Prohexadione-Calcium Enhances the Cropping Potential and Yield of Strawberry

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1 Europ.J.Hort.Sci., 73 (5). S , 2008, ISSN Verlag Eugen Ulmer KG, Stuttgart Prohexadione-Calcium Enhances the Cropping Potential and Yield of Strawberry T. Hytönen 1,2), K. Mouhu 1,3), I. Koivu 1) and O. Junttila 4) ( 1) Department of Applied Biology, University of Helsinki, Finland, 2) Viikki Graduate School in Biosciences, University of Helsinki, Finland, 3) Finnish Graduate School in Plant Biology, University of Helsinki, Finland and 4) Faculty of Science, Department of Biology, University of Tromsø, Tromsø, Norway) Summary The yield of strawberry (Fragaria x ananassa Duch.) is dependent on the number of inflorescences initiated in the apical meristems of the main and branch crowns. Short-day conditions induce crown formation from the axillary buds of the main crown while long-day conditions promote runner formation. The fate of axillary buds is also affected by the plant hormone gibberellin (GA). Exogenously applied GA enhances runner formation in short daylength. Our aim was to test whether the cropping potential of strawberry could be increased by the GA biosynthesis inhibitor, prohexadione-calcium (ProCa), under northern long-day conditions. Six experiments at five different locations were carried out with Polka or Honeoye using ProCa concentrations of 100 and 200 mg l 1. ProCa reduced runner formation in five experiments, and increased crown branching in three experiments. Furthermore, the 200 mg l 1 ProCa treatment increased the number of inflorescences and berry yield by 6 50 % and 7 53 %, respectively. No negative effects on development or yield were observed. In conclusion, ProCa treatment of strawberry during the planting year has the potential to reduce runner development and enhance berry yield in northern long-day conditions. Key words. axillary bud crown flowering Fragaria gibberellin runner Introduction Strawberry (Fragaria x ananassa Duch.) is the most important small soft fruit in the Nordic countries. In Finland, as in many other European countries, mainly short-day cultivars of strawberry are grown (TANSKA 2007). Daylength strongly determines the growth of these cultivars (HEIDE 1977; SØNSTEBY and NES 1998; KON- SIN et al. 2001; HYTÖNEN et al. 2004); in long days, the axillary buds differentiate to runners, and in short days to branch crowns. Adequate crown branching is a prerequisite for satisfactory flowering, because the inflorescences are formed from the apical meristems of the crown in short day conditions (GUTTRIDGE 1985; HYTÖNEN et al. 2004). Exposure of strawberry plants to continuous short days and low temperature induces a rest period that is broken by adequate chilling (GUTTRIDGE 1985). The plant hormone gibberellin (GA) has been shown to affect axillary bud differentiation and flowering in strawberry. The application of active GA enhances runner formation and prevents flowering (THOMPSON and GUT- TRIDGE 1959; PAROUSSI et al. 2002). In contrast, the GA biosynthesis inhibitors paclobutrazol (NISHIZAWA 1993) and AMO-1618 (AVIGDORI-AVIDOV et al. 1977) both increase crown branching at the expense of runner production. Since the increased number of branch crowns has been shown to correlate positively with the number of inflorescences in strawberry (HYTÖNEN et al. 2004), chemical control of GA biosynthesis could be used to enhance cropping potential. The above-mentioned chemicals have long half-lives, making them unsuitable for strawberry cultivation (REEKIE and HICKLENTON 2002). In contrast, the half-life of prohexadione-calcium (ProCa) is short (EVANS et al. 1999). ProCa prevents the formation of the biologically active GA 1 from the biologically in-active GA 20 (NAKAYAMA et al. 1992; RADEMACHER 2000). ProCa has been shown to inhibit shoot elongation for example in apple and pear that are members of the Rosaceae (RADEMACHER et al. 2004). In these species, ProCa may have slight positive effect on generative growth (RADEMACHER et al. 2004; SUGAR et al. 2004; GLENN and MILLER 2005). ProCa has been shown to inhibit growth of petioles, to decrease runner formation, and to enhance branch crown formation in the Californian strawberry cvs. Chandler, Sweet Charlie and Camarosa (REEKIE and HICKLENTON 2002; BLACK 2004; REEKIE et al. 2005a, 2005b). In addition, REEKIE et al. (2003) showed that Pro- Ca treatment given in the nursery field improved the establishment of strawberry plants in the production field, with positive effects on crop earliness and marketable berry yield. However, BLACK (2004) reported that although autumn application of ProCa reduced runner growth and increased branch crown formation in Chandler, it did not affect the number of inflorescences during the following year.

