OXYGEN SENSITIVITY OF C-4 PHOTOSYNTHESIS - EVIDENCE FROM GAS-EXCHANGEAND CHLOROPHYLL FLUORESCENCE ANALYSES WITH DIFFERENT C-4 SUBTYPES

Because photosynthetic rates in C-4 plants are the same at normal leve ls of O-2 (c. 20 kPa) and at c. 2 kPa O-2 (a conventional test for eva luating photorespiration in Cg plants) it has been thought that C-4 ph otosynthesis is O-2 insensitive. However, we have found a dual effect of O-2 on the net rate of CO2 assimilation among species representing all three C-4 subtypes from both monocots and dicots, The optimum O-2 partial pressure for C-4 photosynthesis at 30 degrees C, atmospheric C O2 level, and half full sunlight (1000 mu mol quanta m(-2) s(-1)) was about 5-10 kPa, Photosynthesis was inhibited by O-2 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 inhi bition of photosynthesis by supra-optimum levels of O-2 and a large de crease in the ratio of quantum yield of CO2 fixation/quantum yield of photosystem II (PSII) measured by chlorophyll a fluorescence, Photosys tem II activity, measured from chlorophyll a fluorescence analysis, wa s not inhibited at levels of O-2 that were above the optimum for CO2 a ssimilation, which is consistent with a compensating, alternative elec tron flow as net CO2 assimilation is inhibited, At suboptimum levels o f O-2, however, the inhibition of photosynthesis was paralleled by an inhibition of PSII quantum yield, increased state of reduction of quin one A, and decreased efficiency of open PSII centres, These results wi th different C-4 types suggest that inhibition of net CO2 assimilation with increasing O-2 partial pressure above the optimum is associated with photorespiration, and that inhibition below the optimum O-2 may b e caused by a reduced supply of ATP to the C-4 cycle as a result of in hibition of its production photochemically.