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Nitrogen Isotope Fractionation Associated with Nitrate Reductase Activity and Uptake of NO₃^– by Pearl Millet ¹

Université Pierre et Marie Curie, Laboratoire de Géologie dynamique, 75230 PARIS Cedex 05, France, Université de Paris-Sud, Laboratoire de Physiologie Cellulaire Végétale, 91405 ORSAY Cedex, France, Laboratoire de Microbiologie des Sols, Institut National de la Recherche Agronomique, 21034 DIJON Cedex, France

Nitrogen isotope fractionation by Pearl Millet (Pennisetum 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), for instance in young seedlings having a low nitrate reductase activity or in seedlings grown on high nitrate concentration.

When the substrate concentration was low (and, hence, 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. It is concluded that the uptake of nitrate does not discriminate among nitrogen isotopes.

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