Effects of severe CO(2) starvation on the photosynthetic electron transport chain in tobacco plants.

Tobacco plants (Nicotiana tabacum) were kept in CO(2) free air for several days to investigate the effect of lack of electron acceptors on the photosynthetic electron transport chain. CO(2) starvation resulted in a dramatic decrease in photosynthetic activity. Measurements of the electron transport activity in thylakoid membranes showed that a loss of Photosystem II activity was mainly responsible for the observed decrease in photosynthetic activity. In the absence of CO(2) the plastoquinone pool and the acceptor side of Photosystem I were highly reduced in the dark as shown by far-red light effects on chlorophyll fluorescence and P700 absorption measurements. Reduction of the oxygen content of the CO(2) free air retarded photoinhibitory loss of photosynthetic activity and pigment degradation. Electron flow to oxygen seemed not to be able to counteract the stress induced by severe CO(2) starvation. The data are discussed in terms of a donation of reducing equivalents from mitochondria to chloroplasts and a reduction of the plastoquinone pool via the NAD(P)H-plastoquinone oxidoreductase during CO(2) starvation.