The assimilation of nitrate, nitrite and ammonia in barley, wheat, corn and bean leaves was studied using15N-labelled molecules and either leaf chamber experiments with the uptake of the nitrogen species in the transpiration stream, or vacuum-infiltration experiments. The assimilation of15NO3−into amino nitrogen was strictly dependent on light and ceased abruptly when the light was extinguished. If the leaves were exposed to air, CO2-free air or N2there was no effect on the rate of NO3−assimilation over 0.5 h. After 1.25 h of CO2-free air, NO3−assimilation into amino acids was sharply reduced. Resupply of air at this time stimulated NO3−assimilation and restored it to the rate observed in leaves exposed to air only. There was no recovery by tissue pretreated for 1.25 h in N2and subsequently resupplied with air. Incorporation of15NO2−was also markedly dependent on light with little reduction occurring in the dark. Incorporation of15NH4+into amino acids was stimulated 5 fold by light but considerable incorporation occurred in the dark. The presence of 100 mM NO3−had no effect on the rate of incorporation of15NO2−or15NH4+. Nitrite at 1 mM had no effect on15NO3−incorporation but at 10 mM inhibited it completely after 0.5 h. Ammonia at 1 mM had no effect on15NO3−or15NO2−incorporation and while 10 mM inhibited incorporation for 0.5 h this inhibit
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