Evidence for a physiological role of CO 2 in the regulation of photosynthetic electron transport in intact leaves

Abstract

The effect of CO 2 upon photosynthetic electron transport was examined in wheat and maize leaves in order to establish whether CO 2 had a direct role in electron-transport regulation in vivo. When intercellular CO 2 was depleted a transient rise in chlorophyll fluorescence occurred which correlated with an increase in the reduction of the Photosystem II primary quinone acceptor, Q A, and a decrease in CO 2 fixation rate. However, when intercellular CO 2 was reduced from an already low level (50 μmol·mol −1) towards zero a substantial further reduction in Q A occurred with little change in fluorescence or CO 2 fixation. In very low intercellular CO 2 when no measurable CO 2 fixation was sustained, an appreciable fraction of Q A still remained oxidised, however, maximal reduction of Q A occurred when O 2 was also removed. Q A could then be substantially reoxidised by the readdition of small amounts of CO 2 (20–40 μmol) which only facilitated a very small increase in CO 2 fixation. Changes in the kinetics of the fast rise in fluorescence emission indicated that Q A-to-Q B electron transfer was decreased in a CO 2-free atmosphere and Q B was poised in a more oxidised state. Electron transport that was independent of CO 2 fixation was measured in methyl viologen-treated leaf discs. In 1% O 2, but not in 21% O 2, light-dependent electron transport to methyl viologen was decreased significantly by the depletion of CO 2. It is concluded that CO 2 can modify the redox state of Photosystem II electron transport acceptors in vivo independently of carbon assimilation and that there is a complex interaction between CO 2 and O 2 in the regulation of photosynthetic electron transport. The possibility that CO 2 operates via the reversible binding to PS II and thereby acts as a cofactor for efficient PS II electron transport in the leaf is discussed.