Flower-bud formation in apple under various day and night temperature-regimes

Starting at full bloom, 4 temperature treatments were applied to 3-year-old ‘Golden Delicious’ and ‘Cox's Orange Pippin’ trees. Either 17 or 24° C were applied in 3 successive periods of 5–6 weeks each.In ‘Golden Delicious’, exposure to 24° C during the first 5 weeks after full bloom enhanced shoot growth and reduced flower-bud formation in spur buds. The difference in temperature-regime during the third period did not affect either growth or flowering. Almost all apical shoot buds became floral, irrespective of treatment.‘Cox's Orange Pippin’ trees maintained at 24° C throughout grew more vigorously than did those kept at 17° C continuously, but flowering-abundancy was the same. Lowering of the temperature in the last period before harvest did not influence shoot growth, but markedly reduced flowering of both spur buds and apical shoot buds.In a second experiment, a night temperature of either 20 or 10° C was applied in 2 successive periods to 3-year-old ‘Cox's Orange Pippin’ trees kept at a day temperature of 20° C throughout. Lowering of the night temperature in the middle of the season reduced flower-bud production, but there was no difference in growth vigour compared with 20° C continuously.It is postulated that temperature affects flowering in two opposite ways, whose relative importance determines the net result.