Growth and Gravitational Response in the Development of Leaf Blade Hyponasty

Rapid (4 hr) auxin‐induced hyponastic curvature of primary leaves of Phaseolus vulgaris is shown to depend on a positive increase in growth of the lower portion of the blade. The curvature involves laminar growth as well as vein growth and is not due to simple turgor changes. The response is sensitive to gravitational orientation, as inversion and horizontal rotation reduce 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 hr) effects of 1 mM auxin depend on the surface of the leaf treated. Application to the upper surface results in epinasty, lower surface application in hyponasty, although the initial response in each case is a hyponastic curvature. A dorsi‐ventral auxin transport system and differential auxin sensitivity of upper and lower portions of the leaf blade are postulated to account for these responses.