Physiological Indicators of Plant Water Status of Irrigated and Non-irrigated Grapevines Grown in a Low Rainfall Area of Portugal

Water is a key resource in commercial wine production and both large excesses and deficits have undesirable effects upon the amount and quality of the wine produced. A balance between the water requirements of a fully developed canopy and the induced stress necessary for the commercial quality of the wine must be reached. Thus we need a physiological indicator that integrates both soil and climatic conditions to use as a management tool. An experimental field was established in the eastern part of the Demarcated Region of Douro – Portugal, to study the effect of water supply on the quality of the musts produced and we need a physiological indicator that relates to the water use and stress of the grapevines (Vitis vinifera L.) and to the later evaluation of the effect of irrigation practices upon the quality of the musts. We chose as indicators sap flow, leaf water potential at pre-dawn (0600 h), mid-morning (1000 h), solar noon (1400 h) and sunset (1900 h), stomatal conductance and leaf transpiration both measured at mid-morning and at solar noon, and related them to our experimental treatments that induce differences in soil water content, evaluated with time-domain reflectometry probes, with the objective of selecting the indicator that best describes the plant water status under different amounts of available water. Sap flow, leaf water potential and leaf transpiration rate measured at solar noon had highly significant correlations with soil water content and their regression on soil water content was also highly significant. Each of these parameters has shortcomings and none has a clear advantage over the other two as an integrator of the environmental conditions under these experimental conditions. Further studies of the parameters and their relationship with the quality characteristics of the produced musts are needed to achieve the ultimate objective of manipulating the soil water content.