AUXIN-INDUCED HYPONASTY OF THE LEAF BLADE OF PHASEOLUS VULGARIS

The lateral margins of immature primary leaf blades of Phaseolus vulgaris L. cv. ‘Pinto’ curve up and in toward the midrib when auxin is applied to the leaf. The leaves are most sensitive to auxin shortly after they first unfold and leaves which have grown to about 60 % or more of their ultimate area no longer give this hyponastic response. The response is specific for auxins and is inhibited by the anti-auxins, trans-cinnamic acid and para-chlorophenoxyisobutyric acid. Ethylene and ethylene-generating compounds failed to induce hyponasty, suggesting the response is due to a positive growth promotion by auxin. Measurements of the distance between the lateral margins of the leaf at its maximum width were used to provide quantitative estimates of the degree of hyponasty. Between 2 and 4 hr after auxin application a direct proportionality was found between the amount of curvature and the logarithm of the indoleacetic acid concentration over the range of 10⁻⁶ to 10⁻³ m. The relative sensitivity of the leaves to different auxins was qualitatively similar to that observed in many straight-growth bioassays. Similar responses were obtained when auxin was applied by a carborundum wounding procedure. Potential applications of this auxin bioassay for investigations of the role of auxin in the normal plagiotropic growth behavior of leaf lamina and of the role of auxin in the initiation of various plant diseases are suggested.