Photogravitropic equilibrium in Avena coleoptiles: Fluence rate-response relationships and dependence on dark adaptation and clinostating

Phototropism of Avena coleoptiles was measured in response to blue-light irradiation lasting between 2 and 24h. During this time the coleoptiles established a bending angle of photogravitropic eauilibrium that was dependent on the time of irradiation and also on the pretreatment in light or darkness prior to stimulation. The absolute thresh-old for the photogravitropic eauilibrium in response to blue light was 10E-8 micro molmE-2sE-1. Photon fluence rate-response curves, which were generated after several hours of dark adaptation, had a characteristic shape with a prominent optimum in the middle of the dynamic range. Curves which were generated without prior dark adaptation displayed no such optimum. Clinostating dark-adapted coleoptiles caused an increase of sensitivity and responsiveness during a 2-h period of unilateral irradiation. The advantages and the drawbacks of long-term irradiation experiments for the investigation of phototropism and the generation of action spectra are discussed.