Regulation of nitrate reduction by light, ATP and mitochondrial respiration in wheat leaves

Abstract

ASSIMILATION of nitrate in leaves is closely linked with photosynthesis^1,2 because ammonia, the end product of the former process, is incorporated into amino acids by means of carbon skeletons derived from the assimilation of CO₂. Canvin and Atkins^3,4 have shown that the reduction of nitrate to nitrite in leaves of barley is dependent on light and ceases when the light is extinguished. On the other hand, in the in vivo⁵ method for assay of nitrate reductase, nitrate is reduced to nitrite by leaf disks even in the dark. As Canvin and Atkins^3,4 pointed out, the in vivo method is not a true reflection of what happens in plants in physiological conditions. A regulatory mechanism must exist in leaves, which shuts off nitrate reduction immediately when light is extinguished, so that the accumulation of toxic amounts of nitrite, which can only be reduced by photosynthetic reactions⁶, is avoided. We now propose that this regulation functions through mitochondrial respiration, which operates in the dark, but is inhibited in light, because of the increased cytoplasmic adenylate charge due to photosynthesis.