Anthocyanin Composition of Merlot is Ameliorated by Light Microclimate and Irrigation in Central California

An experiment was conducted in central California on Merlot to determine the interaction of time of mechanical leaf removal (control, prebloom, or post fruit-set) and irrigation amount (sustained deficit irrigation (SDI) at 0.8 of estimated vineyard evapotranspiration (ET_c) or regulated deficit irrigation (RDI) at 0.8 from budbreak to fruit set, 0.5 from fruit set to veraison, and 0.8 from veraison to leaf fall) on productivity, berry skin anthocyanin concentration and composition, and unit cost per hectare. Prebloom leaf removal (applied ~100 GDD prior to bloom) consistently maintained at least 20% of photosynthetically active radiation in the fruit zone in both years of the study, while post fruit-set leaf removal was inconsistent across years. The RDI treatments reduced berry mass, while post fruit-set leaf removal reduced berry skin mass. Prebloom leaf removal did not affect yield in either year. Exposed leaf area and leaf area to fruit ratio (m²/kg) were reduced with leaf removal. The RDI consistently increased juice soluble solids. Anthocyanin concentration increased with prebloom leaf removal in both years, but irrigation treatments had no effect. The proportions of acylated and hydroxylated anthocyanins were not affected by leaf removal. In both years, SDI increased di-hydroxylated anthocyanins while RDI increased tri-hydroxylated anthocyanins. Prebloom leaf removal when combined with RDI optimized total skin anthocyanins (TSA) per hectare, while no leaf removal and SDI produced the lowest TSA. The cost to produce one unit of TSA was reduced 35% by combining prebloom leaf removal and RDI when compared to no leaf removal and SDI. This study provides information to red winegrape growers in warm regions on how to manage fruit to enhance anthocyanin concentration and the proportion of hydroxylation, while reducing input costs through mechanization and reduced irrigation.