Tropic leaf movements, photosynthetic gas exchange, leaf δ13C and chlorophyll a fluorescence of three soybean species in response to water availability

Midday leaf angle, photosynthetic gas exchange, stable carbon isotope ratio (delta(13)C), and chlorophyll a fluorescence among three wild soybeans, Glycine soja, G. tomentella and G. tabacina, from habitats with different water availability were examined. Plants grown under low water availability had reduced leaf area, photosynthetic and electron transport rates, more positive delta(13)C values, and more vertical midday leaf angles. The three species differed in midday leaflet orientation, leaf size, photosynthesis and fluorescence responses to water availability. The species differences were consistent with the water availability of their habitat, G. soja, which grows in the wettest habitats, was shown to be the most photosynthetically susceptible to low water treatment and tended to have the most vertical midday leaflet angles. In contrast, G. tabacina, distributed in the driest habitats, had the least vertical midday leaflet angles and the lowest photosynthetic sensitivity to low water availability. G. tomentella, inhabiting an intermediate habitat, had intermediate midday leaf angles and photosynthetic responses. Our results support the hypothesis that paraheliotropic leaf movements respond in concert with photosynthetic characteristics in soybean leaves such that water use efficiency is enhanced and the risk of photoinhibition under water stress conditions is reduced.