CONTROL OF PARAHELIOTROPISM IN 2 PHASEOLUS SPECIES

Paraheliotropic (light-avoiding) leaf movements have been associated w ith high light intensity, high temperature, and drought. We investigat ed leaf elevation for intact plants, pulvinus bending for excised moto r organs, and size change for protoplasts from motor tissue for two Ph aseolus species: Phaseolus acutifolius A. Gray, native to hot, arid re gions, and Phaseolus vulgaris L, the common bean. Leaf angles above ho rizontal were measured for central trifoliolate leaflets of intact pla nts at 24, 27, and 30 degrees C at 500 and 750 mu mol photons (400-700 nm) m(-2) s(-1) over a range of water potentials; equivalent angles w ere determined for excised motor organs under similar conditions. Diam eters were measured for protoplasts from abaxial and adaxial motor tis sue over a range of photon flux density values, temperatures, and wate r potentials. In general, higher photon flux density and temperature r esulted in elevation of leaves, bending of excised pulvini, and equiva lent changes in protoplast volume (swelling of abaxial protoplasts and shrinking of adaxial protoplasts). In intact plants, lower water pote ntials yielded greater paraheliotropism; abaxial protoplasts increased in size, whereas adaxial ones did not change. P. acutifolius typicall y exhibited greater paraheliotropism than did P. vulgaris under the sa me conditions, a set of physiological responses likely to be highly ad aptive in its native arid habitat.