Abstract
The coleoptile of dark-grown rice (Oryza sativa L.) seedlings shows regular circumnutation. Light-grown coleoptiles do not show circumnutation, because it is inhibited by light. The inhibition is mediated mainly by phytochrome A and B. Investigation of the effects of gravi-stimulation revealed: (1) the phase of circumnutation is synchronized after gravitropic curvature, (2) the revolutionary direction is reversed, depending on the timing of gravi-stimulation, and (3) circumnutation is abolished by rotation on horizontal clinostats. The coleoptiles treated with a red-light pulse and subsequently submerged grow at a high rate but show no circumnutation. Upon brief gravi-stimulation, these coleoptiles reinitiate circumnutation. The amplitude of the reinitiated circumnutation depends on the intensity of gravi-stimulation. It is concluded that graviperception plays a central role in circumnutation. The coleoptile of the agravitropic rice mutant lazy was found to show no circumnutation. The LAZY gene probably contributes to the graviperception mechanism shared between gravitropism and circumnutation.
