Photosynthesis and Activity of Ribulose Bisphosphate Carboxylase of Wheat and Maize Seedlings during and following Exposure to O2-Low CO2-Free N2

摘要

Seven day old wheat and maize seedlings were exposed to 1300 or 2000 microeinsteins per square meter per second photosynthetically active radiation in CO2-free air for 3 hours with either 1% O2 in N2 or N2-only and then returned to normal air of 340 microliters per liter CO2, 21% O2 in N2. Activity of the ribulose bisphosphate carboxylase and amount of the substrate, ribulose 1,5-bisphosphate, were measured during and following the CO2-free treatments as was photosynthetic CO2 fixation. Photoinhibition of photosynthesis was observed only with wheat seedlings following the N2 only treatment. During the CO2-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 CO2-free conditions. The carboxylase total activity increased in minutes in the leaf during and following the CO2-free treatments. The specific activities of the wheat carboxylase went from 0.16 to 1.06 micromoles CO2 fixed per milligram protein per minute while the maize carboxylase varied from 0.05 to 0.36 micromole CO2 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 CO2 and Mg²⁺. 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.