Abstract
Available soil nitrogen usually limits improved pasture production. Perennial, high-nitrogen-fixing forage legumes are desirable for tropical swards. The drought-tolerant Phaseoleae, ‘Copada’ (Cratylia floribunda Mart. ex. Benth.) is well adapted to a wide range of soil texture, pH. and fertility levels of Oxisol-Ultisols in the subhumid neotropics. It is highly competitive and has vigorous regrowth and high nitrogen-fixing capabilities under intensive grazing regimes. Our objective was to study interrelationships of plant growth and the characteristics of determinate mature and distinctive nonmature nodules for enhanced nitrogen fixation. Determinations included nitrogenase activity (C2H2 reduction), nodule cytosol ureide composition, and selected enzyme components associated with symbiotic nitrogen fixation. Plants were grown in the greenhouse, with ages of individual plants recorded as days from emergence at germination. Copada top growth increased from 0.34 to 2.03 grams from 24 to 62 days from emergence. Nodule fresh weight increased from 124.7 to 786.6 milligrams per plant, and nodule numbers approximately doubled, from 69.4 to 135.0. Nitrogenase activity (C2H2 reduction) increased from 28.4 to 200.9 ± mole C2H4 h-1 g-1. Total cytosol ureides of determinate mature nodules increased from 0.21 to 2.59 ± mole g-1 fresh nodule. Differential activity levels of mature nodule cytosol enzymes included increased aspartate aminotransferase (AST) 25.6 to 47.3 IU, glutamate dehydrogenase (GDH) 1.51 to 2.94 IU, and glutamine synthetase (GS) 6.71 to 20.58 IU during the 6-week period. Glutamate synthase (GOGAT) decreased from 3.99 to 1.75 IU. The principal factors influencing nitrogenase activity (C2H2 reduction) were apparent, with a highly significant regression for mature nodule cytosol as: nitrogenase activity (C2H2 reduction) = 2.57 nodular weight + 3.27 ureide + 2.67 GDH - 1.98 GOGAT, R2 = 82.6, CV = 9.24 percent. Differences were highly significant for determinate mature nodule enzyme activity levels between Copada plants during the plant growth cycle. Differences were not statistically significant for comparative composition between mature and distinctive nonmature nodule cytosol from the same Copada plant for GDH and GOGAT. AST and GS were significantly higher within mature nodule cytosol. Nitrate reductase (NR) was significantly higher, with 103.3 IU g-1 for nonmature compared with 4.5 IU g-1 for mature nodules. No significance was apparent for cytosol levels of 2-oxoglutarate (αKG), pyridoxal-5-phosphate, and pyridoxamine-5-phosphate (PLP), and total nonstructural carbohydrates (CHO) as a function of nodule maturity. Components of nonmature nodules significantly influencing cytosol nitrate reductase were determined to be NR = 2.01 AST - 2.95 nodular weight + 2.89 PLP + 1.85 ureide, R2 = 54.2, CV = 39.8 percent. Mature nodule cytosol ureide composition, 1.37 ± mol, was highly significant over 0.01 ±mol g-1 nonmature nodule. A highly significant multiple regression was determined for total cytosol ureide = 4.71 GS + 1.39 nodular weight - 1.32 AST - 1.56 αKG, R2
Published Version
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