Regulation of Ribulose-1,5-Bisphosphate Carboxylase Activity in Response to Changing Partial Pressure of O2 and Light in Phaseolus vulgaris

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Abstract
The regulation of ribulose-1,5-bisphosphate (RuBP) carboxylase (rubisco) activity in Phaseolus vulgaris was studied under moderate CO2 and high light, conditions in which photosynthesis in C3 plants can be insensitive to changes in O2 partial pressure. Steady state RuBP concentrations were higher, the calculated rate of RuBP use was lower and the activation state of rubisco was lower in low O2 relative to values observed in normal O2. It is suggested that the reduced activity of rubisco observed here is related to feedback effects which occur when the rate of net CO2 assimilation approaches the maximum capacity for starch and sucrose synthesis (triose phosphate utilization). The activation state of rubisco was independent of O2 partial pressure when light or CO2 was limiting for photosynthesis. Reduced activity of rubisco was also observed at limiting light. However, in this species light dependent changes in the concentration of an inhibitor of rubisco controlled the apparent Vmax of rubisco in low light while changes in the CO2-Mg2+ dependent activation of rubisco controlled the apparent Vmax in high light.