The rate of nitrite reduction in leaves as indicated by O(2) and CO(2) exchange during photosynthesis

Light response (at 300 ppm CO(2) and 10–50 ppm O(2) in N(2)) and CO(2) response curves [at absorbed photon fluence rate (PAD) of 550 μmol m(−2) s(−1)] of O(2) evolution and CO(2) uptake were measured in tobacco (Nicotiana tabacum L.) leaves grown on either NO(3)(−) or NH(4)(+) as N source and in potato (Solanum tuberosum L.), sorghum (Sorghum bicolor L. Moench), and amaranth (Amaranthus cruentus L.) leaves grown on NH(4)NO(3). Photosynthetic O(2) evolution in excess of CO(2) uptake was measured with a stabilized zirconia O(2) electrode and an infrared CO(2) analyser, respectively, and the difference assumed to represent the rate of electron flow to acceptors alternative to CO(2), mainly NO(2)(−), SO(4)(2−), and oxaloacetate. In NO(3)(−)-grown tobacco, as well as in sorghum, amaranth, and young potato, the photosynthetic O(2)–CO(2) flux difference rapidly increased to about 1 μmol m(−2) s(−1) at very low PADs and the process was saturated at 50 μmol quanta m(−2) s(−1). At higher PADs the O(2)–CO(2) flux difference continued to increase proportionally with the photosynthetic rate to a maximum of about 2 μmol m(−2) s(−1). In NH(4)(+)-grown tobacco, as well as in potato during tuber filling, the low-PAD component of surplus O(2) evolution was virtually absent. The low-PAD phase was ascribed to photoreduction of NO(2)(−) which successfully competes with CO(2) reduction and saturates at a rate of about 1 μmol O(2) m(−2) s(−1) (9% of the maximum O(2) evolution rate). The high-PAD component of about 1 μmol O(2) m(−2) s(−1), superimposed on NO(2)(−) reduction, may represent oxaloacetate reduction. The roles of NO(2)(−), oxaloacetate, and O(2) reduction in the regulation of ATP/NADPH balance are discussed.