Physiology of Flowering

Abstract

Much of the current work in flowering physiology has focused on its underlying mechanism. Although some evidence suggests that histones play a role in floral differentiation, other postulated regulatory mechanisms must also be taken into account. Initiation of flowering in photoperiodically sensitive plants is under the control of a stimulus, so-called florigen, generated in the leaves and translocated to the bud. That florigen may function in close association with the differentiation process is inferred from the requirement for active cell division in the bud at the time the stimulus is present. Flowering, nucleic acid synthesis, and cell division are all inhibited by certain nucleic acid antimetabolites. Florigen may have either a positive role, as in the activation of genes previously quiescent, or a negative one, acting against gene repressors. Some experiments point to a regulant function for transmissible floral inhibitors produced in non-induced tissue and acting at the bud. The sequence of reactions in the leaf leading to the biosynthesis and transport of florigen in photoperiodic plants is regulated by the leaf''s ability to detect small differences in the length of the night. One of the keys to this function is the leaf-pigment phytochrome, an enzyme that exists in 2 forms each convertible to the other by light in the red and far-red spectral regions corresponding to their respective absorption maxima. The far-red absorbing form controls the flowering stimulus and reverts slowly in the dark to the inactive form. The precision timing feature appears to result from the interaction and meshing of this dark-decay process with florigenic reactions which are coupled with the persistent endogenous circadian rhythms that comprise the ubiquitous biological clock, the nature of which remains obscure. Nutrients and plant hormones have been shown to play important but essentially secondary roles in the floral initiation process. The florigenic reactions are being studied by probing different stages of the inductive process with biospecific reagents, but as yet no single biochemical reaction or intermediate has been identified. The forward progress in flowering physiology over the past half-dozen years has opened new prospects for eventual understanding of the complex floral initiation processes at the cellular and biochemical levels. There is a bibliography of 171 references.