Different responses to solar ultraviolet (UV) and blue radiation in two accessions of Vicia faba

A plant depends on sunlight not only as its primary energy source but also as a signal from the environment. UV light, although it constitutes a small part of solar spectrum, has a large influence on plant growth, and has been associated with pre-adapting plants to drought. Our study aims at understanding the effect of solar UV and blue radiation on two accessions of faba bean (Vicia faba L.). Accession ILB938 originates from high altitude in Ecuador, where drought is frequent and UV irradiation is strong, whereas cultivar Aurora originates from Sweden, where drought is rarer and UV levels are lower. The experiment was done outdoors with four optical filters that attenuate different wavelengths from the sunlight. The deprivation of blue light increased plant height and leaf area in both accessions. Blue light increased stomatal conductance in Aurora but decreased it in ILB938. Short UV and blue light strongly induced flavonoid synthesis in upper epidermis of leaves. Detailed leaf phenolic analysis with HPLC-DAD-ESI-MSn showed that there was more quercetin than kaempferol in both accessions. Total kaempferol content did not respond to light in either accession, and the molar concentration of kaempferol did not differ between them, but the derivatives differed. In Aurora, the derivatives were mostly tri-glycoside, tetraglycoside and acetylated-glycoside, whereas there was mainly di-glycoside in ILB938. Total quercetin concentration was increased strongly by short UV (P<0.0001) in both accessions. The variation in total quercetin was higher in ILB938 (P=0.0387), because of the large variation in quercetin-3-O-rhamnoglucoside concentration (P=0.0021). Different quercetin derivatives in the two accessions responded differently to light. In Aurora, quercetin-3- O-rhamno-7-O-galactoside was induced upon short UV (P=0.0287), and this is the most abundant quercetin in Aurora. In ILB938, quercetin-3-O-rhamnoside-7-O-rhamnoside and quercetin-3-O-rhamno-rhamno-galactoside were increased upon both short UV (P<0.0001) and blue light (P<0.0001) exposure. The most abundant quercetin derivative in ILB938 did not respond to light. Thus, these two faba bean accessions responded differently to solar blue and short UV light. Our study will continue to explore these responses at both the molecular and whole plant level and to uncover if maternal-plant exposure to short UV has an epigenetic impact on the progeny.