Response of Rhizobium leguminosarum isolates to different forms of inorganic nitrogen during nodule development in pea ( Pisum sativum L.)

Abstract

Combined nitrogen inhibits nodule development and nitrogen fixation in Rhizobium -legume symbioses. Isolates of R. leguminosarum which had been shown to vary in symbiotic effectiveness in the presence of NN 4NO 3 were used as inoculum for peas grown with NH 4NO 3 KNO 3 or NH 4Cl and harvested between 14 and 24 days after planting. Plants grown in 2m m NH 4NO 3 showed inhibition of nodule development and function 17 days after seeding but there were no significant differences (SDs) between isolates of high and low effectiveness. In the presence of 5 m M NH 4NO 3 nodule development and C 2H 2 reduction were severely inhibited from 14 days onwards. Plants inoculated with isolates of high effectiveness had more tap-root nodules, higher C 2H 2 reduction rates and higher leghemoglobin content than those inoculated with isolates of low effectiveness. In plants grown with 5 m m KNO 3 or 5 m m NN 4Cl, NO 3 − had the major inhibitory effect on nodulation and nodule development, but NH 4 + began to show inhibitory effects on nodule development after 17 days. There were no SDs between isolates of low and high effectiveness at these concentrations of NO 3 − and NH 4 +. Differences in symbiotic response of R. leguminosarum to combined nitrogen are small and appear during nodule development rather than at nodule initiation.