2 Hytönen et al.: Prohexadione-Calcium Enhances the Cropping Potential of Strawberry 211 At high latitudes, the growing season is characterized by long days resulting in vigorous runner growth, inhibition of inflorescence development and hence, low yields (S. Karhu, unpublished). In this study, we show that Pro- Ca can be used to reduce runner production, and to enhance both branch crown formation and berry yield under northern long-day conditions. Materials and Methods Plant material and experimental fields The effect of ProCa on the vegetative and generative growth of strawberry was studied in six field experiments located at the University of Helsinki, at the Gardenknow-howcentre in Suonenjoki and in three commercial strawberry fields in Porvoo, Tuusula and Suonenjoki. Experimental locations, plant material, time of planting and treatment times are shown in Table 1. In all experiments, plants were planted in raised beds covered with plastic mulch with drip irrigation placed under the mulch. The plants were fertilized with soluble fertilizer for strawberries (Mansikan puutarhalannos 7N 4P 27K, Kemira GrowHow Oyj, Helsinki, Finland) and soluble calcium nitrate (15.5N 0P 0K 16Ca, Kemira GrowHow Oyj) when needed. Treatments Two series of experiments were conducted. In 2004 (Exps. 1 3) the plants were sprayed individually to drip-off with 0, 100, and 200 mg l 1 of ProCa (Bas W, BASF, Ludwigshafen, Germany) diluted in tap water with a few drops of Tween20 (Table 1). In 2006 (Exps. 4 6), 1 kg ha 1 of the commercial compound Regalis (BASF), containing 10 % (w/w) of ProCa, in 500 l of water (200 mg l 1 ProCa), was sprayed onto the strawberry rows. Observations, measurements and analyses The effect of ProCa on vegetative growth was quantified in Exp. 1 by measuring petiole lengths in The petiole of one emerging leaf per plant was marked at the time of the treatment, and 2 and 4 weeks after the treatment in The following year, the first emerging leaf (2 May) and one emerging 4 weeks later (2 June) were tagged on each plant. The lengths of petioles were measured when the leaves were fully expanded. The numbers of runners and branch crowns were recorded in all experiments at the end of growing season of the treatment year. In Exp. 1, the runners were also removed and weighed. In Exp. 1, samples of 15 to 18 plants per treatment (control and 100 mg l 1 ProCa) were taken for measurements of fresh and dry weight, and levels of glucose, sucrose, starch, and mineral nutrients (Mg, Al, Si, P, S, Cl, K, Ca, Ti, Mn, Fe, Cu, Zn, Sr, N, B). Plants were dug up three weeks after the ProCa treatment, and divided into leaves, runners, branch crowns, and roots. Plant parts of 5 to 6 plants were pooled and samples of g were dried at 65 ºC for 42 h in a forced-air oven (Memmert GmbH+Co ULM 800, Schwabach, Germany) for dry weight and carbohydrate measurements. For nutrient measurements, samples of 2 4 g were freeze-dried (Heto Drywinner FD 8, Allerød, Denmark) for 48 h. Sucrose and glucose contents were measured using a Sucrose/Glucose Assay Procedure Kit (Megazyme, Wicklow, Ireland) and starch content using a Total Starch Assay Procedure Kit (Amyloglucosidase/α-amylase Method, Megazyme). For the measurement of mineral nutrient contents, the samples were applied to capsules with Spectrolene six film (FSL06-44 Spectrolene TM Micron Film, VHG LABS, Manchester, USA) and analyzed by X-ray Diffractometer (ARL ADVANT XP, Thermo Fisher Scientific, Waltham, USA). Flowering and/or yield accumulation was observed in all experiments in the season following the treatments. The number of inflorescences with the first flower opened was counted weekly until the start of the harvest in Exp. 1, and during full bloom in Exps In addition, the accumulation of marketable berry yield was followed in Exp. 1, and total berry yield was determined in Exps. 2 and 3. Experimental design and statistics Exps. 1 3 were arranged as a randomized complete block design with one three-level factor. Four blocks of ten or 16 plants per treatment plot were planted in Exps. 1 and 3, respectively, whereas in Exp. 2 three blocks consisting of 20 plants per treatment plot were used. The treatment plots were separated with untreated plants between the plots in Exp In 2006, the treatment was applied to separate rows with untreated rows as controls. For the observations, five to six sections of the treated and untreated rows (plots) containing 10 (Exps. 5 and 6) or 20 (Exp. 4) plants were randomly selected for observations. Table 1. Experimental sites, plant material, planting times and treatment times in different experiments. Experiment Experimental site Cultivar Plant type Planting time Time of the treatment Exp. 1 Helsinki, N E Polka Runner plants 15 June July 2004 Exp. 2 Porvoo, N E Polka Runner plants 23 May June 2004 Exp. 3 Suonenjoki, N E Polka Runner plants 25 May June 2004 Exp. 4 Suonenjoki, N E Honeoye Runner plants 21 July August 2006 Exp. 5 Tuusula, N E Honeoye Frigo A+ 10 May July 2006 (ah*) Exp. 6 Tuusula, N E Polka Frigo A+ 10 May July 2006 (ah) *ah = after harvest.

