085 mg Nl 1 )) for 3 days (Tuesday through Thursday), while they received 100 ml of tap water daily for the remaining 3 days of the week. Outside the N feeding periods the plants received 100 ml of tap water daily for 6 days a week throughout the experiment (100 ml of Superba solution on one day). A volume of 100 ml of liquid was adequate to saturate the entire pot soil volume and produce a net run-off of excessive solution Experimental design, data observation and analyses The experiment was a fully factorial design with three replications consisting of three plants each. Starting 2 weeks before commencement of the SD period and lasting until 2 weeks after its termination, samples of 3 3 plants were harvested as shown in Table 1 for monitoring of fresh and dry weights, chlorophyll content, etc. At harvest the plants were partitioned into three components: green leaves (leaves), stem (crowns), and roots. Roots were washed clean of soil material and, after blotting on tissue paper, fresh weight was determined for each component. Leaf area was measured with a LI-COR Inc. Model LI 3100 area meter, and dry weight was determined after drying at 70 8C. At each harvest chlorophyll concentration of the leaves was assessed by light transmission at 650 nm using a Minolta SPAD-502 handheld leaf chlorophyll meter (Wood et al., 1993; Markwell et al., 1995). SPAD values were determined on three leaves of each plant. In each treatment group nine plants remained throughout the experiment for recording of flowering. In these plants new runners were recorded and removed weekly, the time of flowering (first anthesis) was recorded in each plant, and the number of crowns, number of inflorescences, and the total number of flowers in each plant were recorded when all flower buds were well developed and the experiment was terminated. Experimental data were subjected to analysis of variance (ANOVA) by standard procedures using a MiniTab 1 Statistical Software program package (Release 14; Minitab Inc., State College, PA, USA). Table 1 Calendar of treatments showing the timing of N fertilization and plant sampling relative to the 4-week flower-inducing SD period. On weeks marked with X the respective groups of plants received an extra N fertilization for 3 days weekly, starting on the date indicated. Likewise, sampling of plants for determination of fresh and dry weight, chemical contents, etc., were performed on the days marked with an asterisk (*). Start of N fertilization, weeks from beginning of SD Week number and date 32, 5/8 33, 12/8 34, 19/8 35, 26/8 36, 2/9 37, 9/9 38, 16/9 39, 23/9 40, 30/9 2 X* X* X* * * * 1 X* X* X * * 0 X* X* X * 1 X* X* X * 2 X* X* X * 3 X* X* X * SD control ( N) * * * * * LD control ( N) * * * * * * *

4 206 A. Sønsteby et al. / Scientia Horticulturae 123 (2009) Fig. 1. Time courses of shoot (Panel A) and root (Panel B) dry weight and leaf area (Panel C) increments, and changes in chlorophyll concentration (Panel D) of Korona strawberry plants as affected by N fertilization started at different times before and during a 4-week floral-inducing SD period (indicated by vertical lines). Each point represents the mean of three treatment replicates with three plants each. 3. Results 3.1. Growth and chlorophyll concentration Some of the N fertilized plants developed scorched margins of young leaves, due to submergence of the smallest leaves during application and subsequent concentration of the nutrient solution during drying. This suggests that a slightly weaker nutrient solution would have been preferable. A general enhancement of plant growth took place after N fertilization. While the rate of fresh and dry weight increment of leaves and crowns gradually levelled off with time in the control plants in both photoperiods, root weight increments were more linear, and in fact started to increase markedly after 3 weeks of SD exposure (Fig. 1A and B). However, N fertilization always resulted in an almost immediate restoration of weight gains of leaves and crowns (shoot). Leaf area growth showed much the same dynamics as leaf weight, and was strongly stimulated by N fertilization (Fig. 1C). These dynamic growth effects were closely reflected in changes in leaf colour and chlorophyll concentration. Control plants were pale green with mild symptoms of N deficiency which, however, disappeared within 3 4 days after N application. Similarly, leaf chlorophyll concentration increased markedly within 1 week after N application, but levelled off and actually decreased again when N fertilization was terminated after 3 weeks (Fig. 1D). Apparently, this was due to dilution of chlorophyll in the tissue due to growth, and depletion of N in the soil by leaching. The final number of crowns per plant at termination of the experiment was not significantly affected by N fertilization before and during SD exposure, whereas LD almost completely blocked crown branching (Table 2). On the other hand, petiole length of the youngest fully developed leaves at termination was more than doubled by N application, the effect increasing with lateness of application, while photoperiod had no effect in the absence of extra N(Table 2). The total number of runners produced during the experimental period was doubled by early N fertilization in the SD treated plants (P < 0.001) with little effect of timing of application, whereas unfertilized plants in LD produced almost as many runners as the fertilized plants in SD (Table 2). Towards the end of the SD period runner formation was temporarily stopped, to