SHORT-TERM NITRATE EFFECTS ON HYDROPONICALLY-GROWN SOYBEAN CV BRAGG AND ITS SUPERNODULATING MUTANT .1. CARBON, NITROGEN AND MINERAL ELEMENT DISTRIBUTION, RESPIRATION AND THE EFFECT OF NITRATE ON NITROGENASE ACTIVITY

A comparison between two hydroponically-grown soybean genotypes (Glycine max [L.] Merr.) cv. Bragg and the supernodulating mutant nts 1007 was made in terms of dry matter accumulation, carbon, nitrogen, and mineral element distribution. N-15 natural abundance and the effect of short-term treatment with 4.0 mol m-3 KNO3 on nitrogenase activity and respiration. Differences were most pronounced in nodule dry weight and plant nitrogen content, both of which were recorded to be substantially elevated in the mutant. Mineral element concentrations in different plant parts proved to be rather similar with the exception of Ca, found to be lower in leaves of the mutant, and Mn concentrations which were twice as high in roots of nts 1007. The values of N-15 natural abundance showed that both genotypes were equally dependent on nitrogen fixation when nitrate was absent. The results of the acetylene reduction assays indicated similar specific nodule activity, while on a per plant basis nitrogenase activity of the mutant proved to be more than twice the amount of Bragg. This effect was also reflected in higher nodule respiration while root respiration remained below that of Bragg. Nitrate induced a substantial reduction in nitrogenase activity not only in Bragg, but also in nts 1007. Nodule respiratory activity of Bragg was reduced by nitrate from 1.27 to 0.34 mg C h-1 plant-1. In nts 1007 corresponding values were 2.70 to 1.52 mg C h-1 plant-1. Starch concentration in nodules was decreased in both genotypes, but nevertheless remained higher in nts 1007. Values for soluble sugars in nodules even increased in the mutant in response to nitrate while the same treatment caused a reduction in Bragg. The data indicate that nitrogenase activities of Bragg and nts 1007 are equally sensitive to short-term application of nitrate.