CO2 and O2 dependence of PS II activity in C4 plants having genetically produced deficiencies in the C3 or C4 cycle

The CO2 dependence of rates of CO2 fixation (A) and photochemistry of PS II at 5, 15 and 30% O-2 were analyzed in the C-4 plant Amaranthus edulis having a C-4 cycle deficiency [phosphoenolpyruvate carboxylase (PEPC) mutants], and in the C-4 plant Flaveria bidentis having a C-3 cycle deficiency [Rubisco small subunit antisense (alpha SSU)]. In the wild type (WT) A. edulis and its heterozygous mutant having less than 50% WT PEPC activity there was a similar dependence of A and PS II photochemistry on varying CO2, although the CO2 saturated rates were 25% lower in heterozygous plants. The homozygous plants having less than 2% PEPC of the WT had significant levels of photorespiration at ambient levels of CO2 and required about 30 times ambient levels for maximum rates of A. Despite variation in the capacity of the Cs cycle, more than 91% of PS II activity was linearly associated with A under varying CO2 at 5, 15 and 30% O-2. However, the WT plant had a higher PS II activity per CO2 fixed under saturating CO2 than the homozygous mutant, which is suggested to be due to elimination of the C-4 cycle and its associated requirement for ATP from a Mehler reaction. In the alpha SSU F. bidentis plants, a decreased rate of A (35%) and PS II activity (33%) accompanied a decrease in Rubisco capacity. There was some increase in alternative electron sinks at high CO2 when the C-3 cycle was constrained, which may be due to increased flux through the C-4 cycle via an ATP generating Mehler reaction. Nevertheless, even with constraints on the function of the C-4 or C-3 cycle by genetic modifications, analyses of CO2 response curves under varying levels of O-2 indicate that CO2 assimilation is the main determinant of PS II activity in C-4 plants.