Soil potassium effects on nitrogenase activity with associated nodule components of hairy vetch at anthesis

Acetylene reduction techniques are frequently utilized to estimate legume nodule nitrogenase activity levels. However, the known symbiotic nitrogen fixation reactions have no equivalent for the rapid permeastic transport of C₂H₄ reduced by nitrogenase of rhyzobial cells through the cortex tissues with the subsequent volatile excretion that is essential for GC quantitation procedures. The objective of this study was to determine interrelationships of nitrogenase (C₂H₂ reduction) with associated cytosol enzyme components from morphologically homologous nodules of Madison hairy vetch (Vicia villosa, Roth) at anthesis as influenced by soil potassium levels. The vetch plants were grown in a siliceous thermic Psammentic Paleustalf, Eufaula, and inoculated with Rhizobium leguminosarum Frank, ATCC 10314.

Highly significant enhancement of nitrogenase activity progressed from quadratic to linear with increased soil K levels in time‐course samplings at 30, 60, 90 and 120 min. incubations at 27C. Means as C₂H₄ μmole g^‐1 fresh nodule wt. were 25.1, 38.8, 50.1 and 92.2 for 0, 100, 200, and 300 mg K/kg soil, respectively.

Activity levels of four cytosol enzymes, aspartate aminotransferase (AST), glutamate dehydrogenase (GDH) glutamine synthetase (GS) and glutamate synthase (GOGAT), increased significantly with increased K soil levels. These are requisite to enzymatic pathways for fixed N ammonia biotransformations with subsequent xylem translocation from the legume nodule. The transaminase (AST) and ligase (GS) were dominant at all K levels with GS increasing linearly to six fold levels over the check treatment. Cytosol composition of total ureides and αKG increased significantly with increased soil K. levels. Cytosol Ca and Mg increases were not significant but highly significant increased K content with reciprocal decreased Na resulted from increased soil K levels. Multiple regression for the most reliable response surface equation within a general linear model with R² = 60.3% was: Nitrogenase (C₂H₂ reduction) = 2.84 nod. wt. + 1.05 GS + 8.08 αKG + 0.11 ureide, CV = 16.2%. Practical application of these data include need for more than single time‐course C₂H₄ determinations from one culture incubation in order to reliably estimate C₂H₂ reduction capabilities of legume nodules. Adequate levels of available soil potassium were necessary for sustained high nltrogenase activity levels.