Phototropism and geotropism in hypocotyls of cress (Lepidium sativum L.)

Abstract. The phototropic and the geotropic sensitivities of cress hypocotyls differed in etiolated and in green seedlings. In etiolated seedlings, phototropism was poorly developed and geotropism seemed to be the dominant orientation mechanism. In green seedlings, although geotropic sensitivity was slightly greater, phototropic responses were markedly enhanced, such that photo-signals could over-ride, or even reverse, geocurvature. The 'de-etiolation' light pretreatments required to bring about these changes in the photo-and geo-systems were different. The slight enhancement of geocurvature could be achieved by short-term exposure to red light and was reversed by far-red irradiation. The marked stimulation of photocurvature required extensive irradiation pretreatments.