The photoperiodic control of plant reproduction

Photoperiodism is important in controlling many aspects of plant development and in coordinating certain responses, such as flowering, with different times of the year. There are three main photoperiodic response types, short-day, long-day and day-neutral. The responses of SDP and LDP have many features in common such as the importance of the dark period (the critical night length), the involvement of phytochrome and the interaction with a circadian oscillator. There are, however, other features that are peculiar to LDP such as the greater influence of the light period and the possible involvement of a blue-light receptor in the photoperiodic responses of long-day requiring Cruciferae. In SDP and LDP the mechanism of perception of the photoperiodic stimulus and the production of the transmissible signal(s) in the leaf is the same as has been demonstrated by grafting experiments. There is evidence for both an inducing signal and an inhibitory signal, the natures of which are not yet known. These signals appear to be fairly universal in plants and affect a wide variety of processes. Evidence is accumulating for the involvement of gibberellins in photoperiodic responses and that phytochrome B may be involved in regulating the level of, or sensitivity to, certain types of gibberellins. Finally, the isolation of genes from flowering-time mutants is going to significantly advance our knowledge about the mechanism of the response to photoperiod, and is going to enable the manipulation of plant photoperiodic responses for agricultural and economic benefit