EVALUATION OF THE PRESENT INFORMATION ON THE MECHANISMS LEADING TO FLOWER BUD INDUCTION, EVOCATION AND DIFFERENTIATION IN OLEA EUROPAEA

Abstract

Environmental factors such as suitable light and thermal conditions are essential for the reproductive development of olive buds. However, the tree and its buds have to be in a suitable responsive physiological state in order to respond. Furthermore sufficient well developed buds from the previous season with a good nutritive balance, capable to differentiate are required the tree. In the present paper, we are demonstrating the changes in the nutritional state of the tree developing in ON and OFF years affecting the responsiveness of the buds and level of vegetative growth to the environmental conditions. After a heavy crop a nutrient dilution occurred and vegetative growth is inhibited. Thus, a following growing season is required to regain a good nutrient balance for generating vegetative growth with sufficient buds responsive to environmental differentiation stimuli for the initiation of the next ON year. INTRODUCTION The lack of regularity in reproductive organ development leading in many cases to severe alternate bearing is one of the major economical drawbacks of today's olive industry. Although a considerable amount of work has been devoted during the second half of the last century to this problem, our understanding of the metabolism involved and ability to control alternate bearing is still limited. The potential to induce flower bud formation is governed both by the endogenous metabolic processes and the environment, mainly the thermal conditions. It is presently quite clear that both the late spring and winter temperatures play a significant role in controlling the differentiation of reproductive buds. However, neither the required summer nor the low winter temperature regimes are clearly defined. Even the nature of the responses to temperature is still disputed among the various workers in the field. Laboratory experiments with cuttings indicated that lateral bud can develop vegetatively or reproductively according to the thermal conditions during fall and winter. Thus, the genetic potential to initiate flower bud induction and differentiation developing shoots is environmentally dependent. Reproductive buds were reported to show some changes in their RNA already at the end of the summer, however, these buds will develop reproductively only if suitable winter conditions will prevail. The present available data indicate that neither summer-fall nor mid-winter seasons could individually be responsible for flower bud initiation. The developing fruits the tree have a major effect controlling shoot growth as well as the metabolism leading to reproductive development in the buds. The protein composition in leaves of fruiting and non-fruiting trees differs, with specific ones developing in the leaves of each group (Lavee and Avidan, 1994). Furthermore, a conjugated polyphenol, chlorogenic acid, related to the cinnamic acid pass-way is accumulating in leaves of fruiting trees and specifically inhibits the winter differentiation of the buds (Lavee, 1989). The messenger from the developing fruits to change the leaves' metabolism is believed to be a specific gibberellin. While harvest time is effecting bud differentiation fruit load effects has in addition also a major effect shoot growth. The lack of shoot growth a heavily fruit bearing olive tree is directly related to fruiting alternation as no potential buds for differentiation are developing. This lack of growth occurs although the level of carbohydrates in the leaves was found similar in on and off years. The aim of this work was to study the situation of the olive tree and its fluctuation during and after a high