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

Leaf movements, water status, and gas exchange were measured in soybean inoculated with Bradyrhizobium and grown under high and low soil water and nitrate availabilities. We hypothesized that paraheliotropism in low NO₃-grown plants (which have greater N₂ fixation rates) would differ from that of high NO₃-grown plants (which have lower N₂ fixation rates), such that carbon return on nitrogen investment into photosynthesis would be enhanced. Low NO₃-low water plants had more vertical leaf angles and received lower solar irradiances at midday than high NO₃-low water plants. Under constant, vertical illumination, low NO₃-low water plants had steeper leaf angles, increased rates of leaf movement, lower photosaturated photosynthetic rates, and lower stomatal conductances for a given leaf water potential than high NO₃-low water plants. Leaves of high NO₃ plants had lower photosynthetic nitrogen use efficiencies than did low NO₃ plants. Low water plants had lower leaf osmotic potentials and ratios of intercellular/ambient CO₂ concentration than high water plants, but NO₃ treatment did not affect these parameters. Results provided support for our initial hypothesis, and demonstrated a high degree of correspondence between gas exchange and heliotropic response to soil nitrogen and water availabilities in soybean.