Floral induction in mature, perennial angiosperm fruit trees: Similarities and discrepancies with annual/biennial plants and the involvement of plant hormones

Abstract

Floral induction (FI) in perennial fruit trees is distinct from that of annual/biennial (a/b) plants because it is a quantitative process with a significant proportion of the above-ground meristems remaining vegetative, while in a/b-plants all the meristems are induced at once, which terminates the life of the plant. Currently, regulatory mechanisms of floral induction in perennial angiosperm trees are inadequately described, particularly with respect to physiological/molecular mechanisms that prevent the partial or total inaccessibility of genes or meristems for reacting to particular floral promoters, one of the most important prerequisites of perenniality. Epigenetic modifications, which have been studied almost exclusively in a/b-plants and not in perennial trees, are a likely possibility for achieving this “perenniality” of trees and are therefore described in some detail in the following review. Due to the lack of research on this phenomenon, the description of this paragraph is largely speculative. In trees, these epigenetic modifications as well as other regulatory events require endogenous long-distance signals (LDS) due to their physical size and the much larger distances that often separate receptive buds from the origin of the LDS participating in FI. Plant hormones are the most likely candidates as LDS, and due to their general significance in plant physiology, they are given particular attention in this review. Gibberellins and the polar transport of indoleacetic acid are presented as possible inhibitors, and cytokinins are examined as being possible promoters of floral induction in angiosperm trees, whereas hormones that are less likely to be involved in FI are only mentioned briefly. Finally, a method to induce out-of-season flowering for mango and longan is presented. This “out-of-season” technology provides an exceptional possibility to study hormonal changes that occur after, e.g., natural low temperature vs. artificial, e.g., paclobutrazol- or potassium chlorate-induced FI.