Oxygen sensitivity of C4 photosynthesis: Evidence from gas exchange and chlorophyll fluorescence analyses with defferent C4 subtypes

Abstract

Because photosynthetic rates in C4 plants are the same at normal levels of O2 (c. 20 kPa) and at c. kPa O2 (a conventional test for evaluating photorespiration in C3 plants) it has been thought that C4 photosynthesis is O2 insensitive. However, we have found a dual effect of O2 on the net rate of CO2 assimilation among species representing all three C4 subtypes from both monocots and dicots. The optimum O2 partial pressure for C4 photosynthesis at 30 °C, atmospheric CO2 level, and half full sunlight (1000 μmol quanta m-2 s-1) was about 5-10 kPa. Photosynthesis was inhibited by O2 below or above the optimum partial pressure. Decreasing CO2 levels from ambient levels (32·6 Pa) to 9·3 Pa caused a substantial increase in the degree of inhibition of photosynthesis by supra-optimum levels of O2 and a large decrease in the ratio of quantum yield of CO2 fixation/quantum yield of photosystem II (PSII) measured by chlorophyll a fluorescence. Photosystem II activity, measured from chlorophyll a fluorescence analysis, was not inhibited at levels of O2 that were above the optimum for CO2 assimulation, which is consistent with a compensating, alternative electron flow as net CO2 assimilation is inhibited. At suboptimum levels of O2, however, the inhibition of photosynthesis was paralleled by an inhibition of PSII quantum yield, increased state of reduction of quinone A, and decreased efficiency of open PSII centres. These results with different C4 types suggest that inhibition of net CO2 assimilation with increasing O2 partial pressure above the optimum is associated with photorespiration, and that inhibition below the optimum O2 may be caused by a reduced supply of ATP to the C4 cycle as a result of inhibition of its production photochemically.