The photosynthetic organs of the barley spike (lemma, palea, and awn) are considered resistant to drought. However, there is little information about gene expression in the spike organs under drought conditions. We compared response of the transcriptome of the lemma, palea, awn, and seed to drought stress using the Barley1 Genome Array. Barley plants were exposed to drought treatment for 4 days at the grain-filling stage by withholding water. At the end of the stress, relative water content of the lemma, palea, and awn dropped from 85% to 60%. Nevertheless, the water content of the seed only decreased from 89% to 81%. Transcript abundance followed the water status of the spike organs; the awn had more drought-regulated genes followed by lemma and palea, and the seed showed very little change in gene expression. Despite expressing more drought-associated genes, many genes for amino acid, amino acid derivative, and carbohydrate metabolism, as well as for photosynthesis, respiration, and stress response, were down-regulated in the awn compared with the lemma, palea, and seed. This suggests that the lemma and the palea are more resistant to drought stress compared with the awn.

No abstract

Solar tracking by unifoliolate leaves of small red kidney bean (Phaseolus vulgaris L.) seedlings was studied in the field in the summer, using a simple solar‐foliar inclinometer to measure the angle between the leaf surface and the sun's rays. Changes in leaf orientation increased light interception in the morning and late afternoon, while reducing light interception at midday. Light avoidance occurred even in well‐watered plants, but was more marked for plants growing in drier soil. Light avoidance was also observed under diffused but directional light in a growth chamber, where changes in leaf angle were proportional to changes in shoot water potential factors. Temperature differences between leaves of tracking plants in well‐watered soil and those in drier soil were small due to the compensating change in light interception. Holding leaves horizontal increased leaf temperatures in the hottest part of the day slightly above those of tracking leaves for well‐watered plants, but considerably more for plants with lower water potentials. When leaves were held horizontal, water potentials were lower at midday and in early afternoon than they were for tracking leaves. Photosynthesis was enhanced at midday in both wet and dry treatments for leaves held horizontal. At 1600 h horizontal leaves of plants in both treatments, and tracking leaves of plants in the dry treatment, photosynthesized at a low rate, although tracking leaves intercepted more light. Tracking leaves of plants in the wet treatment assimilated at a substantially higher rate.

Unifoliolate leaves of seedlings of soybean [Glycine max (L.) Merr. cv. Northrup King S1346] exhibit diaheliotropic movements when well watered and paraheliotropic movements when water stressed. The movements originate at the pulvinus at the base of each leaf lamina. The receptor site for the light stimulus was located by covering portions of the leaf surface with India ink and observing the effect on leaf movement; the leaf lamina, leaf veins, and pulvinus were tested. The pulvinus was the receptor site for light that stimulates both diaheliotropic and paraheliotropic leaf movement. The role of different wavelengths of light in diaheliotropism and paraheliotropism was determined using filters that remove broad bands of light wavelengths. Blue light was necessary for both diaheliotropism and paraheliotropism. Interference filters were used to expose soybean leaves to selected blue‐light wavelengths. The amount of diaheliotropic leaf movement at each wavelength was used to obtain an action spectrum. There was one peak between 410 nd 440 nm and one between 470 and 490 nm. A nonphytochrome photoreceptor located in the pulvinus used blue light, in conjunction with other factors, to determine leaf orientation and thereby regulate light interception of leaves of soybean seedlings.

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 intad plants at 24, 27, and 30°C at 500 and 750 amo1 photons (400-700 nm) m-'s-' 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 intad 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.