Gibberellins and flowering in Citrus and other fruit trees: A critical analysis

In spite of the large variation within fruit trees (deciduous vs. evergreen, time of floral induction etc.), gibberellins (GAs) were generally found to strongly inhibit flowering. Similarly, growth retardants were shown in numerous cases to promote flowering. The current status of the knowledge in this area is discussed, with a special emphasis on recent progress in Citrus research. The low temperature induction of flowering in Citrus has been shown to be accompanied by a decrease in endogenous GAs. Research in herbaceous plants has shown that GAs may be differentially involved in flowering and stem elongation. Similarly, in fruit trees not all GAs are inhibitory to flowering and some GAs derivatives have been shown to promote flower formation. Special attention has been paid to the antagonism between flowering and shoot elongation in fruit trees and the role of GAs in this relationship. Whereas in herbaceous angiosperms flowering is closely associated with elongation of the inflorescence axis, in woody angiosperms (fruit trees) flowering is generally associated with reduced stem elongation. It may be proposed that the roles of GA^s in the flowering of different plant groups reflect the relationship between stem elongation and flowering. A model describing the GA - flowering - stem elongation relationship is presented and discussed. During the late fifties it became apparent that gibberellins (GA^s) are capable of inducing flowering in a large' number of plant species, particularly LD and/or cold requiring herbaceous angiosperms. Soon afterwards it was reported that GAs inhibit flowering of many fruit trees. (Table 1). Although almost all early experiments were conducted with GA³, GA^4/7 mixtures also proved to be inhibitory. Citrus was among the first to be investigated in some detail. In their classical paper Monselise and Halevy (1964) sprayed GA ³ during the presumed time of flower induction and differentiation Late autumn-early winter decreasing temperatures are believed to be the environmental factor bringing about floral induction. The main idea of the experiment (Fig. 1) was to ensure the presence of GA throughout the whole period of flower formation, from the induction through flower bud differentiation to early floral development. The effect of GA during this long period could be attributed to one of several factors or their combination. On one hand, the advent of floral induction throughout the bud population of the tree may require several months. On the other hand, GA may be inhibitory at several stages, from evocation to floral development. It was indeed demonstrated by Lord and Eckard (1987) that GA could prevent flower formation as long as sepals have not formed.