PSII Activity Was Inhibited at Flowering Stage with Developing Black Bracts of Oat

The color of bracts generally turns yellow or black from green during cereal grain development. However, the impact of these phenotypic changes on photosynthetic physiology during black bract formation remains unclear. Two oat cultivars (Avena sativa L.), ‘Triple Crown’ and ‘Qinghai 444’, with yellow and black bracts, respectively, were found to both have green bracts at the heading stage, but started to turn black at the flowering stage and become blackened at the milk stage for ‘Qinghai 444’. Their photosynthetic characteristics were analyzed and compared, and the key genes, proteins and regulatory pathways affecting photosynthetic physiology were determined in ‘Triple Crown’ and ‘Qinghai 444’ bracts. The results show that the actual PSII photochemical efficiency and PSII electron transfer rate of ‘Qinghai 444’ bracts had no significant changes at the heading and milk stages but decreased significantly (p < 0.05) at the flowering stage compared with ‘Triple Crown’. The chlorophyll content decreased, the LHCII involved in the assembly of supercomplexes in the thylakoid membrane was inhibited, and the expression of Lhcb1 and Lhcb5 was downregulated at the flowering stage. During this critical stage, the expression of Bh4 and C4H was upregulated, and the biosynthetic pathway of p-coumaric acid using tyrosine and phenylalanine as precursors was also enhanced. Moreover, the key upregulated genes (CHS, CHI and F3H) of anthocyanin biosynthesis might complement the impaired PSII activity until recovered at the milk stage. These findings provide a new insight into how photosynthesis alters during the process of oat bract color transition to black.