resume again within 3 4 weeks after transfer to LD (time courses not shown) Flowering Plants grown under LD conditions remained vegetative throughout the experiment, while flowering in SD was significantly modified by N application and its timing. Compared with the SD control, time of flowering varied from a delay by 7 days when fertilization was started 2 weeks before beginning of the SD treatment, to 8 days advancement when fertilization was started 3 weeks after beginning of SD (Figs. 2 and 3). Fertilization at intermediate times did neither delay nor advance flowering compared with the SD control with no fertilization, the point of intersection occurring at about 1 week after start of SD. The amount of flowering was also significantly affected by N fertilization, but exhibited a different fertilizer timing dependence. While the number of flowering crowns and the number of inflorescences per plant was not significantly different from the SD control when fertilization was started 2 weeks before SD, later

5 A. Sønsteby et al. / Scientia Horticulturae 123 (2009) Table 2 Effects of timing of N fertilization on number of crowns per plant, petiole length of the youngest fully developed leaves, and the accumulated number of runners per plant as recorded at the final harvest at the stage when each plant was in full bloom. Start of N fertilization, weeks from beginning of SD Number of crowns Petiole length (cm) Number of runners a 18.6 d 9.2 b a 21.1 bc 9.8 ab a 19.4 cd 9.6 ab a 22.5 ab 8.6 bc a 22.1 b 10.8 a a 24.3 a 7.6 c SD control ( N) 5.1 a 9.3 e 4.4 d LD control ( N) 1.4 b 8.0 c 8.6 bc P value (total ANOVA) <0.001 <0.001 <0.001 P value (ANOVA excluding LD) <0.001 n.s. <0.001 The data are means of three replicates with three plants each. Numbers in a column followed by different letters are significantly different at P = 0.05 according to Tukey s test. n.s. denote not significant. applications successively increased flowering to an optimum when fertilization was started 1 week after SD had begun (Fig. 4A and B). At this time flowering was more than doubled by N fertilization. With later commencement of N applications towards the end of the SD period, stimulation of flowering was gradually reduced again. N fertilization generally increased the number of flowers per inflorescence at all times of application, although less so when started at 1 and 3 weeks after start of SD (Fig. 4C). Similarly, the total number of flowers also exhibited a general increase with N applications, the effect being most apparent at intermediate times of N application (Fig. 4D). 4. Discussion The results demonstrate that SD induction of flowering in strawberry plants can be markedly modified by N fertilization and its timing in relation to the floral-inducing SD period. The growth performance and chlorophyll concentration responses clearly demonstrate that the plants were in a mild state of N deficiency before the fertilization treatments were started. For each new group of plants there was a consistent increase in leaf area and plant mass following fertilization (Fig. 1A C). However, the performance of the control plants indicates that the basal nutrition applied to all plants was sufficient to prevent severe N deficiency and support normal growth and development. The results are therefore considered relevant to common commercial production conditions. Although flowering was generally increased by N fertilization of the plants, the degree of stimulation varied significantly with the time of N application (Fig. 4A D). The effect on flower emergence was particularly marked and varied from a delay (inhibition) when N was applied 2 weeks before beginning of the SD period to an enhancement (promotion) when fertilization was started towards the end of the SD period (Figs. 2 and 3). Since all fertilized plants received the same amount of N regardless of its time of application, these effects are not likely to be indirect growth effects, but appear to be effects on flower formation per se. Effects on the amount of flowering are more complex to interpret since such quantitative characters may be modified by plant growth (Guttridge, 1985). For example, if a dominant response to nutrients is the production of more crowns and therefore more flower sites, fertilization would have a non-specific impact on flowering (cf. Breen and Martin, 1981). However, in the present experiment, the number of crowns did not increase significantly with the fertilization treatments (Table 2). Therefore, the observed timing-dependent increase in inflorescence and flower numbers produced by N fertilization do not appear to be due to production of additional flowering sites. The results in Fig. 4 also show that number of flowering crowns and number of inflorescences per plant did not increase significantly when N was applied 2 weeks before beginning of SD, while the numbers were doubled when the same fertilization started 1 week after Fig. 2. Effects of timing of N fertilization on time of flowering of Korona strawberry plants relative to the SD control plants (horizontal line) which flowered after 77 days from start of the SD. Each value represents the mean of three treatment replicates with three plants each. Vertical bars denote S.E. Fig. 3. Appearance of Korona strawberry plants photographed on November 4, 77 days after start of the SD treatment. Fertilization of the plant to the left was started 3 weeks after beginning of SD, while the plant to the right was first fertilized 2 weeks before the SD period began.

