Effects of severe CO2 starvation on the photosynthetic electron transport chain in tobacco plants

Tobacco plants (Nicotiana tabacum) were kept in CO2 free air for several da ys to investigate the effect of lack of electron acceptors on the photosynt hetic electron transport chain. CO2 starvation resulted in a dramatic decre ase in photosynthetic activity. Measurements of the electron transport acti vity in thylakoid membranes showed that a loss of Photosystem II activity w as mainly responsible for the observed decrease in photosynthetic activity. In the absence of CO2 the plastoquinone pool and the acceptor side of Phot osystem 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 CO2 free air retarded photoinhibitory loss of ph otosynthetic activity and pigment degradation. Electron flow to oxygen seem ed not to be able to counteract the stress induced by severe CO2 starvation . The data are discussed in terms of a donation of reducing equivalents fro m mitochondria to chloroplasts and a reduction of the plastoquinone pool vi a the NAD(P)H-plastoquinone oxidoreductase during CO2 starvation.