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Articles

Expression of Nitrate and Nitrite Reductase Activities under Various Forms of Nitrogen Nutrition in Phaseolus vulgaris L. ¹

Department of Biochemistry and Microbiology, Cook College, New Jersey Agricultural Experiment Station, Rutgers University, New Brunswick, New Jersey 08903

The main objectives of this work were to study the effect of different N sources on plant growth, N accumulation, and on the expression of nitrate reductase activity in Phaseolus vulgaris L. leaves. Plants were grown under greenhouse conditions (15 to 25 kilolux; 16/8 hour day/night cycles) in plastic pots filled with perlite: vermiculite (1:1) and watered daily with a minus N solution (N₂ plants) or supplemented with either KNO₃, (NH₄)₂SO₄, or urea as combined N sources.

Significant levels of nitrate reductase activity in trifoliolate leaves of N₂-, NH₄⁺-, urea-, or NO₃^–-dependent plants was demonstrated throughout this work. Leaves from the urea- or NH₄⁺-grown plants accumulated NO₂^– in the dark but not in the light when NO₂^– was supplied by vacuum infiltration. These results indicated that the potential for reduction of NO₃^– or NO₂^– was not impaired by growing the plants on NH₄⁺ or urea and, in addition, provided evidence for the occurrence of a non-nitrate-inducible nitrite reductase. The nitrate reductase activities associated with N₂-, NH₄⁺-, or urea-dependent plants are tentatively regarded as `constitutive' to differentiate from the widely occurring NO₃^–-inducible nitrate reductase activity.

Plants grown on NO₃^– or urea accumulated significantly larger amounts of reduced N and dry matter as compared to NH₄⁺- and N₂-dependent plants. Regardless of N treatment, or size of plants, about 50% of the N accumulated by the plant was allocated to the leaves.

¹ New Jersey Agricultural Experiment Station Publication No. D-01103-2-82, supported by State funds and by United States Hatch Act funds. Partial support was provided by a Rutgers Council grant to C. A. N.