Blue light as an important factor increasing plant tolerance to acute photooxidative stress

Previous studies have confirmed the stimulating effect of blue light on phenolic compound accumulation and emphasized that sufficient dose of blue light is essential for biosynthesis of B-dihydroxylated flavonoids with enhanced antioxidant properties (under UV-lacking conditions). This study investigates the importance of blue light and complex role of phenolic compounds in plant tolerance against acute photooxidative stress. Hordeum vulgare (L. Cv. Bojos) seedlings were acclimated to different light spectra (blue, green:red 1:1, and white composed of blue:green:red 1:1:1) at total irradiance 400 µmol.m⁻².s⁻¹. Subsequently, they were subjected to a 3-hour combined stress induced by high photosynthetically active (850–950 µmol.m⁻².s⁻¹) and UV-B (2.0–2.5 W.m⁻²) radiation. Content of flavonoids, expression of genes involved in their biosynthesis (phenylalanine ammonia-lyase, chalcone synthase, flavonoid 3′-hydroxylase), and antioxidant activity of plant extracts were significantly highest in plants acclimated to blue light. As an indicator of reactive oxygen species interaction with biomolecules, the content of lipid hydroperoxides was estimated. It was demonstrated that plants acclimated to blue light revealed significantly lower extent of lipid peroxidation compared to those acclimated to white or green:red light. Plants exposed to combined light-induced stress for 3 hours exhibited pronounced disruption of PSII function: F_V/F_M tended to decrease proportionally with decreasing amount of blue photons in the treatments. Additionally, stress exposure upregulated the expression of genes related to phenolic compounds but not genes encoding antioxidant enzymes. We confirmed higher resistance of plants acclimated to blue light and presume that phenolic compounds are significantly involved in protection during the acute phase of stress.