Evapotranspiration-Based Irrigation Scheduling for Syrah: Assessing Vine Water Status by Petiole Electrical Potential

Abstract

Six irrigation treatments consisting of three reference evapotranspiration (ET_o) values (100%, 50%, 25%) and two crop coefficients (K_c) estimated from canopy volume (0.6, or “fixed,” and 0.2 to 0.9, or “variable”) were imposed on Syrah/110R vines in the Dunnigan Hills (California) in a randomized block containing four replicates and 12 equally spaced vines per three-row treatment replicate during 2004 to 2007. Treatment imposition began at fruit set and continued until one week preharvest. Midday leaf water potentials (ψ) were lowest in 25ET/variable vines, and were frequently <-1.5 MP. Petiole conductance followed the same pattern as ψ, particularly in 2005 and 2006, suggesting that electropotential technology has possibilities for monitoring vine water status. Irrigation did not affect most yield components. Berry weight decreased linearly with reducing ET_o and was lowest in 25ET treatments. A variable K_c increased berry weight. Soluble solids (Brix) increased with decreasing ET_o in 2004 and 2006 (>25 Brix); in 2005, 25ET treatments were lowest (<25 Brix). A variable K_c decreased Brix. Titratable acidity decreased with decreasing ET_o in two seasons, but effects of K_c were inconsistent. pH increased linearly with decreasing ET_o in two seasons but effects of K_c were inconsistent. Highest pH occurred in 2004 and 2006 in 25ET/0.6 (>4.15) and was lowest in 100ET/variable and 50ET/0.6 berries. Total anthocyanins and total phenols increased with decreasing ET_o in 2004 but decreased relative to decreasing ET_o in 2005 and 2006. Sensorially, 25ET treatments diminished red fruit aroma/flavor without impacting intensity of other fruit components and had least vegetal and most lavender and chocolate aromas; retronasally, they had most dark fruit, anise, alcohol/heat, viscosity, and length.