3 212 Hytönen et al.: Prohexadione-Calcium Enhances the Cropping Potential of Strawberry The results were subjected to the GLM procedure of the SAS statistical program package. The pairwise comparisons with control were performed as one-tailed (upper or lower) or two-tailed using the PDIFF option in the LSMEANS statement. The sum curves of open inflorescences and marketable yield in Exp. 1 were analyzed using the PROFILE option in the REPEATED statement. Results The effect of ProCa on strawberry growth during the planting year Petiole length was measured in Exp. 1, and the development of runners and branch crowns was observed in all experiments during the planting year. Both ProCa concentrations (100 and 200 mg l 1 ) affected vegetative growth, but the effect was more pronounced after the stronger dose. ProCa had a transient effect on petiole length: petioles that emerged at the time of the treatment remained shorter in ProCa-treated plants than in the control, but this effect disappeared after 2 weeks (Table 2). In addition, an opposite effect was found in petioles emerging 4 weeks after the treatment, but the differences were not statistically significant. ProCa had a highly variable effect on axillary bud differentiation in both cultivars (Table 3 and 4). A transient reduction in the number and growth of runners was found in all experiments (data not shown). At the end of the growing season of the treatment year, a significant reduction in the number of runners (16 72 %) and an increase in the number of branch crowns ( %) were found in five and three locations, respectively (Table 3 and 4). ProCa had a stronger effect on runner fresh weight than on the number of runners: 100 mg l 1 and 200 mg l 1 ProCa reduced the runner fresh weight by 33 % and 45 %, respectively, whereas the number of runners was reduced by only 9 % and 32 %, respectively (Table 3). The effect of ProCa on carbohydrate and nutrient partitioning In Exp. 1, the effects of 100 mg l 1 ProCa on the levels of carbohydrates and mineral nutrients in different plant organs were studied. Three weeks after the treatment, the fresh weight of runners was reduced by 41 % in Pro- Ca-treated plants compared to the control plants. ProCa did not have significant effects on the dry weight of leaves, branch crowns, or roots per plant or the proportion of dry matter in any plant part at this stage (data not Table 2. The effect of planting year application of ProCa on the length of petioles (mm) in Polka strawberry in Exp. 1 (Helsinki). Young petioles were tagged 0, 2, or 4 weeks from the treatment in 2004 (planting year), and on 2 May and 2 June 2005, and petiole lengths were measured when elongation growth had ceased (leaflets were fully expanded). Values are mean ± SD. ProCa (mg l 1 ) 0 weeks 2 weeks 4 weeks 2 May 2 June 0 87 ± 8 98 ± ± ± ± ± 4 ** x 93 ± 2 NS 137 ± 6 NS 130 ± 6 * 224 ± 21 * ± 3 ** 90 ± 3 NS 135 ± 6 NS 146 ± 10 ** 228 ± 6 * xsignificance of two-tailed pairwise comparisons with control. NS, *, ** Non-significant, significant at P 0.05 or 0.01, respectively. Table 3. The effect of planting year application of ProCa on the number of runners per plant, fresh weight (g) per runner, and the number of branch crowns per plant in Polka strawberry at the end of growing season 2004 in Exps. 1, 2 and 3 (Helsinki, Porvoo and Suonenjoki, respectively). Values are mean ± SD. Location ProCa (mg l 1 ) Runners g per runner Branch crowns Helsinki ± ± ± ± 0.7 * x 11.2 ± 2.3 NS y 5.4 ± 0.7 ** z ± 1.2 *** 9.2 ± 2.3 * 7.4 ± 1.0 *** Porvoo ± ± ± 3.3 NS 6.5 ± 1.3 *** ± 2.8 * 7.6 ± 1.1 *** Suonenjoki ± ± ± 3.0 NS 5.3 ± 0.3 NS ± 4.2 NS 5.7 ± 0.6 NS x, y, z Significance of one-tailed (lower), two-tailed, or one-tailed (upper) pairwise comparisons, respectively, with control. NS, *, **, *** Non-significant, significant at P 0.05, 0.01, or 0.001, respectively.

