LAUSR

PREMISE While essential for photosynthesis, excess light can damage plants. We investigated how growth light conditions affect two photoprotective strategies, chloroplast movement and nonphotochemical quenching (NPQ), as well as electron transport rates (ETR), and the relative importance of these processes in the short-term stress tolerance of Arabidopsis thaliana. METHODS We grew wild-type (WT) and mutant plants with impaired chloroplast movement (phot1, phot2, phot1 phot2, chup1) or NPQ (npq1) at low (160 μmol photons m-2 s-1 ) or intermediate light (400 μmol photons m-2 s-1 ) before quantifying transmission changes due to chloroplast movement, NPQ, ETR, and the ability to recover from a short-term high-light treatment. RESULTS Plants with impaired chloroplast avoidance movement (phot2, phot1 phot2, chup1) did not recover as well from a short-term high light treatment as the WT or npq1 and phot1 mutants. Plants grown at intermediate light recovered more completely from the same stress treatment regardless of their genotype and despite reduced degrees of transmission changes due to chloroplast movement. This result was due in part to all genotypes having up to a 2-fold increase in ETRmax and a slight increase in NPQmax . CONCLUSIONS Growth light conditions affect which mechanisms are important in dealing with short-term high-light stress. The chloroplast avoidance response is important for low-light-grown plants, while increases in ETRmax and NPQmax allow plants grown at intermediate light intensities to avoid being damaged.