Timing of GA4/7 application and the flowering of Pinus sylvestris grafts in the greenhouse

Effects of varying the time of stem injection of gibberellin A4/7 (GA4/7) on male and female flowering in potted grafts of Pinus sylvestris L. in a greenhouse were monitored in two consecutive years. Flowering results for each application time were correlated with phenological characteristics and abiotic factors in an attempt to find reproducible indicators of the most effective time of GA4/7 treatment. The GA4/7 treatment had a significant effect on female flowering, but no effect on male flowering. In both years, the largest effect on female flowering was obtained with the latest application. Percent shoot and needle elongation (relative to final length) were poor indicators of the optimal time of GA4/7 application. Three heat sum accumulation models gave equally accurate predictions of the optimal time for GA4/7 application; therefore, the simplest model, i.e., the sum of degree-days, is recommended. The timing window for stimulation of female flowering with GA4/7 had a lower limit of about 500 degree-days (threshold +5 degrees C), and the optimal time of GA4/7 application was after about 700 degree-days.