The effects of ammonium ion and glutamate on CO_2 fixation abilities and related carbon metabolism were investigated in pea (Pisum sativum L. cv. Sparkle) leaf discs under conditions favoring photorespiration (21% O_2, 0.03% CO_2) and nonphotorespiration (5% O_2, 0.03% CO_2). A concentration of more than 10 mM of NH_4^+ decreased the photosynthetic CO_2 fixation and those inhibitory effects were more remarkable in 21% O_2 than in 5% O_2 conditions. The effect of glutamate on CO_2 fixation was found to be independent of the O_2 level, as glutamate increased the CO_2 fixation under both 21% and 5% O_2 conditions. L-methionine-dl-sulfoximine, an irreversible inhibitor of glutamate synthetase, however, inhibited the CO_2 fixation markedly under 21% O_2, but did not affecte it under 5% O_2 conditions.
The treatment with NH_4^+ elevated the relative amounts of ^14C incorporated into soluble components from ^14CO_2 with no relation to O_2 levels, while glutamate increased ^14C into insoluble components and neutral sugars. Glutamate, especially, seemed to stimulate the biosynthesis of starch under 5% O_2 condition. These results indicated that NH_4^+ stimulated the degradation of sugar or starch and this proposal was condifirmed by the increasing of pyruvate kninase activity in leaf discs treated with ammonium ion.
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