The effects of leaf removal and UV-B on grape biochemical composition

Abstract

Leaf removal is a common practice in viticulture to reduce disease pressure. To study the effects of this practice on berry biochemical composition we have removed leaves over the fruiting zone of both Sauvignon Blanc and Pinot Noir grapes. In addition, we have used screening materials to investigate the specific role of UV-B radiation in these responses. Three major groups of chemicals have been focused on; flavonoids, amino acids and aroma compounds. In addition to changes in the biochemical composition we have measured gene expression for important biosynthetic pathways using qPCR and a relatively new Nanostring nCounter technology to simultaneously investigate multiple known and putative genes involved in amino acid metabolism. The major flavonoids identified are flavonols, quercetin and kaempferol glycosides and our results show that UV-B exposure is a significant determinant in their accumulation. Of the five flavonol synthase genes identified only two were active. We have also monitored gene activity for the signal transduction cascade from the recently discovered UV-B photoreceptor vvUVR8, through interacting partners, vvCOP1/vvHY5 and transcription factors, vvWD40/vvMYCA1/vvMYB12 to flavonol synthase. In contrast, amino acid biosynthesis is not affected by UV-B, but overall levels decline due to leaf removal. Amino acids play important roles in the wine making process, including acting as nitrogen source for fermentation and as precursors of important flavour and aroma compounds. We are therefore investigating how specific amino acid metabolism takes place in the berry, particularly for arginine and the non-YAN amino acid proline. Methoxypyraziness are important aroma compounds in Sauvingnon Blanc grapes and wine and their accumulation increases substantially pre-veraison. Leaf removal can significantly reduce this accumulation, but this response is varied and may be correlated with seasonal temperature variations. An interesting observation is that our leaf removal protocol has little effect on total soluble solid levels in the fruit, suggesting that primary metabolism can be decoupled from secondary metabolism to influence fruit biochemical composition.