GROWTH AND GRAVITATIONAL RESPONSE IN DEVELOPMENT OF LEAF BLADE HYPONASTY

Rapid (4 h) auxin-induced hyponastic curvature of primary leaves of Phaseolus vulgaris apparently depended on a positive increase in growth of the lower portion of the blade. The curvature involved laminar growth as well as vein growth and was not due to simple turgor changes. The response was sensitive to gravitational orientation, as inversion and horizontal rotation reduced the auxin-induced curvature. The ethylene-generating compound, 2-chloroethylphosphonic acid, had no hyponastic effect on the leaves when applied to either the upper or lower surface and it inhibited auxin-induced hyponasty. This inhibition was additive to that of inversion. Long-term (24-48 h) effects of 1 mM auxin depended on the surface of the leaf treated. Application to the upper surface resulted in epinasty and lower surface application in hyponasty, although the initial response in each case was a hyponastic curvature. A dorsi-ventral auxin transport system and differential auxin sensitivity of upper and lower portions of the leaf blade were postulated to account for these responses.