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Oxygen sensitivity of C 4 photosynthesis: evidence from gas exchange and chlorophyll fluorescence analyses with different C 4 subtypes
Author(s) -
MAROCO J. P.,
KU M. S. B.,
EDWARDS G. E.
Publication year - 1997
Publication title -
plant, cell and environment
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.646
H-Index - 200
eISSN - 1365-3040
pISSN - 0140-7791
DOI - 10.1046/j.1365-3040.1997.d01-41.x
Subject(s) - photosynthesis , photorespiration , photosystem ii , chlorophyll fluorescence , quantum yield , chlorophyll , oxygen , chemistry , chlorophyll a , c4 photosynthesis , photochemistry , botany , fluorescence , analytical chemistry (journal) , biology , environmental chemistry , physics , organic chemistry , quantum mechanics
Because photosynthetic rates in C 4 plants are the same at normal levels of O 2 ( c , 20 kPa) and at c , 2 kPa O 2 (a conventional test for evaluating photorespiration in C 3 plants) it has been thought that C 4 photosynthesis is O 2 insensitive. However, we have found a dual effect of O 2 on the net rate of CO 2 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 °C, atmospheric CO 2 level, and half full sunlight (1000 μmol quanta m −2 s −1 ) was about 5–10 kPa. Photosynthesis was inhibited by O 2 below or above the optimum partial pressure. Decreasing CO 2 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 O 2 and a large decrease in the ratio of quantum yield of CO 2 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 O 2 that were above the optimum for CO 2 assimilation, which is consistent with a compensating, alternative electron How as net CO 2 assimilation is inhibited. At suboptimum levels of O 2 , 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 C 4 types suggest that inhibition of net CO 2 assimilation with increasing O 2 partial pressure above the optimum is associated with photorespiration, and that inhibition below the optimum O 2 may be caused by a reduced supply of ATP to the C 4 cycle as a result of inhibition of its production photochemically.