Induction of phototropism by multiple pulses of unilateral light

Abstract

The effects of four sequential pulses of unilateral monochromatic blue light on phototropism were studied using darkgrown buckwheat ( Fagopyrum esculentum) hypocotyls and a variety of fluence rates, pulse lengths and dark interval durations. Pulse stimuli can induce substantial curvature (>70°), obey the law of reciprocity, and are more effective than continuous irradiation. The optimal pulse length was inconsistently variable, while the optimal dark interval duration increases from 15 min to 45 min as the light pulse becomes more optimal. The magnitude of curvature is dependent on the length of the stimulus period, defined as the temporal sum of the light pulses and dark intervals. The complexity of the fluence-response relationships observed using long, continuous exposures is reduced; in addition to a fluence-independent region, only a single bell-shaped curve is generated by pulse stimuli. These data extend those reported for Arabidopsis (B. Steinitz and K. Poff, Planta, 168 [1986] 305) and support the proposal that a single mechanism, consisting of both light-dependent and light-independent events, exists for positive phototropism, at least of dicots.