Insights from a recent alternate bearing (AB) review: a new conceptual model

Two major unresolved alternate bearing (AB) issues were highlighted in a recent review; the difficulties in elucidation of genetic control and the source of differences between cultivars. Apple (Malus × domestica Borkh.) has been and still is the fruit tree system for studies of these interrelated problems. Resource depletion has been assumed to play a major role in regulation of masting and alternate bearing, leading to the development of resource budget models (RBM). However, as of recently, there is a growing conviction that extreme resource depletion is rather rare and cannot be considered as a general masting and alternate bearing mechanism. Examination of the interplay between vegetative and reproductive growth in fruit trees, as modified by plant growth regulators, indicated that it might be responsible for alternate bearing fluctuations. A conceptual model based on the ‘REP/VEG ratio’ of fruit tree units has been proposed as a general alternate bearing mechanism in fruit trees. Natural processes as well as agrotechnical practices are modeled. The REP/VEG ratio model is confined to fruit trees’ alternate bearing and does not involve broader evolutionary considerations.