Physiological Signals That Induce Flowering.

Abstract

The timing of the transition from vegetative growth to flowering is of paramount importance in agriculture, horticulture, and plant breeding because flowering is the first step of sexual reproduction. Studies to understand how this transition is controlled have occupied countless physiologists during the past half century and have produced an almost unmanageably large amount of information (Bernier et al., 1981a; Halevy, 1985-1989; Bernier, 1988; Kinet, 1993). A majority of plants use environmental cues to regulate the transition to flowering because all individuals of a species must flower synchronously for successful outcrossing and because all species must complete their sexual reproduction under favorable externa1 conditions. Any environmental variables exhibiting regular seasonal changes are potential factors that control the transition to flowering. The major factors are photoperiod, temperature, and water availability. Plants that do not require a particular photoperiod or temperature to flower, i.e., the so-called “autonomous-flowering” plants, are usually sensitive to irradiance. The environmental factors are perceived by different parts of the plant. Photoperiod and irradiance are perceived mainly by mature leaves in intact plants. Temperature is perceived by all plant parts, although low temperature (vernalization) is often perceived mainly by the shoot apex. Water availability is perceived by the root system. There are strong interactions between these different factors, so that each factor can change the threshold value for the effectiveness of the others. Plants, as opportunists, will thus make use of a different critical factor in different environments. Melilotus officinalis, for example, is a biennial with a vernalization requirement in temperate zones and an annual long-day (LD) plant with no cold requirement in arctic regions. In photoperiodic species, such as the short-day (SD) plant Pharbitis nil and the LD plant Silene armeria, flowering in unfavorable photoperiods can be caused by changing temperature, irradiance, or nutrition or by removing the roots. Similarly, in some late-flowering mutants of Arabidopsis, vernalization and an increase in the proportion of far-red light in the light source can substitute for one another in promoting the transition to flowering (Martinez-Zapater and Somerville, 1990; Bagnall, 1992). Clearly, there are alternate pathways to flowering in most, if