REDUCED LEVELS OF CYTOCHROME-B(6)LF IN TRANSGENIC TOBACCO INCREASES THE EXCITATION PRESSURE ON PHOTOSYSTEM-II WITHOUT INCREASING SENSITIVITY TO PHOTOINHIBITION IN-VIVO

We have examined tobacco transformed with an antisense construct again st the Rieske-FeS subunit of the cytochrome b(6)f complex, containing only 15 to 20% of the wild-type level of cytochrome f. The anti-Rieske FeS leaves had a comparable chlorophyll and Photosystem II reaction c enter stoichiometry and a comparable carotenoid profile to the wild-ty pe, with differences of less than 10% on a leaf area basis. When expos ed to high irradiance, the anti-Rieske-FeS leaves showed a greatly inc reased closure of Photosystem II and a much reduced capacity to develo p non-photochemical quenching compared with wild-type. However, contra ry to our expectations, the anti-Rieske-FeS leaves were not more susce ptible to photoinhibition than were wild-type leaves. Further, when we regulated the irradiance so that the excitation pressure on photosyst em II was equivalent in both the anti-Rieske-FeS and wild-type leaves, the anti-Rieske-FeS leaves experienced much less photoinhibition than wild-type. The evidence from the anti-Rieske-FeS tobacco suggests tha t rapid photoinactivation of Photosystem II in vivo only occurs when c losure of Photosystem II coincides with lumen acidification. These res ults suggest that the model of photoinhibition in vivo occurring princ ipally because of limitations to electron withdrawal from photosystem II does not explain photoinhibition in these transgenic tobacco leaves , and we need to re-evaluate the twinned concepts of photoinhibition a nd photoprotection.