4 Hytönen et al.: Prohexadione-Calcium Enhances the Cropping Potential of Strawberry 213 Table 4. The effect of ProCa on the number of runners, branch crowns and inflorescences in Exps. 4, 5 and 6 (Tuusula and Suonenjoki) in Regalis, 1 kg ha 1 in 500 l of water (200 mg l 1 ProCa), was sprayed onto the strawberry rows. Control rows remained untreated. Values are mean ± SD. Location Treatment Runners Branch crowns Inflorescences Tuusula, Polka Control 16.0 ± ± ± 0.6 (Exp. 6) ProCa 8.5 ± 0.7 ** x 5.5 ± 0.3 NS 13.2 ± 0.6 ** Tuusula, Honeoye Control 7.6 ± ± ± 0.1 (Exp. 5) ProCa 6.4 ± 0.4 ** 3.2 ± 0.1 NS 5.2 ± 0.3 NS Suonenjoki, Honeoye Control 1.8 ± ± ± 0.1 (Exp. 4) ProCa 0.5 ± 0.1 ** 1.4 ± 0.1 * 1.8 ± 0.1 ** xsignificance of two-tailed pairwise comparisons with control. NS, *, ** Non-significant, significant at P 0.05 or 0.01, respectively. shown). Furthermore, ProCa had no significant effects on the contents of sucrose, glucose, starch, or most of the mineral nutrients in any plant parts calculated on dry weight basis (data not shown). The only exception was potassium content, which decreased from 2.32 % in the control plants to 2.18 % in the plants treated with 100 mg l 1 ProCa. The effect of planting year ProCa treatments on the growth during the following season A possible after-effect of ProCa treatment on petiole elongation was studied in Exp. 1. The young petioles emerging in early spring (2 May) grew longer in the plants treated with ProCa in the previous year than in the control plants (Table 2). The effect was present even in the leaves emerging in June. In general, the overall growth was more vigorous in ProCa treated plants than in the control plants. The numbers of inflorescences were counted weekly in Exp. 1 and during the full bloom in Exps. 4, 5 and 6. In Exp. 1, the first flowers of Polka opened during week 21 both in control plants and in those that received 100 mg l 1 ProCa, whereas in the 200 mg l 1 ProCa treatment the first open flowers were observed a week later. By 27 June, 9.8±0.9, 12.2±2.7 and 14.2±2.9 (mean ± SD) inflorescences per plant were present in the control, 100 mg l 1 ProCa and 200 mg l 1 ProCa treatments, respectively. The emergence of inflorescences continued after that time, but counting was stopped due to the start of harvest. A similar increase in the number of inflorescences was also found in Honeoye in Exp. 4 and in Polka in Exp. 6, but not in Honeoye in Exp. 5 (Table 4). ProCa treatment delayed the start of the harvest by a few days in Exp. 1 (Fig. 1), but not in Exps. 2 and 3 (data not shown). In Exp. 1, the accumulation profile of marketable yield was similar in all treatments, except between 11 and 18 July, when the plants treated with the 200 mg l 1 dose yielded significantly more than those in the other treatments (Fig. 1). The 200 mg l 1 dose gave clearly the best yield and increased the marketable yield per plant by 53 % in Exp. 1 and total berry yield by 23 % in Exp. 2 compared to the control (Fig. 1, Table 5). In contrast, no statistically significant yield increase was found in Exp. 3. In Exp. 1, the average berry weight was also calculated, but no significant differences were observed (data not shown). The berry weight decreased quickly after the start of harvest, but ProCa had no detectable effect on the profile of the change in size (data not shown). Discussion We show here that under northern long-day conditions, a single ProCa treatment in the planting year had two main effects on strawberry; first, a reduction of runner growth, and, secondly, an increased formation of branch crowns with an increase in the flowering and berry yield during the following year. In our study, treatment with 200 mg l 1 of ProCa increased the number of inflorescences or berry yield in four of the six experiments, conducted with Polka or Honeoye. No negative effects were observed. In