Photoprotection and resistance to very high light in Chlamydomonas reinhardtii: Effects of CO2 supply

Abstract

We have isolated over thirty very high light resistant mutants (VHLR) in Chlamydomonas reinhardtii that grow in 1500 to 2000 µmol photons m-2 s-1 (VHL) lethal to wildtype (Förster et al., 1999, J. Photochem. Photobiol. 48: 127-135). Although growth rates were CO2 dependent, the four nonallelic nuclear mutants examined survived equally well in air and 5% (v/vair) CO2 under all light conditions. Photoprotective processes defined by nonphotochemical quenching (NPQ) and xanthophyll pigment composition were markedly affected by CO2 supply. The de-epoxidation state (DPS) of the xanthophyll cycle pigments and NPQ were depressed in 5% CO2, whereas in air, wildtype and the derived VHLR mutant S9 showed high DPS and NPQ under high light (HL, 600 µmol photons m-2 s-1) and VHL. About 85% of the NPQ in S9 and about 50% of the NPQ in wildtype were discharged by the addition of nigericin under HL in air, showing that NPQ at high DPS was ?pH dependent. The herbicide resistant A251L D1 mutant L* and the derived VHLR mutant L4 also showed a similar effect of CO2 on DPS but did not engage high NPQ under all conditions examined. Perhaps the impaired PSII electron transfer in L* and L4 was insufficient to generate the ?pH required for high NPQ at high DPS. Thus, there is little evidence that this form of photoprotection determines VHL resistance (Förster et al., 2001, Photosynth. Res., still in press!).