Abstract
Phototropic sensitivity in hypocotyls of dark-grown, 4-d-old radish seedlings (Raphanus sativus L. cv. Miyashige) was investigated with reference to the influence of pre-exposure treatment. The magnitude of the second positive phototropic curvature in response to continuous unilateral irradiation was increased when the seedlings were pre-exposed with white light from above. This effect of the pre-exposure on the phototropic sensitivity increased with duration of the treatment and reached a plateau 3 h after the onset of the treatment. The pre-exposure treatment on the hypocotyls was only efficient to enhance the phototropic sensitivity, but the treatments on the apex and cotyledons have no effect. These results suggest that the site of perception of the pre-exposure light to enhance the phototropic sensitivity could be located within the phototropically responding organ, hypocotyl itself.