6 208 A. Sønsteby et al. / Scientia Horticulturae 123 (2009) Fig. 4. Effects of timing of N fertilization on quantitative flowering parameters of Korona strawberry plants. Horizontal lines denote the appropriate values for control plants in SD. Each value represents the mean of three treatment replicates with 3 plants each. Vertical bars denote S.E. beginning of the SD period. With still later applications flowering decreased again to nearly the control level. Such dynamic changing responses suggest that these effects are mainly direct effects on inflorescence initiation. On the other hand, enhancement of flower numbers per inflorescence, which was mainly observed in plants that received an early application of nitrogen (Fig. 4C), could well be a result of increased plant vigour affecting floral differentiation after inflorescences had been initiated. The literature on fertilization of strawberry is extensive (for a review, see May and Pritts, 1990). Seasonal effects on fertilizer efficiency are common, and these have been related to seasonally dependent changes in allocation of nitrogenous and other resources to the crowns (Archbold and Mackown, 1997; Fernandez et al., 2001; Tagliavini et al., 2005). Although such seasonal effects are likely to be photoperiodically controlled, investigations to explore this relationship directly have not been reported before, while Lieten (2002) studied the effects of nutritional timing under naturally changing photoperiodic conditions in autumn. Although not directly comparable, the present results and those of Lieten (2002) are highly compatible. Thus, Lieten (2002) observed by serial dissections that flower initiation had started by mid- September. This means that the plants must have received SD at least 2 weeks earlier, i.e. by the end of August. He further observed that high nutrition during the month of August reduced fruit number and yield, while withholding nutrition until end of August followed by complete nutrition at that time, always produced the highest fruit number and yield. In other words, flowering was negatively affected by high plant nutritional status in the period immediately before SD induction, while high fertility during the early part of the SD induction period, significantly enhanced flowering, even though fertilization at any time enhanced flowering compared with the nutrient deficient control plants. This is in close agreement with our results. Also, the present results with strawberry are directly comparable with, and in full agreement with, the findings of Rünger (1961) with the SD plant Kalanchoë blossfeldiana. When N-deficient plants were fertilized with ammonium nitrate during a 20-day period starting 20 or 10 days before beginning of the SD period, flowering was significantly delayed and reduced, while the same treatments started at the early part of the SD period had the opposite effect and advanced and increased flowering significantly. The largest stimulation of flowering occurred when fertilization was started on the first SD. Both inhibition and stimulation increased with the amount of N applied. Although the inhibitory effect of N application before start of the SD was larger in Kalanchoë than in strawberry, the timing effects were similar, indicating related regulatory mechanisms in these two SD species. Suggestions for a possible mechanism underlying these flowering responses can only be tentative at this stage. It is commonly accepted that flowering in SD strawberry cultivars is regulated by an inhibitory LD process (Guttridge, 1985; Taylor, 2002), the effect of SD being to overcome this inhibition and releasing the plants from LD floral inhibition. Based on evidence from experiments with donor receptor pairs of runner plants joined by the stolon, Guttridge (1959) proposed that a transmissible hormone produced in LD enhances petiole elongation and runnering and inhibits flowering in