Carbon and nitrogen partitioning in young nodulated pea (wild type and nitrate reductase-deficient mutant) plants exposed to NO3− or NH4+

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.