We have isolated very high light resistant nuclear mutants (VHLR) in Chlamydomonas reinhardtii, that grow in 1500–2000 μmol photons m−2 s−1 (VHL) lethal to wildtype. Four nonallelic mutants have been characterized in terms of Photosystem II (PS II) function, nonphotochemical quenching (NPQ) and xanthophyll pigments in relation to acclimation and survival under light stress. In one class of VHLR mutants isolated from wild type (S4 and S9), VHL resistance was accompanied by slower PS II electron transfer, reduced connectivity between PS II centers and decreased PS II efficiency. These lesions in PS II function were already present in the herbicide resistant D1 mutant A251L (L*) from which another class of VHLR mutants (L4 and L30) were isolated, confirming that optimal PS II function was not critical for survival in very high light. Survival of all four VHLR mutants was independent of CO2 availability, whereas photoprotective processes were not. The de-epoxidation state (DPS) of the xanthophyll cycle pigments in high light (HL, 600 μmol photons m−2 s−1) was strongly depressed when all genotypes were grown in 5% CO2. In S4 and S9 grown in air under HL and VHL, high DPS was well correlated with high NPQ. However when the same genotypes were grown in 5% CO2, high DPS did not result in high NPQ, probably because high photosynthetic rates decreased thylakoid ΔpH. Although high NPQ lowered the reduction state of PS II in air compared to 5% CO2 at HL in wildtype, S4 and S9, this did not occur during growth of S4 and S9 in VHL. L* and VHLR mutants L4 and L30, also showed high DPS with low NPQ when grown air or 5% CO2, possibly because they were unable to maintain sufficiently high ΔpH due to constitutively impaired PS II electron transport. Although dissipation of excess photon energy through NPQ may contribute to VHL resistance, there is little evidence that the different genes conferring the VHLR phenotype affect this form of photoprotection. Rather, the decline of chlorophyll per biomass in all VHLR mutants grown under VHL suggests these genes may be involved in regulating antenna components and photosystem stoichiometries.