strawberry. These effects could be mimicked by application of the plant hormone gibberellin (GA) (Thompson and Guttridge, 1959; Guttridge and Thompson, 1963). Furthermore, inhibitors of GA biosynthesis such as prohexadione-calcium (Pro- Ca) (Rademacher, 2000) have the opposite effect. Being a potent inhibitor of the GA 3-oxidase enzyme that catalyses the final conversion step in the biosynthesis of active GAs, Pro-Ca effectively inhibits runner formation and enhances crown branching in strawberry under field conditions (Black, 2004) with a concomitant increase in berry yield (Hytönen et al., 2008). The GA

7 A. Sønsteby et al. / Scientia Horticulturae 123 (2009) biosynthesis pathway in strawberry has been established (Taylor et al., 2000) and in a recent paper Hytönen et al. (2009) demonstrated that GA synthesis is tightly controlled by photoperiod. The paper provides strong molecular and other evidence indicating that GA functions as one of the key signals controlling developmental fate in strawberry. In light of this predominant role of GA in the mediation of photoperiod-dependent developmental events in the strawberry plant, it is logical to assume that also the photoperiod-dependent effect of N fertilization is somehow connected to GA metabolism. Possibly, fertilization under LD conditions might be boosting LDcontrolled GA biosynthesis and thereby, strengthening the LDcontrolled floral inhibition. Conversely, the same fertilization applied under SD conditions, when GA biosynthesis is blocked (Hytönen et al., 2009), might well have a completely different effect and enhance the flower initiation and differentiation processes then in progress. Blocking of side-crown formation under LD conditions as reported by Hytönen et al. (2009) was fully confirmed by the present results (Table 2). The fact that this process is mediated by GA (Hytönen et al., 2009) supports the suggestion that GA metabolism is somehow mediating also the photoperiod-dependent effects of N fertilization. The demonstrated timing effect of N fertilization in relation to the floral-inducing SD period has potential for practical application. Clearly, N availability in the autumn immediately before and at the time when the photoperiod becomes effective for floral induction can have considerable effects on flowering in strawberry, and technologies are now available for its precise control. The fertigation technique allows precise control of amount and time of fertilization, and hopefully, chlorophyll assessment by a chlorophyll meter may be used to monitor N status and aid fertilization of strawberry in the same way as for other crops (Wood et al., 1993). An interesting observation is that the high number of inflorescences formed when N fertilization started 1 week after beginning of the SD period, was associated with a marked reduction in the number of flowers per inflorescence. Since Korona plants are known to produce an excessive number of flowers per inflorescence, and hence many small berries, optimal timing of N fertilization in the autumn might represent a lead to enhancement of both berry number and size in this cultivar. This and other aspects of adaptation of the present findings to commercial strawberry production merit further investigations under field conditions. Such investigations are now under way. References Abbott, A.J., Growth of the strawberry plant in relation to nitrogen and phosphorus nutrition. J. Hortic. Sci. 43, Archbold, D.D., Mackown, C.T., Nitrogen availability and fruiting influence nitrogen cycling in strawberry. J. Am. Soc. Hortic. Sci. 122, Black, B.L., Prohexadione-calcium decreases fall runners and advances branch crowns of Chandler strawberry in a cold-climate annual production system. J. Am. Soc. Hortic. Sci. 129, Breen, P.J., Martin, L.W., Vegetative and reproductive growth responses of three strawberry cultivars to nitrogen. J. Am. Soc. Hortic. Sci. 