N₂Fixation (C₂H₂-Reducing Activity) and Leghaemoglobin Content during Nitrate- and Water-Stress-Induced Senescence ofMedicago sativaRoot Nodules

Nitrate and water stress were used to induce senescence in root nodules of alfalfa (Medicago sativa L. cv. Aragon). Nodule senescence was assessed by determinations of the nitrogenase (C₂H₂-reducing) activity, and the leghaemoglobin (LHb) and total soluble protein contents of the nodules. Nodules responded similarly to $NO3-$ and water stress in many respects, but there was a significant difference. All parameters of nodule activity, expressed on the basis of nodule dry weight (DW), consistently decreased following treatment with $NO3-$ or during drought; there was a significant interaction (synergism) between the inhibitory effects of $NO3-$ and water stress on nitrogenase activity, but such effects were merely additive in the case of LHb content or LHb/soluble protein ratio. However, $NO3-$ caused the selective decay of LHb with respect to other nodular soluble proteins, whereas the decrease of LHb during water stress was due to a general inhibition of protein synthesis and to an increased proteolytic activity in the nodule cytosol rather than to a specific proteolysis of LHb.