ABSTRACT
Seven day old wheat and maize seedlings were exposed to 1300 or 2000 microeinsteins per square meter per second photosynthetically active radiation in CO(2)-free air for 3 hours with either 1% O(2) in N(2) or N(2)-only and then returned to normal air of 340 microliters per liter CO(2), 21% O(2) in N(2). Activity of the ribulose bisphosphate carboxylase and amount of the substrate, ribulose 1,5-bisphosphate, were measured during and following the CO(2)-free treatments as was photosynthetic CO(2) fixation. Photoinhibition of photosynthesis was observed only with wheat seedlings following the N(2) only treatment. During the CO(2)-free treatments, the levels of RuBP rose during all experiments except when wheat was photoinhibited. The activity of the ribulose bisphophate carboxylase, measured directly upon grinding the leaves, declined during the CO(2)-free conditions. The carboxylase total activity increased in minutes in the leaf during and following the CO(2)-free treatments. The specific activities of the wheat carboxylase went from 0.16 to 1.06 micromoles CO(2) fixed per milligram protein per minute while the maize carboxylase varied from 0.05 to 0.36 micromole CO(2) fixed per millogram protein per minute. This suggests that in these seedlings considerable inactive carboxylase must be stored in a form not activatable in extracts by CO(2) and Mg(2+). Possible mechanisms of regulation of photosynthesis by the ribulose bisphosphate carboxylase must consider not only the amount of active enzyme, but the amount of enzyme which the plant can make activatable upon demand.