earlier studies, ProCa application in the nursery field was shown to increase berry yield of strawberry plants established in a winter annual-hill production system (REEKIE et al. 2005a). However, BLACK (2004) reported that planting year applications of ProCa had no effect on the number of inflorescences in Chandler in a cold-climate annual production system, although the number of branch crowns increased during the fall. This variation in the responses may in part be cultivar-dependent, but ob- Fig. 1. The effect of planting year treatment of ProCa on the marketable yield accumulation in Polka strawberry the following year in Exp.1 (Helsinki). Symbols: 0mgl 1 ; ProCa 100 mg l 1 ; ProCa 200 mg l 1

5 214 Hytönen et al.: Prohexadione-Calcium Enhances the Cropping Potential of Strawberry Table 5. The effect of planting year (2004) treatment of ProCa on the berry yield per plant in Polka strawberry during the following year in Exps. 1, 2 and 3 (Helsinki, Porvoo and Suonenjoki). Values are mean ± SD. ProCa Berry yield (g plant 1 ) (mg l 1 ) Helsinki (Exp. 1) Porvoo (Exp. 2) Suonenjoki (Exp. 3) ± ± ± ± NS x ± 35.0 NS ± 31.6 NS ± ** ± 37.2 NS ± 40.1 NS xtwo-tailed pairwise comparisons with control in columns. NS, ** Non-significant, significant at P 0.01, respectively. viously it can also be affected by climatic conditions. In Southern Finland, the daily photoperiod during the summer months is h. Consequently, crown branching and flower initiation occur only during a short, 6- to 8-week-long period in the fall. Under these growing conditions, the potential number of sites for inflorescences to develop in the crown is often a key factor limiting cropping potential. ProCa treatment given during the long-day conditions in the summer enhanced crown branching and increased the number of meristems that could initiate floral differentiation in the fall. ProCa clearly promoted the formation of branch crowns in three experiments. The 100 mg l 1 dose doubled and 200 mg l 1 dose even tripled the number of branch crowns compared to the control plants in one experiment (Exp. 1). BLACK (2004) also reported that branch crown number was increased by treating the plants a week after an autumn planting with 120 and 240 mg l 1 ProCa. Nevertheless, BLACK (2004) and REEKIE (2005) reported that doses lower than 120 mg l 1 had no effect on the number of branch crowns. In contrast, in our studies even a 50 mg l 1 ProCa treatment enhanced crown branching in Polka and Korona under greenhouse conditions (unpublished). The higher cropping potential of ProCa-treated plants correlated with reduced runner growth in all cases. A single 200 mg l 1 ProCa application decreased the number of runners by % by the end of the growing season, but the 100 mg l 1 dose did not effectively suppress runner formation. Similarly, REEKIE and HICKLENTON (2002), and BLACK (2004) reported that a single low dose ProCa treatment (under 100 mg l 1 ) had no significant effect on the number of runners in Chandler, Camarosa and Sweet Charlie, but concentrations of 120 mg l 1 or higher repressed the runner growth in dose-dependent manner. In addition to concentration, proper timing of the treatment is crucial for successful reduction of runner growth. The fact that plants were already vigorously runnering at the time of treatment in the experiments carried out in may partially explain the relatively mild effect of ProCa on runner inhibition in these cases. Clearly, ProCa should be applied before runner growth begins. In the Exps. 5 and 6, the grower used frigo-plants of Polka and Honeoye instead of vegetative runner plants in order to get berry yield during the planting year. Consequently, plants could not be treated before the end of the harvest. During cropping, runnering is normally suppressed, but it rapidly accelerates at the end of the cropping period (HYTÖNEN et