Oxygen and electron flow in C 4 photosynthesis: Mehler reaction, photorespiration and CO 2 concentration in the bundle sheath

Abstract

The photosynthetic linear electron transport rate in excess of that used for CO2 reduction was evaluated in Sorghum bicolor Moench. [NADP-malic enzyme (ME)-type C4 plant], Amaranthus cruentus L. (NAD-ME-type C4 plant) and Helianthus annuus L. (C3 plant) leaves at different CO2 and O2 concentrations. The electron transport rate (JF) was calculated from fluorescence using the light partitioning factor (relative PSII cross-section) determined under conditions where excess electron transport was assumed to be negligible: low light intensities, 500 μmol CO2 · mol−1 and 2% O2. Under high light intensities there was a large excess of JF/4 at 10–100% O2 in the C3 plant due to photorespiration, but very little in sorghum and somewhat more in amaranth, showing that photorespiration is suppressed, more in the NADP-ME- and less in the NAD-ME-type species. It is concluded that when C4 photosynthesis is limited by supply of atmospheric CO2 to the C4 cycle, the C3 cycle becomes limited by regeneration of ribulose 1,5-bisphosphate (RuBP) which in turn limits RuBP oxygenase activity and photorespiration. The rate of excess electron transport over that consumed for CO2 fixation in C4 plants was very sensitive to the presence of O2 in the gas phase, rapidly increasing between 0.01 and 0.1% O2, and at 2% O2 it was about two-thirds of that at 21% O2. This shows the importance of the Mehler O2 reduction as an electron sink, compared with photorespiration in C4 plants. However, the rate of the Mehler reaction is still too low to fully account for the extra ATP which is needed in C4 photosynthesis.