Interactive Effects of Deficit Irrigation and Berry Exposure Aspect on Merlot and Cabernet Sauvignon in an Arid Climate

The objective of this study was to determine the main and interactive effects of a sustained water deficit during berry development and berry exposure aspect on yield and berry composition in the red winegrape cultivars Merlot and Cabernet Sauvignon. Standard (STD) and reduced (RED) levels of sustained deficit irrigation were imposed throughout berry development over four growing seasons in a field trial located in the Snake River Valley American Viticultural Area (AVA) of southwestern Idaho, United States. The STD irrigation provided ~70% of crop evapotranspiration (ETc) and the RED treatment provided ~50% of the STD. Midday leaf water potential (Ψl) was measured weekly and the diameter of exposed east- and west-facing berries was nondestructively measured during berry development. Yield and berry maturity and the phenolic composition of exposed east- and west-facing berries were measured at harvest for each irrigation treatment. Vines under RED had lower Ψl than vines under STD irrigation. Deficit severity and west-facing berry exposure had an additive effect on reducing berry growth. East-facing berries under STD irrigation grew faster than west-facing berries under RED irrigation. Yield per vine and berry growth were reduced in both cultivars under RED irrigation and west-facing berries contained less total monomeric anthocyanin per berry than east-facing berries. Reduction in total anthocyanin content was due to lower concentration per gram berry fresh weight in Merlot and reduced berry fresh weight in Cabernet Sauvignon. The lack of significant irrigation effect and significant exposure effect observed in this study supports the hypothesis that temperature is a major factor influencing berry compositional development.