al. 2004). In Exps. 5 and 6, Pro- Ca reduced effectively post-harvest runnering in Polka, but little effect was found in Honeoye. The reduced runnering of Polka in the year of treatment clearly correlated with increased flowering during the following year. In Honeoye, the mild effect might be attributed to the lateness of treatment time along with too small dose of Pro- Ca. Only single applications were used in this study. In strawberry, repeated applications lead to an increase in the number of branch crowns and to a decrease in the number of runners compared to single applications, even at low 62.5 mg l 1 doses (BLACK 2004; REEKIE et al. 2005a). In apple and pear as well, repeated treatments were more effective than single treatments in controlling the shoot growth (RADEMACHER et al. 2004; SUGAR et al. 2004; MEDJDOUB et al. 2005). The effectiveness of multiple ProCa treatments for reducing excessive runnering should also be tested in northern long-day conditions. ProCa reduced the growth of young petioles, that were elongating at the time of the treatment, but petioles that started to emerge 2 or 4 weeks after the treatment were no longer significantly affected. This is consistent with earlier observations that the height of strawberry plants returns to the level preceding the treatment about 3 weeks after the treatment (REEKIE and HICKLENTON 2002). The transient growth reduction effect of ProCa and subsequent vigorous growth can be explained by the short half-life of ProCa of about 2 weeks in higher plants (EVANS et al. 1999). Inhibition of 3β-hydroxylation by ProCa leads to a rapid accumulation of GA 20 and GA 19, the immediate precursors of the active GA 1, in strawberry Korona (Hytönen, Moritz, Elomaa and Junttila, unpublished) as in some other plants (RADEMACHER 2000). Due to this accumulation of precursors, a gradual breakdown of ProCa may result in a transient stimulation of the formation of active GA and, consequently, an enhanced elongation growth above the normal level (Table 2). ProCa has been shown to increase dry matter partitioning to the roots from leaves in strawberry (REEKIE et al. 2007) and to increase the content of total non-structural carbohydrates in apple trees, without affecting the partitioning pattern (GUAK et al. 2001). We did not find clear changes in glucose, sucrose, starch and mineral nutrient partitioning in ProCa-treated plants compared to the controls, except the slight reduction of potassium content in the whole plant. However, ProCa reduced the total amount of nutrients and carbohydrates partitioned to the runners, because of reduced runner growth. In conclusion, runner formation and crown branching in strawberry grown under long-day conditions can be

6 Hytönen et al.: Prohexadione-Calcium Enhances the Cropping Potential of Strawberry 215 regulated with a single ProCa treatment resulting in a remarkable increase in flowering and berry yield. The combination of a relatively strong dose with early treatment is crucial for the good control of runners. Finally, ProCa is clearly beneficial in strawberry production, but proper timing, concentration, and number of applications need to be optimized for different cultivars, plant types and climatic conditions. Acknowledgements We thank Dr. Wilhelm Rademacher, BASF Corp., for kindly providing us the growth regulator. References AVIGDORI-AVIDOV, H., E.E. GOLDSCHMIDT and N. KEDAR 1977: Involvement of endogenous gibberellins in the chilling requirements of strawberry. Ann. Bot. 41, BLACK, B.L. 2004: Prohexadione-calcium decreases fall runners and advances branch crowns of Chandler strawberry in a cold-climate annual production system. J. Am. Soc. Hort. Sci. 129, EVANS, J.R., R.R. EVANS, C. REGUSCI and W. RADEMACHER 1999: Mode of action, metabolism, and uptake of BAS 125W, prohexadione-calcium. HortSci. 