N-2 FIXATION DURING DIFFERENT PHYSIOLOGICAL STAGES OF PHASEOLUS-VULGARIS OAC RICO AND ITS SUPERNODULATING MUTANT R32BS15 - THE ROLE OF ASSIMILATE SUPPLY TO AND EXPORT FROM NODULES

In this study possible reasons for unreliable N-2 fixation during the pod filling of common bean were examined. The experiment was conducted in water culture without mineral N under natural light conditions in the greenhouse. A supernodulating mutant of common bean Phaseolus vulg aris L. R32BS15 and its wild type cv. OAC Rico were used. Plants were inoculated with Rhizobium leguminosarum by. phaseoli during germinatio n. The plants were cultivated until maturity. Four harvests were taken at several stages of plant development. N concentrations, N contents, amounts of total amino acids, ureides and sucrose were determined in roots, shoots, and nodules. Although the number of nodules of the muta nt was higher there were no differences in Nz fixation during the expe riment: between the wild type and the mutant. While the supply of the host planes with N was sufficient in the vegetative growth phase the p erformance of bacteroids to fix N-2 decreased markedly in the generati ve growth phase. A lack of carbohydrates in nodules as a result of sin k competition among nodules and fruits was nor responsible for this de cline since sucrose in the nodules was not limited. However, amino aci ds that accumulated in the nodules at the end of the experimental phas e and a disturbance of ureide synthesis are discussed as possible inhi bit ors of nitrogenase activity.