THE TIMING OF AND EFFECT OF TEMPERATURE ON AUXIN-INDUCED HYPONASTIC CURVATURE OF THE BEAN PRIMARY LEAF BLADE

Detailed examination of the hyponastic curvature of the primary bean leaf blade in response to indoleacetic acid (IAA) shows that curvature begins within 15 min after application and increases to a maximal rate at 20 to 30 min. A second application of IAA results in a second curvature maximum when applied 1.5 hr or more after the first. Washing experiments indicate IAA uptake is largely complete by about 20 min after application, suggesting the return to planar form is accompanied by the uptake and passage of a wave of IAA through the responding cells. The rate of curvature decreases as the temperature is lowered, particularly below 14 C; at low concentrations (10^–4 m) the rate of response to 2,4-dichlorophenoxyacetic acid and 2,4,5-trichlorophenoxypropionic acid is slower than that for IAA and naphthaleneacetic acid. These differences are proposed to reflect the involvement of the polar auxin transport system in the response. The leaves of bean seedlings exposed to 4 C develop hyponastic curvatures when returned to normal growth temperature; 5 min treatment is sufficient to induce this response, and with longer treatments, greater curvatures are obtained. This curvature is inhibited by application of 2,3,5-triiodobenzoic acid (TIBA) to the undersurface of the leaf at the beginning of the cold treatment. The results are consistent with a model of planar plageotropic growth regulation in the leaf blade in which auxin produced by cells in the upper portion of the blade is transported by the polar transport system through cells in the lower portion that are growth limited by auxin supply. The hyponastic and epinastic effects caused by exogenous application of auxin or TIBA and of cold treatments are considered to result from changes in this auxin supply.