Suitability of pre-dawn and stem water potential as indicators of vineyard water status in cv. Tempranillo

Background and Aims: Irrigation management requires the collection of up-to-date information that allow growers to make rapid decisions, water potential being one of the most used parameters in irrigation scheduling. The goal of this research was to determine whether predawn (Psi pd) and stem (Psi s) water potential can be used equally well, quantifying the effect external factors have on the relationship between them. Methods and Results: Psi pd, mid-morning (Psi s-m) and noon (Psi s-n) stem water potential were measured at seven cv. Tempranillo vineyards. Climatic conditions, vine growth and fruit load affected daily water potential dynamics. A greater decline of Psi s occurred when temperatures were higher and there was no dew formation, as well as under larger canopies and fruit load. While the relative impact of these variables on water potential was not particularly high, it was by no means negligible. Conclusions: Data collected in this study did not favour Psi pd, Psi s-m or Psi s-n as a preferred measure to evaluate plant water status. It is important to remark that similar Psi pd values do not necessarily indicate the same level of deficit, and that similar Psi s values do not always mean similar water availability. Therefore, a straightforward interpretation of Psi pd and Psi s can sometimes be misleading. Significance of the Study: The results obtained provide significant explanation to the lack of agreement in the literature regarding the most suitable time to measure water potential. Vineyard managers and scientists should select it according to the level of stress expected, climatic conditions and irrigation frequency.