, Volume 116, Issue 3, pp 207-224

Nitrate, nitrite and ammonia assimilation by leaves: Effect of light, carbon dioxide and oxygen

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The assimilation of nitrate, nitrite and ammonia in barley, wheat, corn and bean leaves was studied using 15N-labelled molecules and either leaf chamber experiments with the uptake of the nitrogen species in the transpiration stream, or vacuum-infiltration experiments. The assimilation of 15NO3 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 N2 there 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 N2 and subsequently resupplied with air. Incorporation of 15NO2 was also markedly dependent on light with little reduction occurring in the dark. Incorporation of 15NH4 + 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 of 15NO2 or 15NH4 +. Nitrite at 1 mM had no effect on 15NO3 incorporation but at 10 mM inhibited it completely after 0.5 h. Ammonia at 1 mM had no effect on 15NO3 or 15NO2 incorporation and while 10 mM inhibited incorporation for 0.5 h this inhibition did not persist.