OXYGEN AND ELECTRON FLOW IN C-4 PHOTOSYNTHESIS - MEHLER REACTION, PHOTORESPIRATION AND CO2 CONCENTRATION IN THE BUNDLE-SHEATH

The photosynthetic linear electron transport rate in excess of that us ed for CO2 reduction was evaluated in Sorghum bicolor Moench. [:NADP-m alic enzyme (ME)-type C-4 plant], Amaranthus cruentus L. (NAD-ME-type C-4 plant) and Helianthus annuus L. (C-3 plant) leaves at different CO 2 and O-2 concentrations. The electron transport rate (J(F)) was calcu lated 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 mu mol CO2 mol(-1) and 2% O-2 Under high light intensities there was a la rge excess of J(F)/4 at 10-100% O-2 in the C-3 plant due to photorespi ration, but very little in sorghum and somewhat more in amaranth, show ing that photorespiration is suppressed, more in the NADP-ME- and less in the NAD-ME-type species. It is concluded that when C-4 photosynthe sis is limited by supply of atmospheric CO2 to the C-4 cycle, the C3 c ycle becomes limited by regeneration of ribulose 1,5-bisphosphate (RuB P) which in turn limits RuBP oxygenase activity and photorespiration. The rate of excess electron transport over that consumed for CO2 fixat ion in C-4 plants was very sensitive to the presence of O-2 in the gas phase, rapidly increasing between 0.01 and 0.1% O-2, and at 2% O-2 it was about two-thirds of that at 21% O-2 This shows the importance of the Mehler O-2 reduction as an electron sink, compared with photorespi ration in C-4 plants. However, the rate of the Mehler reaction is stil l too low to fully account for the extra ATP which is needed in C-4 ph otosynthesis.