Pathway for Nitrate Assimilation in Corn (Zea mays L.) Leaves

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Abstract
The localization of enzymes responsible for nitrate assimilation and the generation of NADH for nitrate reduction were studied in corn (Z. mays L.) leaf blades. The techniques used effectively separated mesophyll and bundle sheath cells as judged by microscopic observations, enzymic assays, chlorophyll a/b ratios and photochemical activities. Nitrate reductase, nitrite reductase and the nitrite content of leaf blades were localized primarily in the mesophyll cells, although some nitrite reductase was found in the bundle sheath cells. Glutamine synthetase, NAD-malate dehydrogenase, NAD-glyceraldehyde-3-phosphate dehydrogenase and NADP-glutamate dehydrogenase were found in both types of cells. However, more NADP-glutamate dehydrogenase was found in the bundle sheath cells than in the mesophyll cells. The mesophyll cells are the major site for nitrate assimilation in the leaf blade because they contained an ample supply of nitrate and the enzymes considered essential for the assimilation of nitrate into amino acids. Because the specific activity of nitrate reductase was several-fold lower than other enzymes involved in nitrate assimilation, nitrate reduction is indicated as the rate-limiting step in situ. A sequence of reactions is proposed for nitrate assimilation in the mesophyll cells of corn leaves are related to the C-4 pathway of photosynthesis.