Nitrogen Fertiliser Effects on Grain Anthocyanin and γ-Oryzanol Biosynthesis in Black Rice

Accumulation of phytochemicals in vegetative tissue under nitrogen (N) stress as an adaptive strategy has been investigated in various crops, but the effect of applied N on grain phytochemicals is poorly understood. This study investigated the effect of applied N on the biosynthesis and accumulation of rice ( Oryza sativa L.) grain anthocyanin and γ-oryzanol under different ultraviolet-B (UV-B) conditions in a controlled pot trial using two distinct black rice genotypes. The response of grain anthocyanin and γ-oryzanol content to applied N was genotype-dependent but was not altered by UV-B conditions. Applied N increased grain anthocyanin and decreased γ-oryzanol content in genotype SCU212 but had no significant effect in genotype SCU254. The expression of the OsKala3 regulatory gene was significantly upregulated in response to applied N in SCU212, while the expressions of OsKala4 and OsTTG1 were unchanged. The expression of all three regulatory genes was not significantly affected in SCU254 with applied N. Key anthocyanin biosynthesis genes were upregulated in grain by N application, which indicates that the common increase in anthocyanin in vegetative tissues under N deprivation does not hold true for reproductive tissues. Hence, any future approach to target higher content of these key phytochemicals in grains should be genotype-focused.