Nitrate effects on nodule oxygen permeability and leghemoglobin: Nodule oximetry and computer modeling

Two current hypotheses to explain nitrate inhibition of nodule function both involve decreased O₂ supply for respiration in support of N₂ fixation. This decrease could result from either (a) decreased O₂ permeability (P_O) of the nodule cortex, or (b) conversion of leghemoglobin (Lb) to an inactive, nitrosyl form. These hypotheses were tested using alfalfa (Medicago sativa L. cv Weevlchek) and birdsfoot trefoil (Lotus corniculatus L. cv Fergus) plants grown in growth pouches under controlled conditions. Nodulated roots were exposed to 10 mM KNO₃ or KCl. Fractional oxygenation of Lb under air (FOL_air), relative concentration of functional Lb, apparent P_O, and O₂-saturated central zone respiration rate were all monitored by nodule oximetry. Apparent P_O and FOL_air in nitrate-treated nodules decreased to <50% of values for KCl controls within 24 h, but there was no decrease in functional Lb concentration during the first 72 h. In nitrate-treated alfalfa, but not in birdsfoot trefoil, FOL_air, apparent P_O, and O₂-saturated central zone respiration rate decreased during each light period and recovered somewhat during the subsequent dark period. This species difference could be explained by greater reliance on photoreduction of nitrate in alfalfa than in birdsfoot trefoil. Computer simulations extended the experimental results, showing that previously reported decreases in apparent P_O of Glycine max nodules with nitrate exposure cannot be explained by hypothetical decreases in the concentration or O₂ affinity of Lb.