Flavonol and Anthocyanin Biosynthesis in Grapevine Berry is Ameliorated by Early Season Light Quantity, Not Irrigation Restriction in Warm Climate

The effects of timing of inflorescence or infructescence exposure to photosynthetically active radiation (PAR) and phenology based irrigation restriction on the profile and content of phenolics in ‘Merlot’ grapevine were investigated in a resource-limited, warm climate environment.   Leaves on the morning-sun side of canopy in the fruiting zone were removed with a mechanical leaf remover to ensure at least 20% of PAR was intercepted in the fruiting zone at one week prior to anthesis, 20 days post-anthesis and at veraison, or no leaf removal was done.  The two irrigation treatments were applied to include a control treatment of sustained deficit irrigation (SDI) where mid-day leaf water potential (Y) was maintained at -1.2MPa from fruit set to end of harvest with 80% of ETo replaced based on weekly crop coefficient (Kc) measurement. A regulated deficit irrigation (RDI) maintained Y at -1.2 MPa between bud break and fruit set at 80% ETo, Y at -1.4MPa between fruit set and veraison at 50% ETo based on weekly Kc measurements, but not thereafter.  Phenolic composition was determined with partial and exhaustive grape skin and seed extracts at fruit-set, 3-5 mm berry size, veraison and harvest by HPLC and anthocyanins, benzoic acids, and flavan-3-ol monomers were identified and quantified.  Exposure of inflorescence or infructescence to at least 20% of PAR had no effect on number of berries per length of rachis.  Leaf removal reduced berry and cluster mass but did not affect yield per vine.  PBBLR and RDI increased Brix at harvest. Exposing inflorescence to at least 20% PAR  by leaf removal increased flavonoids, flavan-3-ols, anthocyanins, and tannins in the skin by 30%, and flavanols, in the seed at harvest when compared to control. Increasing PAR  to > 20% post veraison in the fruit zone did not affect flavonol or anthocyanin . Increasing PAR to at least 20% by leaf removal one week prior to anthesis reduced pruning weight by 23% while Ravaz Index and leaf area:fruit increased by 23%, and 16% respectively.  This study provides fundamental information to growers where the cost to produce a unit of total skin anthocyanin per hectare was reduced by 23% without adversely affecting yield utilizing the methods described above