UV-B acclimation promotes photoprotection. (A) The maximum quantum yield of PSII was monitored in cell cultures of WT (WT CC-4533), uvr8, and uvr8/UVR8 complemented line 10 that had been previously exposed for 16 h to UV-B (Fv/Fm +UV) or were left untreated (Fv/Fm –UV). The effect of acclimation is expressed as the ratio (Fv/Fm +UV)/(Fv/Fm –UV) (error bars represent the SEM; n = 5) measured at the end of acclimation (0) and after a subsequent high-light treatment (1 h, 700 µmol⋅m−2⋅s−1). (B) Cell cultures that had been exposed for 16 h to UV-B (+UV) or untreated controls (−UV) were photographed (t = 0) and then exposed to high light (1,000 µmol⋅m−2⋅s−1) for 5 h and photographed again (t = 5 h). Data shown are representative of three independent biological repetitions. (C and D) As A and B, but for the WT (WT 137C) and mutants npq4, lhcsr1, and npq4 lhcsr1. (C) Error bars represent the SEM; n = 3. (D) Data shown are representative of three independent biological repetitions. (E) Scheme of photoreceptor-mediated photoprotection of the photosynthetic machinery after UV-B exposure compared with high light.