Abstract
Paraheliotropic (light-avoiding) leaf movements have been associated with high light intensity, high temperature, and drought. We investigated leaf elevation for intact plants, pulvinus bending for excised motor organs, and size change for protoplasts from motor tissue for two Phaseolus species: Phaseolus acutifolius A. Gray, native to hot, arid regions, and Phaseolus vulgaris L., the common bean. Leaf angles above horizontal were measured for central trifoliolate leaflets of intact plants at 24, 27, and 30[deg]C at 500 and 750 [mu]mol photons (400-700 nm) m-2 s-1 over a range of water potentials; equivalent angles were determined for excised motor organs under similar conditions. Diameters were measured for protoplasts from abaxial and adaxial motor tissue over a range of photon flux density values, temperatures, and water potentials. In general, higher photon flux density and temperature resulted in elevation of leaves, bending of excised pulvini, and equivalent changes in protoplast volume (swelling of abaxial protoplasts and shrinking of adaxial protoplasts). In intact plants, lower water potentials yielded greater paraheliotropism; abaxial protoplasts increased in size, whereas adaxial ones did not change. P. acutifolius typically exhibited greater paraheliotropism than did P. vulgaris under the same conditions, a set of physiological responses likely to be highly adaptive in its native arid habitat.