GENETICS OF LENGTH OF DORMANCY PERIOD IN MALUS VEGETATIVE BUDS

In a study of chilling requirement in Malus, broad-sense heritability estimates for the length of vegetative bud dormancy in 43 clones growing under simulated subtropical winter conditions were 0.76 +/- 0.04 in 1986 and 0.81 +/- 0.04 in 1987. Narrow-sense heritability estimates were 0.66 +/- 0.13 in 1986 and 0.69 +/- 0.13 in 1987. Seedlings with low chilling requirements (CR) were not observed in crosses where both parents had high bud-chilling requirements. 'Koningszuur' did not transmit its long CR to its seedlings. Open-pollinated (OP) seedling populations from the Malus x domestica Borkh. cultivars Anna, Dorsett Golden, Ein Shemer, Khashabi, Winter Banana, and Zabaoani, and the species and interspecific hybrids M. baccata L. DE#98, M. brevipes Rehd., M. xrobusta (Carr.) Rehd. DE#485, M. xrobusta No. 5 ('R5'), M. rockii, M. turesi Rehd. PI 34143, and 'Rosedale' had at least 5% of their descendants in the lower CR classes. In all but one instance, 50% or more of 'Anna' descendants had low CR. Many of these seedlings were within a few classes of the extreme low CR. It is postulated that the low-CR character present in 'Anna' is controlled by at least one major dominant gene and that minor genes interact to modulate its effects. Very low-CR cultivars have a shallow bud dormancy. This highly heritable component for low bud CR is related to a failure to develop a deep dormancy state, rather than to acceleration of the termination of the dormancy process.