Blue light increases anthocyanin content and delays fruit ripening in purple pepper fruit

Abstract

In purple-fruited Capsicum genotypes, fruit anthocyanin content first increases, and later decreases upon ripening. In this study, the response of this transient anthocyanin accumulation as a function of light intensity and light spectrum was investigated. Harvested fruit at the mature purple stage were illuminated by white-red LED lights at a photosynthetic photon flux density of 0 (darkness), 80, 160 and 320 μmol m-2 s-1 or illuminated by 24 (white light), 57%, 72% and 99% blue light (400–500 nm) at a photosynthetic photon flux density of 80 μmol m-2 s-1 for 28 d. Total anthocyanin, chlorophyll, capsanthin and soluble sugar contents, and their related gene expression, were measured. Anthocyanin content was hardly affected by white-red light intensity. Increasing the blue light fraction increased anthocyanin levels, via enhancing anthocyanin biosynthesis. This is supported by kinetic modelling and higher expression levels of the anthocyanin biosynthetic genes CaMYB, CaCHS, CaDFR, CaANS and CaUFGT. A higher blue light fraction delayed fruit ripening. This is supported by delayed sugar accumulation as indicated by kinetic modelling and transcriptome analysis. Transcriptome analysis indicated that 91% of the differentially expressed genes down-regulated during ripening had a higher expression in the 72% compared to 24% blue light treatment. In addition, PCA analysis of transcriptomic data indicated fruit ripening and senescence related genes NCED1, NCED2, NOR and RIN were expressed less in fruit illuminated by 72% blue light. Pathways upstream of ethylene production were downregulated in fruits illuminated with higher blue light fractions that might result in suppressed ethylene production. Higher fractions of blue light during postharvest storage might be applied to improve fruit anthocyanin levels but are not suited to create ripe anthocyanin rich pepper fruit.