Nitrogen Fixation ofNodulation Mutants ofSoybean as Affected byNitrate1

Itwaspreviously reported that three soybean (Glycine max[L.] Merr.) nodulation mutants (NOD1-3, NOD2-4, andNOD3-7) were partially tolerant tonitrate whennitrate wassupplied simultaneously withinoculation atthetimeoftransplanting. Thecurrent studyevaluated theeffect ofshort-term nitrate treatment on nitrogenase activity (C2H2 reduction perplant andpernodule weight) andonrelative abundance ofureides whennitrate application wasdelayed until plants were3weeksoldandnodules werefully developed. Nitrogenase activity ofthemutants was similar tothatofWilliams after aninitial 3-week growth period, prior tonitrate treatment. Application of5millimolar nitrate resulted ingreater inhibition ofnitrogenase activity inWilliams than inthethree mutants. NOD1-3 wasmosttolerant ofnitrate among themutants tested andshowedthehighest relative abundance ofureides. Although C2H2 reduction activity perplant forNOD1-3 washigher thanforWilliams inthepresence ofnitrate, C2H2 reduction activity pergramofnodules waslower forNOD1-3than forWilliams inthepresence andabsence ofnitrate. Compared toWilliams, NOD1-3hadhigher nodule ureide concentration and hadsimilar glutamine synthetase activity innodule tissue, indicating itsnodules havenormal nitrogen assimilation pathways. Nitrate application resulted inureide accumulation innodule tissue aswell asinallplant parts assayed. Unexpectedly, nitrate treatment alsoincreased therateofureide degradative capacity ofleaves inbothNOD1-3andWilliams. Thedataconfirmed that nitrogenase activity oftheselected nodulation mutants wasmore, butstill onlypartially, tolerant ofnitrate compared withtheWilliams parent. also hadhigher ureide concentration inits nodule tissue (13),