Analysis of crop water stress index (CWSI) for estimating stem water potential in grapevines: comparison between natural reference and baseline approaches

The crop water stress index (CWSI) was developed as a normalized index to quantify stress and overcome the effects of other environmental parameters affecting the relationship between stress and plant temperature. This index has been widely used for crop water status monitoring. Nowadays CWSI can be determined by different methodologies being the most used: i) baseline empirical approach and ii) natural and artificial wet and dry reference surfaces. In this study both methodologies were used in the calculation of CWSI. Then, canopy thermal images were used to assess the relationship between grapevine water status and CWSI. Natural references T-dry and T-wet surfaces were obtained by covering the leaves with petroleum jelly and spraying soapy water on leaves, respectively. Non-water stressed and non-transpiring baseline equations were developed using canopy temperature measured from full irrigated and stress plants using infrared thermometers placed on top of grapevines. Stem water potential (Psi(stem)) was determined with a pressure chamber on 32 grapevines in a 'Carmenere' commercial vineyard. Thermal image acquisitions and Psi(stem) measures were obtained simultaneously at midday. Comparison between natural references and baseline approaches for estimating stem water potential showed that both methods presented high and significant correlations with determination confidents (r(2))over 0.78. According to these results, both CWSI approaches, can be accepted as a useful tool to schedule irrigations in grapevines. However, for practical purposes, the use of natural references is more suitable since it does not need field experiments to determine the empirical baseline.