Nitrogen Isotope Fractionation Associated with Nitrate Reductase Activity and Uptake of NO3− by Pearl Millet

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Abstract
N isotope fractionation by pearl millet (P. americanum L. and P. mollissimum L.) grown on nitrate was associated with nitrate reductase activity. Fractionation was evidenced at the step of nitrate reduction when the substrate-to-enzyme ratio was high (possibly saturating for the active sites of the nitrate reductase enzyme), e.g., in young seedlings having a low nitrate reductase activity or in seedlings grown with a high nitrate concentration. When the substrate concentration was low and the active sites of the enzyme were possibly not saturated, the isotopic discrimination could only be associated with the uptake of nitrate into the cell. In that case, isotopic fractionation was null. Evidently, the uptake of nitrate does not discriminate among N isotopes. [To use the variation in the natural abundance of 15N as the basis for estimating N2-fixation, it is necessary to know whether there is a significant alteration of the isotopic content (isotopic fractionation) as atmospheric N2 moves from its origin into plant tissue.].