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

The effect of nitrogen nutrition on the pattern of ¹⁴CO₂ fixationduring photosynthesis by young maize plants was studied. Nitratefeeding increased the incorporation of ¹⁴C into malate and certainamino acids (particularly aspartate) and decreased that intosucrose and starch. Ammonium nutrition caused similar, but notidentical, changes, the major difference being a much smallereffect on incorporation into malate. Tungstate applied to theplants prior to nitrate almost completely inhibited the formationof nitrate reductase and eliminated most, but not all, of theeffects of nitrate on the fixation pattern. The results obtainedare discussed in terms of: (i) more reduced nitrogen as substratefor amino acid synthesis; (ii) the changes in pH and ionic balancecaused by the process of nitrate reduction to ammonium; and(iii) specific enzyme activation or inhibition by nitrate orammonium ions.