Use of Reflectance Indices to Assess Vine Water Status under Mild to Moderate Water Deficits

The monitoring of vine water status is of interest for irrigation management in order to improve water use while optimizing both berry yield and quality. Remote-sensing techniques might provide accurate, rapid, and non-destructive estimates of vine water status. The objective of this study was to test the capability of the reflectance-based water index (WI) and the photochemical reflectance index (PRI) to characterize Vitis vinifera L. cv. Xarel center dot lo water status under mild to moderate water deficits. The study was conducted at the leaf level in irrigated potted plants and at the plant level on five commercial rain-fed vineyards in 2009 and 2010. In potted plants, the reflectance indices PRI and WI closely tracked variation in the leaf-to-air temperature difference (Delta T) with r(2) = 0.81 and r(2) = 0.83, for WI and PRI, respectively (p < 0.01). In addition, in potted plants, both PRI and WI showed significant relationships with light-use efficiency (LUE)-calculated as the ratio between net CO2 assimilation rate (A(n)) and incident photosynthetic active radiation (PAR) at the leaf surface-with r(2) = 0.92 and r(2) = 0.74 for PRI and WI, respectively. At the canopy level, vine predawn water potential (psi(pd)) was related to the canopy-to-air temperature difference (Delta T-m) across years (r(2) = 0.37, p < 0.05). In the years of study, the relationships between PRI and WI showed variable degrees of correlation against psi(pd) and Delta T-m. Across years, PRI and WI showed significant relationships with psi(pd), with r(2) = 0.41 and r(2) = 0.37 (p < 0.01), for WI and PRI, respectively. Indices formulated to account for variation in canopy structure (i.e., PRInorm and WInorm) showed similar degrees of correlation against psi(pd) to their original formulations. In addition, PRI and WI were capable of differentiating (p < 0.01) between mild (psi(pd) > -0.4 MPa) and moderate (psi(pd) < -0.4 MPa) water deficits, and a similar response was observed when PRInorm and WInorm-formulated to account for variation in canopy structure-were considered. Thus, at the leaf level, our result suggest that WI and PRI can be used to adequately predict the diurnal dynamics of stomatal aperture and transpiration. In addition, at the canopy level, PRI and WI effectively differentiated vines under mild water deficits from those experiencing moderate water deficits. Thus, our results show the capability of WI and PRI in characterizing vine water status under mild to moderate water deficits.