Ethylene as a Cause of Transient Petiole Epinasty in Helianthus annuus during Clinostat Experiments

Petiole curvature and elongation growth in Helianthus annuus L. have been recorded for plants rotating with their stems parallel to the horizontal axis of a clinostat at 8 revolutions per hour over 72 hours. When rotation was continuous, dorso-convex curvature (epinasty) developed in the first 12 hours and was followed by recovery (straightening) in the next 36 hours. Thereafter the petioles remained straight. These changes in shape are due to brief consecutive increases in the elongation growth of the upper and lower halves of the petiole. Plants exposed to 10 μl per liter ethylene after 200 hours on the clinostat, developed similar petiole epinasty, followed by straightening when the exposure to ethylene ceased. Interrupting rotation of the plant for 1 hour in 4, did not change the petiole response, whereas the alternation of 4 hour stationary and rotation periods, delayed the straightening process. The axillary angle between the stem and petiole increased from about 40° to 63° during either continuous rotation or rotation with 1 or 4 hour stationary periods. When detached leaves were inverted, the rate of ethylene release approximately doubled after 4 hours and continued to increase thereafter. The results indicate that the development of transient petiole epinasty on the clinostat, is due to ethylene production caused primarily by the disorientation of the plant, rather than to the rotation process.