Alternate bearing of fruit trees is still a serious problem in the cultivation of numerous fruit tree species. In this review the progress in the study of alternate bearing during the past two decades is highlighted and regulatory mechanisms involved are discussed. In studying the alternate bearing phenomenon it is useful to distinguish between the trigger, which induces and initiates the alternation, and the mechanisms operating in the perpetuation of the ‘On/Off’ cyclic condition. Alternate bearing can be triggered in two ways. Alternation can develop gradually, in relation to the annual increase in yield during the first years of fruiting, allowing the co-existence of ‘On’ and ‘Off’ trees during the same year and fairly constant fruit yields. Alternatively, and perhaps more often, a single environmental event, mostly climatic but occasionally pest-related, leads to abrupt, extreme alternation. In such cases the alternate bearing is synchronized for the whole orchard and in most cases for the entire region. Three mechanisms are apparently involved in the maintenance of the alternate bearing condition: a) flowering site limitations; b) hormonal control and c) nutritional control. Following an extreme ‘On’ year with profuse flowering, heavy fruiting, almost no vegetative growth occurs, leading to a shortage of flowering sites which hold the potential for next year’s crop. Evidence indicates that the young fruit somehow interferes with flower bud differentiation, imposing a flowering site limitation. As seedless fruits have little effect on next year’s flowering, whereas seeded fruit are inhibitory, a role for seeds in alternate bearing was hypothesized in several tree crops. Among plant hormones, gibberellins [GAs] have been recognized as inhibitors of flower bud differentiation in several fruit trees. It has been proposed that GAs from apple seeds diffuse to the bourse shoot where they inhibit flowering. Auxins may also be involved in the fruit induced inhibition of flowering. Chlorogenic acid, a phenolic which has some auxins affects, has been suggested to play a role in the alternate bearing of olive. It has been frequently reported that the reproductive effort during the ‘On’ year leads to a serious depletion of carbohydrate (CHO) reserves and mineral nutrients. Carbohydrate starvation involves dramatic changes in gene expression. Although the intensity of flower bud differentiation is not in direct correlation with CHO levels, threshold levels of CHO might be required for flower bud differentiation. Fruit set and abscission are closely dependent upon CHO levels. The three regulatory mechanisms discussed above are not mutually exclusive, and functional links among these mechanisms are likely. From an evolutionary stand point, alternate bearing appears to be related to the masting phenomenon which is widespread among forest trees. In conclusion, regular fruiting probably evolved as a result of human selection and horticultural practices. The domestication of fruit trees and their cultivation eliminated most natural stresses (drought, pests, etc.), which, for sure, resulted in irregular bearing. In a broader evolutionary sense, alternate bearing should perhaps be understood as a phenomenon of homeostasis. The alternating, homeostatic behavior of trees secures their survival as a long living fruiting organism.
Keywords: carbohydrate reserves, mineral nutrients, flower bud differentiation, homeostasis, hormones