Gas exchange measurements and noninvasive leghemoglobin (Lb) spectrophotometry (nodule oximetry) were used to monitor nodule responses to shoot removal in alfalfa (Medicago sativa L. cv Weevlchek) and birdsfoot trefoil (Lotus corniculatus L. cv Fergus). In each species, total nitrogenase activity, measured as H2 evolution in Ar:O2 (80:20), decreased to 50% of the initial rate within 1 hour after detopping, and net CO2 production decreased to about 65% of the initial value. In a separate experiment in which nodule oximetry was used, nodule O2 permeability decreased 50% within 5 hours in each species. A similar decrease in the O2-saturated respiration rate (Vmax) for the nodule central zone occurred within 5 hours in birdsfoot trefoil, but only after 24 hours in alfalfa. Lb concentration, also measured by oximetry, decreased after 48 to 72 hours. The decrease in permeability preceded the decrease in Vmax in each species. Vmax may depend mainly on carbohydrate availability in the nodule. If so, then the decrease in permeability could not have been triggered by decreasing carbohydrate availability. Both oximetry and gas exchange data were consistent with the hypothesis that, for the cultivars tested, carbohydrate availability decreased more rapidly in birdsfoot trefoil than in alfalfa nodules. Fractional Lb oxygenation (initially about 0.15) decreased during the first 24 hours after detopping but subsequently increased to >0.65 for a majority of nodules of each species. This increase could lead to O2 inactivation of nitrogenase.
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