Assessing vineyard water status using the reflectance based Water Index

In the Mediterranean arc, vines for wine production are mainly grown without the support of irrigation. Under such conditions, site variables affecting the extent and seasonal timing of water deficits are the dominant environmental constraints for grape production. Moreover, water availability and vine water status are the factors most comprehensively determining fruit composition and, thus, wine quality. Therefore, monitoring the extent of water stress in vines might be a valuable tool for the optimisation of grape yield and quality. The objective of this study was to evaluate the feasibility of using the reflectance based Water Index (WI) to estimate vine water status at the leaf and canopy levels. The study was conducted on Vitis vinifera cv. Chardonnay potted plants submitted to contrasting water availability and in ten commercial vineyards in the D.O. Penedes region (Catalonia, Spain) over two consecutive years (2007-2008). In potted plants, differences in water supply were translated into differences in net photosynthetic carbon assimilation, stomata! conductance (g(s)) and WI, despite similar predawn water potentials (Psi(p)). There was a negative close correlation between WI and g(s), although the relationship was treatment dependent (r(2) = 0.81 and r(2) = 0.95, for control and deficit plants, respectively). Similarly the leaf to air temperature difference was inversely related to gs (r(2) = 0.51). In the field study there were significant differences in Psi(s), among vineyards and between years and indicated that vines experienced moderate to severe water stress. Water availability determined vine canopy development as indicated by the close relationship between the Normalized Difference Vegetation Index and in Psi(p) (r(2) = 0.57). The canopy to air temperature difference (determined with an infrared thermometer at midday) was inversely correlated with WI with r(2) = 0.45 and r(2) = 0.56 in 2007 and 2008, respectively. Thus, the reflectance based Water Index effectively tracked variation in both leaf stomatal conductance and canopy transpiration under a range of moderate to severe water stress. The results are discussed in relation to the potential application of WI to irrigation scheduling and to monitoring vineyard performance. (c) 2010 Elsevier B.V. All rights reserved.