Control over the time of flowering is essential for the survival of flowering plants (angiosperms). Insect pollinators may be present only at certain times, unless an insect-pollinated plant is flowering at that time, pollination and the production of the next generation cannot occur. Embryo and seed development may be successful only under certain climatic conditions. The ability to respond to environmental cues is an essential factor in the regulation of flowering.
While the basic biochemical sequence of events may be common to all angiosperms, the specific regulatory steps vary greatly among species. A floral promoter is produced by the leaves and is transported to the shoot apex, which results in the initiation and, ultimately, the production of flowers. To analyze control points in this sequence, it is helpful to focus separately on environmental signals such as temperature and photoperiod and the way organs perceive and respond to these signals.
The regulation of flowering requires interactions between the shoot apex and other organs and thus depends heavily on chemical signals. There is strong evidence for the existence of a floral promoter called florigen, which may be produced in the leaves. The existence of florigen was first proposed by M. Kh. Chailakhyan, a Soviet plant physiologist, in 1937. Florigen was believed to be produced in leaves, because if leaves were removed before the photoperiod was right for flowering (a process called photoinduction), no flowering occurred. Later work by Anton Lang showed that the plant hormone gibberellincould induce flowering in certain plants, even without appropriate photo-induction. This prompted Chailakhyan to consider the possibility that florigen was actually composed of two different substances, gibberellin and a new substance he called anthesin.
In the late 1970’s Lang, Chailakhyan, and I. A. Frolova, working with tobacco plants, discovered that there was also a floral inhibitor they called antiflorigen. Later, several genes controlling the production of an inhibitor in pea cotyledons and leaves were identified in other laboratories. In addition to leaf-derived inhibitors, root-derived inhibitors have been shown to regulate flowering in black currant and tobacco plants. Aside from the clear role of gibberellin in flowering, none of the other promoters and inhibitors has been identified. Nutrient levels and allocation throughout the plant may also control the time of flowering.
One major role of environmental signals is to control the timing of the production of florigen and antiflorigen.