Effects of Carbon Dioxide and Oxygen on the Regulation of Photosynthetic Carbon Metabolism by Ammonia in Spinach Mesophyll Cells

Photosynthetic carbon metabolism of isolated spinach mesophyll cells was characterized under conditions favoring photorespiratory (PR; 0.04% CO(2) and 20% O(2)) and nonphotorespiratory (NPR; 0.2% CO(2) and 2% O(2)) metabolism, as well as intermediate conditions. Comparisons were made between the metabolic effects of extracellularly supplied NH(4) (+) and intracellular NH(4) (+), produced primarily via PR metabolism. The metabolic effects of (14)CO(2) fixation under PR conditions were similar to perturbations of photosynthetic metabolism brought about by externally supplied NH(4) (+); both increased labeling and intracellular concentrations of glutamine at the expense of glutamate and increased anaplerotic synthesis through alpha-ketoglutarate. The metabolic effects of added NH(4) (+) during NPR fixation were greater than those during PR fixation, presumably due to lower initial NH(4) (+) levels during NPR fixation. During PR fixation, addition of ammonia caused decreased pools and labeling of glutamate and serine and increased glycolate, glyoxylate, and glycine labeling. The glycolate pathway was thus affected by increased rates of carbon flow and decreased glutamate availability for glyoxylate transamination, resulting in increased usage of serine for transamination. Sucrose labeling decreased with NH(4) (+) addition only during PR fixation, suggesting that higher photosynthetic rates under NPR conditions can accommodate the increased drain of carbon toward amino acid synthesis while maintaining sucrose synthesis.