Interaction of short day and timing of nitrogen fertilization on growth and flowering of ‘Korona’ strawberry (Fragaria × ananassa Duch.)

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.