Evaluation of the Present Information on the Mechanisms Leading to Flower Bud Induction, Evocation and Differentiation in Olea europaea

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 on 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 on 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 on the tree have a major effect on 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 on shoot growth. The lack of shoot growth on 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 Proc. VI IS on Olive Growing Eds.: E.M. Sampaio and A.C. Pinheiro Acta Hort. 949, ISHS 2012 94 production year, in relation to shoot growth, new lateral bud development, mineral nutrient fluctuations and some biochemical compounds levels and their influence on the following crop formation. MATERIAL AND METHODS The study was conducted over a period of three years (two “on” and one “off”) on ten 20-year-old ‘Manzanillo’ olive trees, growing in the “Aljarafe” zone (Seville, Spain). The number of inflorescences/infruitescences, flowers and later fruits, fruit weight, terminal growth of the reproductive shoots and. the total fruit yield at harvest was recorded on a monthly basis. Samples of leaves and buds were taken for analysis from each tree at the same monthly intervals. The leaves were analyzed for their mineral nutrients (N, P, K, Ca, Mg, Fe, Mn and B) as described elsewhere (García et al., 1999). The buds were fixed in FAA and prepared for observation under an optical microscope according to Troncoso (1967). The general situation of the trees was described at each sampling date. RESULTS AND DISCUSSION Plant Situation after a Heavy Crop Year In a high “on” year almost all the buds on the shoots grown in the previous year developed into inflorescences, except for those on the very short terminal parts of the branches which usually remain vegetative. As our study was carried out on ‘Manzanillo’, a table olive cultivar, the plants were already without fruits in the fall. Still, all the buds of recent formation except for the apical one, were poorly developed. Depending on the time of harvest, in an “on” year the olive tree enters the winter chilling period (decisive period for bud differentiation) with or without fruits. In both cases, there was a negative influence for the crop of the following year. The productive branches of the present year which comprise the majority of a productive tree have no visible auxiliary buds left after the heavy cropping on the shoots. Only on the apical part of the branch where some limited growth might occur even during the period of fruit development viable buds can be found. This growth however, consisted of short (9 cm long) semi herbaceous twigs with still developing young leaves and auxiliary buds and an apical bud responsible of the twig elongation. The leaves at the basal and medium zones of the twigs now two years old, are showing symptoms of ageing and some are falling. The leaf nutrient contents in the winter of a crop year show very low levels of N, P and K and high accumulation of Ca (Table 1). This results in a nutritive imbalance, particularly in relation to Ca/K and N/K ratios, as compared to the optimum levels defined by González-García et al. (1972). These data according to Mengel and Kirby (1982) indicate low activity, cell senescence and represent a negative factor for the development of a new crop. The crop provokes profound alterations in the metabolism of the tree. GonzálezGarcía et al. (1976) found that the general content of phenolic acids at the end of an “on” year was low and linked to low IAA oxidase activity and a marked drop of tryptophane, converted into IAA. This situation is reversed in the winter of “off” years. They concluded that the less enzyme activity results in high levels of IAA which is involved in the initiation of vegetative growth of the trees (González-García et al., 1976; Catalina et al., 1978; González-García and Catalina, 1982). Lavee and Avidan (1994) found low levels of a particular phenolic acid, CHA (chlorogenic acid), after a non-productive season in contrast with a high content of CHA after a high crop year. They also showed that CHA inhibits bud differentiation when injected in the tree, concluding that CHA is associated to alternate bearing. Carbohydrate metabolism is also affected by the crop and at fall the leaf contents of starch and sugars have been depleted as a consequence of the translocation of carbohydrates for the fruit growth (Sarmiento et al., 1976). This however was not confirmed by Stutte and Martin (1986) who did not find a relation between the