Kinetics and Fluence-Response Relationships of Phototropism in the Dicot Fagopyrum esculentum Moench. (Buckwheat)

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Abstract
Phototropism of hypocotyls of light-grown seedlings of buckwheat was studied using narrow-spectrum blue light. Under continuous stimulation at high fluence rates plants curve 90° at a rate of 1.5° min-1, and selected individuals exceed 2° min-1. There is a lag period of approximately 4 min between the onset of unilateral irradiation and the initiation of curvature. The magnitude of curvature at any fluence rate is directly related to the duration of the stimulus and roughly proportional to fluence rate up to 2 × 10-1 μmol m-2 s-1 at non-saturating irradiance times. Buckwheat is sensitive to classic first-positive fluence levels. However, beyond that region, increases in irradiance time or fluence rate cause increased curvature; therefore, there is no pattern comparable to the negative or indifferent response seen in other plants. Reciprocity is valid only at low fluence levels generated by relatively high fluence rates for short periods. Curvature follows Weber-Fechner law predictions only at relatively long irradiance periods. Curvatures of 90° are attained in the lowest fluence rate used (3 × 10-4 μmol m-2 s-1) given an exposure time of 12,000 s.