Control of Paraheliotropism in Two Phaseolus Species.

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.

Patterns of Effective Permeability of Leaf Cuticles to Acids.

Plants in the field are frequently exposed to anthropogenic acid precipitation with pH values of 4 and below. For the acid to directly affect leaf tissues, it must pass through the leaf cuticle, but little is known about the permeability of cuticles to protons, or about the effect of different anions on this permeability. We investigated the movement of protons through isolated astomatous leaf cuticles of grapefruit (Citrus X paradisi Macfady.), rough lemon (Citrus limon [L.] Burm. fils cv Ponderosa), and pear (Pyrus communis L.) using hydrochloric, sulfuric, and nitric acids. Cuticles were enzymically isolated from leaves and placed in a diffusion apparatus with pH 4 acid on the morphological outer surface of the cuticle and degassed distilled water on the inner surface. Changes in pH of the solution on the inner surface were used to determine rates of effective permeability of the cuticles to the protons of these acids. Most cuticles exhibited an initial low permeability, lasting hours to days, then after a short transition displayed a significantly higher permeability, which persisted until equilibrium was approached. The change in effective permeability appears to be reversible. Effective permeabilities were higher for sulfuric acid than for the others. A model of the movement of protons through the cuticle is presented, proposing that dissociated acid groups in channels within the cutin are first protonated by the acid, accounting for the low initial effective permeability; then protons pass freely through the channels, resulting in a higher effective permeability.

Effect of Cations on Effective Permeability of Leaf Cuticles to Sulfuric Acid.

Many plants are exposed to prolonged episodes of anthropogenic acid precipitation with pH values of 4 or less, but there is little evidence of widespread direct damage to the plant cells. Acids appear to permeate leaf cuticle via charged pores, which act as a fixed buffer that delays but does not stop acid movement. We investigated the effect of cations on the movement of protons through astomatous isolated leaf cuticles of pear (Pyrus communis L.) and rough lemon (Citrus limon [L.] Burm. fils cv Ponderosa). Chloride salt solutions of Na, K, Ca, Cd, Mg, Gd, or Y in a diffusion apparatus were applied to the morphological inner surface of the cuticle, while the outer surface faced a large volume of pH 3 or 4 sulfuric acid. Effective permeability was calculated from the change in the pH of the inner solution as measured with a pH microelectrode. Monovalent cations caused either no change (pear) or promotion (rough lemon) of proton movement. Divalent cations reduced proton movement in a concentration-dependent manner (both species), whereas trivalent cations (rough lemon only) caused the effective permeability to decrease to near zero. Inhibition by 10 mM CaCl2 was reversed with water. The effects of these cations on the permeability of cuticles to protons was used to elucidate mechanisms by which cations can protect leaves from acid precipitation in nature.

Assessment of the potential of the blue light gradient in soybean pulvini as a leaf orientation signal.

Blue light gradients in the pulvini of soybean (Glycine max var. Northrup King S1346) leaves with different laminar orientations were examined with a fiber optic microprobe. The gradients changed markedly as a function of both incident light angle and leaf position and were determined largely by the amount of light present in the adaxial side of the pulvinus. The steepest gradient for inclined leaves was with light incident at 90° whereas for declined leaves it occurred when the light was incident at 150°. A proposed pulvinar mechanism which allows for the detection of light direction and leaf position by using the blue light gradient as an orientation signal could account for solar tracking by soybean.