The Effect of Nitrate and Ammonium Feeding on Carbon Dioxide Assimilation in Maize

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Abstract
The effect of nitrogen nutrition on the pattern of 14CO2 fixation during photosynthesis by young maize plants was studied. Nitrate feeding increased the incorporation of 14C into malate and certain amino acids (particularly aspartate) and decreased that into sucrose and starch. Ammonium nutrition caused similar, but not identical, changes, the major difference being a much smaller effect on incorporation into malate. Tungstate applied to the plants prior to nitrate almost completely inhibited the formation of nitrate reductase and eliminated most, but not all, of the effects of nitrate on the fixation pattern. The results obtained are discussed in terms of: (i) more reduced nitrogen as substrate for amino acid synthesis; (ii) the changes in pH and ionic balance caused by the process of nitrate reduction to ammonium; and (iii) specific enzyme activation or inhibition by nitrate or ammonium ions.

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