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The objectives of this study were to select and initially characterize mutants of soybean (Glycine max L. Merr. cv Williams) with decreased ability to reduce nitrate. Selection involved a chlorate screen of approximately 12,000 seedlings (progeny of mutagenized seed) and subsequent analyses for low nitrate reductase (LNR) activity. Three lines, designated LNR-2, LNR-3, and LNR-4, were selected by this procedure.In growth chamber studies, the fully expanded first trifoliolate leaf from NO(3) (-)-grown LNR-2, LNR-3, and LNR-4 plants had approximately 50% of the wild-type NR activity. Leaves from urea-grown LNR-2, LNR-3, and LNR-4 plants had no NR activity while leaves from comparable wild-type plants had considerable activity; the latter activity does not require the presence of NO(3) (-) in the nutrient solution for induction and on this basis is tentatively considered as a constitutive enzyme. Summation of constitutive (urea-grown wild-type plants) and inducible (NO(3) (-)-grown LNR-2, LNR-3, or LNR-4 plants) leaf NR activities approximated activity in leaves of NO(3) (-)-grown wild-type plants. Root NR activities were comparable in wild-type and mutant plants grown on NO(3) (-), and roots of both plant types lacked constitutive NR activity when grown on urea. In both growth chamber- and field-grown plants, oxides of nitrogen [NO((x))] were evolved from young leaves of wild-type plants, but not from leaves of LNR-2 plants, during in vivo NR assays. Analysis of leaves from different canopy locations showed that constitutive NR activity was confined to the youngest three fully expanded leaves of the wild-type plant and, therefore, on a total plant canopy basis, the NR activity of LNR-2 plants was approximately 75% that of wild-type plants. It is concluded that: (a) the NR activity in leaves of NO(3) (-)-grown wild-type plants includes both constitutive and inducible activity; (b) the missing NR activity in LNR-2, LNR-3, and LNR-4 leaves is the constitutive component; and (c) the constitutive NR activity is associated with NO((x)) evolution and occurs only in physiologically young leaves.

Soybean Mutants Lacking Constitutive Nitrate Reductase Activity