Nodule physiology of a supernodulating pea mutant

Abstract

Physiological and biochemical parameters of the supernodulating pea (Pisum sativum L.) mutant nod3 were compared to those of its wild‐type parent cv. Rondo in a nil nitrate environment. Plants of cv. Rondo produced more biomass and accumulated more N than plants of nod3. Accordingly, seed yield of the wild type was twice that of the supernodulating mutant. Although the nodule number of nod3 was 10‐fold that of cv. Rondo, the nodule mass of nod3 was only twice that of cv. Rondo as individual nodules were smaller in nod3 than in cv. Rondo. The maximum rate of acetylene reduction activity, determined in an open flow‐through gas system, was higher in the wild type than in nod3 when expressed on a nodule dry weight basis. However, when expressed on a whole plant basis, the nitrogenase activity (acetylene reduction) was similar in the two symbioses. The net carbon costs of nitrogenase activity was 25% lower in nod3 than in cv. Rondo. An equal proportion of the net CO2 efflux from the root system was for growth and maintenance of the tissue in the two symbioses. However, growth and maintenance respiration was higher in nod3 than in cv. Rondo per gram dry weight of the nodulated root system. The nodules of nod3 had a reduced soluble protein concentration as compared to those of the wild type. The specific activities of nodule glutamine synthetase (EC 6.3.1.2), glutamate synthase (EC 1.4.1.14) and asparagine synthetase (EC 6.3.5.4) were lower in nod3 than in cv. Rondo. The root bleeding sap of nod3 contained lower amounts of glutamine and higher amounts of asparagine than that of cv. Rondo. The results suggest that the use of carbon directly related to the dinitrogen fixation and nitrogen assimilation may be less in nod3 than in cv. Rondo, and that there may be differences between the two symbioses in the pathway for assimilation of fixed nitrogen.