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

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 V_max of rubisco in low light while changes in the CO₂-Mg²⁺ dependent activation of rubisco controlled the apparent V_max in high light.