RESPONSE OF TROPIC LEAF MOVEMENTS AND PHOTOSYNTHETIC GAS-EXCHANGE TO WATER AVAILABILITY IN N2-FIXING AND NO3-FED SOYBEAN

Leaf movements, water status, and ps exchange were measured in soybean inoculated with Bradyrhizobium and grown under high and low soil wate r and nitrate availabilities. We hypothesized that paraheliotropism in low NO3-grown plants (which have greater N2 fixation rates) would dif fer from that of high NO3-grown plants (which have lower N2 fixation r ates), such that carbon return on nitrogen investment into photosynthe sis would be enhanced. Low NO3-low water plants had more vertical leaf angles and received lower solar irradiances at midday than high NO3-l ow water plants. Under constant, vertical illumination, low NO3-low wa ter plants had steeper leaf angles, increased rates of leaf movement, lower photosaturated photosynthetic rates, and lower stomatal conducta nces for a given leaf water potential than high NO3-low water plants. Leaves of high NO, plants had lower photosynthetic nitrogen use effici encies than did low NO3 plants. Low water plants had lower leaf osmoti c potentials and ratios of intercellular/ambient CO2 concentration tha n high water plants, but NO3 treatment did not affect these parameters . Results provided support for our initial hypothesis, and demonstrate d a high degree of correspondence between gas exchange and heliotropic response to soil nitrogen and water availabilities in soybean.