Spur versus Whole-tree Regulation of Apple Flowering

Flowering regulation in apple trees is important because biennial bearing is challenging for the apple industry. Apple flowering is generally thought to be influenced by many factors including crop load, seed number and bourse length. However it is unclear how these factors exert their control. In this study we investigated how these different factors contribute to the control of flower induction and whether this is regulated locally in the spur or if it is under the control of the whole tree system. Seven different thinning treatments were applied manually, which varied in the total number of fruit per tree and fruit number per spur. The appropriate number of fruiting spurs was tagged and fruits were removed from the rest of the tree. Fruit weight, seed number, seed weight, bourse number, bourse length, bud diameter and flowering differentiation per spur were recorded. This experiment was conducted in spring 2011, 2012 and 2013, but no data were collected in 2012 due to a severe spring freeze. In 2013, the methods were modified so that individual spurs could be tracked rather than averages and proportions being calculated. In 2011, there were no significant differences in the proportion of flowers on tagged spurs among treatments suggesting that fruiting on the rest of the tree did not influence flowering on tagged spurs. It appeared that the local, within-spur fruiting status had stronger impact. Bearing one or two fruits on the same spur had a similar effect on return bloom. Fruit weight and seed number had a negative linear relationship with flower formation. This experiment will help us to characterize the local versus general regulation of apple flowering and aims to improve our current understanding of apple tree regulation of flowering.