Regrowth, Nodulation, and Nitrogenase Activity of Strophostyles helvola with Rigorous Defoliation

Indigenous legumes contribute toward increased N nutrition and enhanced grazing productivity of both native and improved pasture swards. However, quantitative assay of symbiotic dinitrogen fixation by pastured legumes is difficult. The objective of these studies was to determine defoliation effects on regrowtb, nodulation, nitrogenase activity (C₂H₂ red.) and associated nodule enzymology of trailing field bean [Strophostyles helvola (L.) Ell.]. Plants were grown in a Psammentic Paleustalf (Eufaula) and inoculated with Rhizobium leguminosarum Frank, ATCC 10314. Characteristics of nondefoliated plants during 3 years of field experiments included near exponential growth with 10-fold increase in dry weight from 1.49 to 17.45 g at 30 to 120 days after emergence (DAE). Number of stems tripled and stem length increased six-fold. Average nodules per plant increased from 80.2 to 239.6. Nodule mass as fresh weight increased from 0.94 to 6.17 g, and mature nodules increased from 13% to 89%. Differences in regrowth after defoliation at 30-day intervals was not significant averaging 4.21 g, CV 11 %. Nodules per plant after regrowth averaged 113 with fresh nodule wt. 2.06 g and 27 % were mature nodules. Nitrogenase (C₂H₂ red.) activity levels were highest at 90 DAE, 168.2 μmols. C₂H₄g⁻¹h⁻¹. Regrowing plants after defoliation averaged 156.3 μmol C₂H₄g⁻¹h⁻¹. Highest cytosol enzyme activity levels were for aspartic transaminase (AST) with 25.0 IU (nondefoliated) and 25.9 IU (defoliated) g⁻¹ fresh nodule. Glutamine synthetase (GS) was 1.7 IU (nondefoliated) and 12.0 IU (defoliated). Multiple regression indicated significance for glutamate dehydrogenase (GDH) but not glutamine 2-oxoglutarate aminotransferase (GOGAT) with interactions for the equation of best fit with P < 0.05 level as nitrogenase (C₂H₂ red.) = 13.3 nodule wt. + 0.5 nodule no. + 4.5 nodule maturity % + 58.6 GDH, R² = 80.9 and CV 11%. More knowledge of indigenous pastured legumes, as S. helvola, could have practical significance for improvement of symbiotic N fixation and grazing production for the extensive pastured regions with severe soil and climate limitations.