| Abstract | Fresh weight accumulation, and carbon and nitrogen partitioning were investigated in wild type and a nitrate reductase-deficient mutant (A317) of Pisum sativum L. (cv. Juneau), effectively inoculated with Rhizobium leguminosarum strain 128C54. The plants were grown hydroponically in medium without combined N for 21 days, followed by a further 7 days in medium without combined N or with 5 mM NO₃− or NH₄+. In the absence of combined N, the nitrogenase activity (measured as acetylene reduction and expressed on a specific nodule basis) of A317 was 53% of the wild type. In the presence of combined N the nitrogenase activity of wild-type plants was reduced by 60%, whereas that of A317 was not affected. The decline in the proportion of ¹⁴C translocated to nodulated roots that was allocated to nodules only was significant in the wild type. Inorganic N accumulated in the nodule. Nodule concentrations of asparagine and glutamine increased dramatically in both genotypes with NH₄+ but not NO₃−. The partitioning of sugar and starch was often dependent on the pea genotype and N form. These data suggest that the assimilation of NO₃− and (or) NH₄+ plays a role in the inhibition of symbiotic N₂ fixation by combined N. Key words: carbon and nitrogen partitioning, nitrate reductase, nodulated pea plants. |
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