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Abstract(s)
The basis for O2 sensitivity of C4 photosynthesis was evaluated
using a C4-cycle-limited mutant of Amaranthus edulis (a phosphoenolpyruvate
carboxylase-deficient mutant), and a C3-cyclelimited
transformant of Flaveria bidentis (an antisense ribulose-1,5-
bisphosphate carboxylase/oxygenase [Rubisco] small subunit
transformant). Data obtained with the C4-cycle-limited mutant
showed that atmospheric levels of O2 (20 kPa) caused increased
inhibition of photosynthesis as a result of higher levels of photorespiration.
The optimal O2 partial pressure for photosynthesis was
reduced from approximately 5 kPa O2 to 1 to 2 kPa O2, becoming
similar to that of C3 plants. Therefore, the higher O2 requirement
for optimal C4 photosynthesis is specifically associated with the C4
function. With the Rubisco-limited F. bidentis, there was less inhibition
of photosynthesis by supraoptimal levels of O2 than in the
wild type. When CO2 fixation by Rubisco is limited, an increase in
the CO2 concentration in bundle-sheath cells via the C4 cycle may
further reduce the oxygenase activity of Rubisco and decrease the
inhibition of photosynthesis by high partial pressures of O2 while
increasing CO2 leakage and overcycling of the C4 pathway. These
results indicate that in C4 plants the investment in the C3 and C4
cycles must be balanced for maximum efficiency.
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Citation
Plant Physiology, 116, 823-832
Publisher
American Society of Plant Biologists