Browsing by Author "Ku, Maurice S. B."
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- Oxygen requirement and inhibition of C4 photosynthesisPublication . Maroco, João; Ku, Maurice S. B.; Lea, Peter J.; Dever, Louisa V.; Leegood, Richard C.; Furbank, Robert T.; Edwards, Gerald E.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.
- Oxygen sensitivity of C4 photosynthesis: Evidence from gas exchange and chlorophyll fluorescence analyses with different C4 subtypesPublication . Maroco, João; Ku, Maurice S. B.; Edwards, Gerald E.Because photosynthetic rates in C4 plants are the same at normal levels of O2 (c. 20 kPa) and at c. 2 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 α fluorescence. Photosystem II activity, measured from chlorophyll α fluorescence analysis, was not inhibited at levels of O2 that were above the optimum for CO2 assimilation, which is consistent with a compensating, alternative electron flow as net CO2 assimilation is inhibited. At suboptimurn 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 CO2assimilation 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 cylcle as a result of inhibition of its production photochemically.
- Utilization of O2 in the metabolic optimization of C4 photosynthesisPublication . Maroco, João; Ku, Maurice S. B.; Edwards, Gerald E.The combined effects of O2 on net rates of photosynthesis, photosystem II activity, steady-state pool size of key metabolites of photosynthetic metabolism in the C4 pathway, C3 pathway and C2 photorespiratory cycle and on growth were evaluated in the C4 species Amaranthus edulis and the C3 species Flaveria pringlei. Increasing O2 reduced net CO2 assimilation in F. pringlei due to an increased flux of C through the photorespiratory pathway. However, in A. edulis increasing O2 up to 5–10% stimulated photosynthesis. Analysis of the pool size of key metabolites in A. edulis suggests that while there is some O2 dependent photorespiration, O2 is required for maximizing C4 cycle activity to concentrate CO2 in bundle sheath cells. Therefore, the response of net photosynthesis to O2 in C4 plants may result from the balance of these two opposing effects. Under 21 versus 5% O2, growth of A. edulis was stimulated about 30% whereas that of F. pringlei was inhibited about 40%.