NITROGENASE ACTIVITY, NODULE RESPIRATION, AND O2 PERMEABILITY FOLLOWING DETOPPING OF ALFALFA AND BIRDSFOOT-TREFOIL

Gas exchange measurements and noninvasive leghemoglobin (Lb) spectroph otometry (nodule oximetry) were used to monitor nodule responses to sh oot removal in alfalfa (Medicago sativa L. cv Weevlchek) and birdsfoot trefoil (Lotus corniculatus L. cv Fergus). In each species, total nit rogenase activity, measured as H2 evolution in Ar:O2 (80:20), decrease d 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 separ ate experiment in which nodule oximetry was used, nodule O2 permeabili ty decreased 50% within 5 hours in each species. A similar decrease in the O2-saturated respiration rate (V(max)) for the nodule central zon e occurred within 5 hours in birdsfoot trefoil, but only after 24 hour s in alfalfa. Lb concentration, also measured by oximetry, decreased a fter 48 to 72 hours. The decrease in permeability preceded the decreas e in V(max) in each species. V(max) may depend mainly on carbohydrate availability in the nodule. If so, then the decrease in permeability c ould not have been triggered by decreasing carbohydrate availability. Both oximetry and gas exchange data were consistent with the hypothesi s 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 ho urs after detopping but subsequently increased to > 0.65 for a majorit y of nodules of each species. This increase could lead to O2 inactivat ion of nitrogenase.