34, GLENN, D.M. and S.S. MILLER 2005: Effects of Apogee on growth and whole-canopy photosynthesis in Spur Delicious apple trees. HortSci. 40, GUAK, S., D. NEILSEN and N.E. LOONEY 2001: Growth, allocation of N and carbohydrates, and stomatal conductance of greenhouse grown apple treated with prohexadione-ca and gibberellins. J. Hort. Sci. Biotech. 76, GUTTRIDGE, C.G. 1985: Fragaria x ananassa. In: HALEVY, A. H. (ed.): CRC Handbook Of Flowering Volume III. CRC Press, Boca Baton HEIDE, O. 1977: Photoperiod and temperature interactions in growth and flowering of strawberry. Physiol. Plant. 40, HYTÖNEN, T., P. PALONEN, K. MOUHU and O. JUNTTILA 2004: Crown branching and cropping potential in strawberry (Fragaria x ananassa Duch.) can be enhanced by daylength treatments. J. Hort. Sci. Biotech. 79, KONSIN, M., I. VOIPIO and P. PALONEN 2001: Influence of photoperiod and the duration of short-day treatment on vegetative growth and flowering of strawberry. J. Hort. Sci. Biotech. 76, MEDJDOUB, R., J. VAL and V. BLANCO 2005: Inhibition of vegetative growth in red apple cultivars using prohexadione-calcium. J. Hort. Sci. Biotech. 80, NAKAYAMA, I., M. KOBAYASHI, Y. KAMIYA, H. ABE and A. SAKURAI 1992: Effects of a plant-growth regulator, prohexadione-calcium (BX-112), on the endogenous levels of gibberellins in rice. Plant Cell Physiol. 33, NISHIZAWA. T. 1993: The effect of paclobutrazol on growth and yield during first year greenhouse strawberry production. Sci. Hort. 54, PAROUSSI, G., D.G. VOYIATZIS, E. PAROUSSIS and P.D. DROGOUDI 2002: Growth, flowering and yield responses to GA 3 of strawberry grown under different environmental conditions. Sci. Hort. 96, RADEMACHER, W. 2000: Growth Retardants: Effects on gibberellin biosynthesis and other metabolic pathways. Ann. Rev. Plant Physiol. Plant Mol. Biol. 51, RADEMACHER, W., K. VAN SAARLOOS, J.A. GARUZ PORTE, F. RIERA FORCADES, Y. SENECHAL, C. ANDREOTTI, F. SPINELLI, E. SABATINI and G. COSTA 2004: Impact of prohexadione-ca on the vegetative and reproductive performance of apple and pear trees. Eur. J. Hort. Sci. 69, REEKIE, J.Y.C. and P.R. HICKLENTON 2002: Strawberry growth response to prohexadione-calcium. Proc. North Am. Strawb. Conf. 5, REEKIE, J.Y., P.R. HICKLENTON, J. DUVAL, C. CHANDLER and P.C. STRUIK 2003: Manipulating transplant morphology to advance and enhance fruit yield in strawberry. Acta Hort. 626, REEKIE, J.Y., P.R. HICKLENTON, J.R. DUVAL, C.K. CHANDLER and P.C. STRUIK 2005a: Leaf removal and prohexadione-calcium can modify Camarosa strawberry nursery plant morphology for plasticulture fruit production. Can. J. Plant Sci. 85, REEKIE, J.Y., P.R. HICKLENTON and P.C. STRUIK 2005b: Prohexadione-calcium modifies growth and increases photosynthesis in strawberry nursery plants. Can. J. Plant Sci. 85, REEKIE, J.Y., P.C. STRUIK, P.R. HICKLENTON and J.R. DUVAL 2007: Dry matter partitioning in a nursery and a plasticulture fruit field of strawberry cultivars Sweet Charlie and Camarosa as affected by prohexadione-calcium and partial leaf removal. Eur. J. Hort. Sci. 72, SUGAR, D., D.C. ELFVING and E.A. MIELKE 2004: Effects of prohexadione-calcium on fruit size and return bloom in pear. HortSci. 39, SØNSTEBY, A. and A. NES 1998: Short days and temperature effects on growth and flowering in strawberry. J. Hort. Sci. Biotech. 73, TANSKA, T. 2007: Polkan valtakausi jatkuu. Puutarha ja Kauppa 7, 41 (in Finnish). THOMPSON, P.A. and C.G. GUTTRIDGE 1959: Effect of gibberellic acid on the initiation of flowers and runners in the strawberry. Nature 184, Received December 19, 2007 / Accepted July 7, 2008 Addresses of authors: Timo Hytönen (corresponding author), Katriina Mouhu, and Ilpo Koivu, Department of Applied Biology, P. O. Box 27, FIN University of Helsinki, Finland, and Olavi Junttila, University of Tromsø, Faculty of Science, Department of Biology, Dramsveien 201, N-9037 Tromsø, Norway, Timo.hytonen@helsinki.fi.