Tropic leaf movements, photosynthetic gas exchange, leaf delta C-13 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, Gl ycine soja, G. tomentella and G. tabacina, from habitats with different wat er availability were examined. Plants grown under low water availability ha d reduced leaf area, photosynthetic and electron transport rates, more posi tive delta(13)C values, and more vertical midday leaf angles. The three spe cies 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 gro ws in the wettest habitats, was shown to be the most photosynthetically sus ceptible to low water treatment and tended to have the most vertical midday leaflet angles. In contrast, G. tabacina, distributed in the driest habita ts, had the least vertical midday leaflet angles and the lowest photosynthe tic sensitivity to low water availability. G. tomentella, inhabiting an int ermediate habitat, had intermediate midday leaf angles and photosynthetic r esponses. Our results support the hypothesis that paraheliotropic leaf move ments respond in concert with photosynthetic characteristics in soybean lea ves such that water use efficiency is enhanced and the risk of photoinhibit ion under water stress conditions is reduced.