Light, Rhythms, and the Problem of Florigen

Abstract

Photoperiodic induction in plants occurs when a change in state of phytochrome or other pigment is synchronized with circadian rhythmic events, leading to the export of a yet uncharacterized mobile stimulus (florigen) from leaf to bud. To analyze the mechanisms involved in this response, we have studied simple systems displaying phytochrome control, phytochrome-rhythm interaction, and the generation of mobile stimuli.

In the leguminous plants Albizzia and Samanea, rhythmic and photoinduced changes in leaflet orientation are caused by alternating net influx and efflux of K⁺ and Cl^- ions into and out of pulvinal motor cells. Influx is characterized by a Q₁₀ > 1 and efflux by a Q₁₀ < 1 in the physiologic temperature range. Together with data from inhibitors and promotors, this implies the simultaneous operation within any one cell of two competing processes, whose alternating predominance leads to cycling. Each pulvinus acts as a unit, unaffected by its neighbors. Excised pulvini continue to oscillate if supplied with far-red phytochrome (P_fr) and sucrose, phytochrome conversion serving to set the clock.

In various plant systems, circadian time or the state of phytochrome can control electrical potential, the degree of electrical coupling, the transport of solutes, and photomorphogenesis. These facts, together with the growing evidence that phytochrome and hormones act on cellular membranes, lead to the postulate that the mobile stimulus connecting leaf and bud controls cellular transport phenomena.