The role of phytochrome in the geotropic behavior of roots of Convolvulus arvensis.

Geotropic behavior in roots of Convolvulus arvensis is influenced by both gravity and light. In darkness, the roots grow horizontally; exposure to light induces a positive orthogeotropic response. The response is elicited by red light of narrow band width and is reversed by a far-red-light treatment following the red exposure. Phytochrome thus mediates this light response. Experiments with microbeam irradiation suggest that this pigment is located in the root apex, probably in the root cap. Statoliths are present and are shown to sediment upon reorientation of the root apex in both light-and dark-grown roots.