Many plant responses important for photoprotection in high light environments are mediated by photoreceptors for blue and UV radiation. Perception and response to these shortwave regions at relatively low irradiances may function as a means of priming leaves for the transition to high irradiance. We tested the roles of phototropin (PHOT), cryptochrome (CRY) and ultraviolet-resistance locus 8 (UVR8) photoreceptors in facilitating acclimation to an increase in solar radiation. Arabidopsis thaliana plants deficient in these photoreceptors were grown in a controlled environment under spectral irradiance treatments with or without blue light for 35 days, followed by additional UV-B radiation for 9 days. All plants were then transferred outdoors. Acute high irradiance outdoors caused an immediate drop in photosynthetic yield across genotypes, followed by a midday decline that was less pronounced in wild-type plants grown with blue light and UV-B. Plants grown with blue light but no UV-B were less affected than those with UV-B but no blue. Plants deficient in both UVR8 and CRY performed worst, with CRY having a stronger effect on photoinhibition. Epidermal anthocyanin and flavonol indices were strongly negatively correlated. This relationship was particularly tight in plants from treatments containing blue light and it followed a genotype-specific pattern; whereby genotypes showing the weakest flavonols accumulation (cry1cry2uvr8-2 and cry1cry2) also had the highest anthocyanins. This pattern suggests metabolic compensation through anthocyanins for the relative lack of flavonol induction. Nevertheless, this compensation mechanism was insufficient to allow plants lacking CRY to maintain photosynthetic function when transferred to outdoor conditions.
Trasser et al. (Fri,) studied this question.