Promotion of flowering in apple trees with gibberellin A4 and C-3 epi-gibberellin A 4.

The proportion of spurs flowering on apple trees (Malus domestica Borkh. cv Golden Delicious) displaying a high degree of alternate-year flowering was increased in the "off" year by gibberellin A4 (GA4) and C-3 epi-GA4 applied in the previous year. When applied 4.5 weeks after anthesis amounts of GA4 ranging from 3 to 300 μg per spur and 25 or 50 μg of C-3 epi-GA4 per spur were effective. Treatments with GA4 made seven weeks after anthesis were less effective. A combination of 30 μg GA4 and 30 μg zeatin (6-(4-hydroxy-3-methylbut-trans-2-enylamino)purine) promoted flowering at both treatment times, and tended to be more effective than GA4 alone.