AUTONOMIC STRAIGHTENING AFTER GRAVITROPIC CURVATURE OF CRESS ROOTS

Few studies have documented the response of gravitropically curved org ans to a withdrawal of a constant gravitational stimulus. The effects of stimulus withdrawal on gravitropic curvature were studied by follow ing individual roots of cress (Lepidium sativum L.) through reorientat ion and clinostat rotation. Roots turned to the horizontal curved down 62 degrees and 88 degrees after 1 and 5 h, respectively. Subsequent r otation on a clinostat for 6 h resulted in root straightening through a loss of gravitropic curvature in older regions and through new growt h becoming aligned closer to the prestimulus vertical. However, these roots did not return completely to the prestimulus vertical, indicatin g the retention of some gravitropic response. Clinostat rotation shift ed the mean root angle -36 degrees closer to the prestimulus vertical, regardless of the duration of prior horizontal stimulation. Control r oots (no horizontal stimulation) were slanted at various angles after clinostat rotation. These findings indicate that gravitropic curvature is not necessarily permanent, and that the root retains some commitme nt to its equilibrium orientation prior to gravitropic stimulation.