Feedback limitation of photosynthesis at high CO acts by modulating the activity of the chloroplast ATP synthase.

It was previously shown that photosynthetic electron transfer is controlled under low CO via regulation of the chloroplast ATP synthase. In the current work, we studied the regulation of photosynthesis under feedback limiting conditions, where photosynthesis is limited by the capacity to utilise triose-phosphate for synthesis of end products (starch or sucrose), in a starch-deficient mutant of Nicotiana sylvestris Speg. & Comes. At high CO, we observed feedback control that was progressively reversed by increasing O levels from 2 to 40%. The activity of the ATP synthase, probed in vivo by the dark-interval relaxation kinetics of the electrochromic shift, was proportional to the O-induced increases in O evolution from PSII (J), as well as the sum of Rubisco oxygenation (v) and carboxylation (v) rates. The altered ATP synthase activity led to changes in the light-driven proton motive force, resulting in regulation of the rate of plastoquinol oxidation at the cytochrome bf complex, quantitatively accounting for the observed control of photosynthetic electron transfer. The ATP content of the cell decreases under feedback limitation, suggesting that the ATP synthesis was downregulated to a larger extent than ATP consumption. This likely resulted in slowing of ribulose bisphosphate regeneration and J). Overall, our results indicate that, just as at low CO, feedback limitations control the light reactions of photosynthesis via regulation of the ATP synthase, and can be reconciled with regulation via stromal P, or an unknown allosteric affector.