106, Fernandez, G.E., Butler, L.M., Louws, F.J., Strawberry growth and development in an annual plasticulture system. HortScience 36, Guttridge, C.G., Further evidence for a growth-promoting and flower inhibiting hormone in strawberry. Ann. Bot. 23, Guttridge, C.G., Fragaria ananassa. In: Halevy, A.H. (Ed.), Handbook of Flowering, vol. 3. CRC Press, Boca Raton, FL, pp Guttridge, C.G., Thompson, P.A., The effects of gibberellins on growth and flowering of Fragaria and Duchesna. J. Exp. Bot. 15, Heide, O.M., Photoperiod and temperature interactions in growth and flowering of strawberry. Physiol. Plant. 40, Hytönen, T., Mouhu, K., Koivu, I., Elomaa, P., Junttila, O., Prohexadionecalcium enhances the cropping potential and yield of strawberry. Eur. J. Hortic. Sci. 73, Hytönen, T., Elomaa, P., Moritz, T., Junttila, O., Gibberellin mediates daylength-controlled differentiation of vegetative meristems in strawberry (Fragaria ananassa Duch.). BMC Plant Biol. 9, 18, doi: / Ito, H., Saito, T., Studies on the flower formation in the strawberry plant. I. Effects of temperature on flower formation. Tohoku J. Agric. Res. 13, Lieten, P., The effect of nutrition prior to and during flower differentiation on phyllody and plant performance of short day strawberry Elsanta. Acta Hortic. 567, Lineberry, R.A., Burkart, L., Collins, E.R., Fertilizer requirements on new land in North Carolina. Proc. Am. Soc. Hortic. Sci. 45, Long, J.H., The use of certain nutrient elements at the time of flower formation in the strawberry. Proc. Am. Soc. Hortic. Sci. 37, May, G., Pritts, M., Strawberry nutrition. Adv. Straw. Prod. 9, Markwell, J., Osterman, J.C., Mitchell, J.L., Calibration of the Minolta SPAD-502 leaf chlorophyll meter. Photosynth. Res. 46, Opstad, N., Sønsteby, A., Flowering and fruit development in strawberry in a field experiment with two fertilizer strategies. Acta Agric. Scand. Section B: Soil Plant Sci. 58, Rademacher, W., Growth retardants: effects on gibberellin biosynthesis and other metabolic pathways. Annu. Rev. Plant Physiol. Mol. Biol. 51, Rünger, W., Über den Einfluss der Stickstoffernährung und der Temperatur während Langtag- und Kurztagperioden auf die Blütenbildung von Kalanchoe blossfeldiana. Planta 56, Sønsteby, A., Heide, O.M., Temperature responses, flowering and fruit yield of the June-bearing strawberry cultivars Florence, Frida and Korona. Sci. Hortic. 119, Tagliavini, M.E., Baldi, E., Lucchi, P., Antonelli, M., Sorrenti, G., Baruzzi, G., Faedi, W., Dynamics of nutrient uptake by strawberry plants (Fragaria ananassa Duch.) grown in soil and soilless culture. Eur. J. Agron. 23, Taylor, D.R., The physiology of flowering in strawberry. Acta Hortic. 567, Taylor, D.R., Blake, P.S., Crisp, C.M., Identification of gibberellins in leaf exudates of strawberry (Fragaria ananassa, Duch.). Plant Growth Regul. 30, Thompson, P.A., Guttridge, C.G., Effects of gibberellic acid on the initiation of flowers and runners in strawberry. Nature 184, Verheul, M.J., Sønsteby, A., Grimstad, S.O., Interaction of photoperiod, temperature, duration of short-day treatment and plant age on flowering of Fragaria ananassa Duch. cv. Korona. Sci. Hortic. 107, Verheul, M.J., Sønsteby, A., Grimstad, S.O., Influence of day and night temperatures on flowering of Fragaria ananassa Duch. cvs. Korona and Elsanta. Sci. Hortic. 112, Wagstaffe, A., Battey, H.B., Tunnel production of strawberry in the UK: a review. In: Takeda, F., Handley, D.T., Poling, E.B. (Eds.), Proc. North Am. Straw. Symp., CAL, pp Way, D.W., White, G.C., The influence of vigour and nitrogen status on the fruitfulness of Talisman strawberry plants. J. Hortic. Sci. 43, Whitehouse, W.G., Nutritional studies with the strawberry. Proc. Am. Soc. Hortic. Sci. 25, Wood, C.W., Reeves, D.W., Himelrick, D.L., Relationship between chlorophyll meter readings and leaf chlorophyll concentration, N status and crop yield: a review. Proc. Agron. Soc. New Zeal. 23, 1 9.