Leaf movement, photosynthesis and resource use efficiency responses to multiple environmental stress in Glycine max (soybean)

Increases in the concentration of greenhouse gases in the atmosphere, may cause a significant increase in temperature, with implications for general wind patterns and precipitation. Reductions in stratospheric ozone will result in increased levels of UV-B reaching earth's surface. During their lifetime plants must deal with a variety of co-occurring environmental stresses. Accordingly, studies into plant responses to multiple environmental factors is important to our understanding of limits to their growth, productivity, and distribution. Heliotropic leaf movements are a generalized plant response to environmental stresses, and the pattern of these movements can be altered by resource availability (e.g., water, and nitrogen). Previous greenhouse and field studies have demonstrated damaging effects of UV-B radiation in crop species, including soybean. Our objectives were to document leaf movement and gas exchange responses of four soybean cultivars with different sensitivity to UV-B radiation to enhanced levels of UV-B, and modifications of these responses caused by water stress and nitrogen fertilization. UV-B radiation had no effect on the patterns of leaf orientation in soybean; however, a ranking of the cultivars based on midday leaf angles was the same as the ranking of these cultivars based on their sensitivity to UV-B radiation. Water and nitrogen altered the leaf movement patterns of soybeans. Gas exchange parameters in all four cultivars responded in a similar fashion to changes in leaf water potential. The cultivar Forrest had enhanced photosynthetic performance under UV-B, while cv Cumberland and Essex were unaffected, and CNS showed reductions in photosynthesis. Reductions of stomatal conductance under increased levels of UV-B radiation, suggest that UV-B effects on gas exchange were mainly through effects on stomata, rather than damage to the photosystems. Water was the main factor reducing photosynthesis through stomatal and non-stomatal factors. Carbon isotope discrimination varied among soybean cultivars, however the discrimination values were not correlated to the UV-B sensitivity ranking of the cultivars. Reductions in water availability resulted in lower discrimination. Nitrogen fertilization in cv Forrest, also resulted in lower discrimination, especially under low water regimes, indicating a higher water use efficiency for fertilized plants. UV-B radiation resulted in lower discrimination in the UV-B sensitive CNS cultivar, indicating a stronger stomatal limitation to photosynthesis under increased UV-B levels. The complementary responses of heliotropism and leaf gas exchange to different water and nitrogen availabilities may provide a basis to enhance carbon return on the investment of limiting resources into photosynthesis. The differences, among the cultivars, in the extent of these responses may partially explain the differential sensitivity of these soybean cultivars to UV-B radiation.