Comparison of Fluence-Response Relationships of Phototropism in Light- and Dark-Grown Buckwheat

Abstract

Fluence-response relationships of phototropism in light-and darkgrown buckwheat (Fagopyrmn escukntum Moench.) were compared using systematically varied fluence rates and irradiation times of unilateral monochromatic blue light. Etiolated seedlings respond to most fluence rates in a tri-phasic manner. Phase one differs from classic first positive in that reciprocity is not observed and the peak occurs at a wide variety of fluences, often orders of magnitude less than those characteristic of first positive. Light-grown plants display this pattern only when stimulated by low fluence rates. Phase three is an ascending arm directly related to irradiance time and is comparable to classic second positive. Phase two is a nearly indifferent zone separating phases one and three. At the lowest fluence rates, the maximal observed curvature is greater for dark-grown than for light-grown plants and the former curve more in response to short (2-second) exposures than do the latter. At the highest fluence rates, the maximal observed curvature is much greater for lightgrown than for dark-grown seedlings, particularly at irradiation times of 2 to 3 minutes or more. Tropic curvatures correlate positively with increasing fluence rate up to some inflection range, above which the relationship becomes negative. This inflection range is approximately two orders of magnitude higher for light-grown plants.

Phototropism: Bending towards Enlightenment

“…In contrast with etiolated seedlings, Pringsheim (1912) observed that light-grown pea seedlings are more responsive to brighter light than to dimmer light. Similar behavior was observed in buckwheat (Ellis, 1987) and Arabidopsis (Whippo and Hangarter, 2005). However, how this developmental shift in phototropic sensitivity occurs is currently unknown.…”

“…However, von Wiesner also believed that the induction of phototropism was closely related to the induction of photomorphogenesis and concluded that the regulation of phototropic sensitivity and responsiveness coincides with light-mediated growth inhibition (von Wiesner, 1878). During the next century, Pringsheim (1912) and Ellis (1987) reconfirmed von Wiesner's initial observation, but a clear explanation of the phenomenon was not evident.…”

Growth Retardant-Induced Changes in Phototropic Reaction of Vigna radiata Seedlings

“…It is well known that a light pretreatment causes a shift in the fluence-response relationship (1,7,11,19) in the range of first positive curvature. The effect of ancymidol on the fluence-response curve is similar to that ofthe light pretreatment, a result that indicates that it might affect the photoreceptor system.…”

Photosensory Adaptation in Aneural Organisms*

“…While this holds true for example for Phycomyces, which has a lower threshold for the bending rate after dark-growth than after light-growth (Galland and Russo, 1984b), the situation can be different in higher plants. The complex fluence-response curves for phototropism of buckwheat (Fagopyrum esculentum) vary considerably with preceding darkor light-growth (Ellis, 1987). In the low-fluence range dark-grown specimens have a lower threshold and react more strongly than light-grown ones.…”

“…In the low-fluence range dark-grown specimens have a lower threshold and react more strongly than light-grown ones. In the high-fluence range, on the other hand, lightgrown seedlings are more sensitive (Ellis, 1984(Ellis, , 1987. The different growth conditions not only influence the threshold (sensitivity) but also the adaptation kinetics.…”