**3.4. Grapevines**

A number of indicators related to plant water status of grapevines have been discussed in the literature such as gs [76, 77], Ψp, or Ψstem [78–81], sap flow and SDV-derived variables. Being very sensitive to transient meteorological conditions, gs at the time of measurement performed poorly in detecting grapevine water stress in Alto Douro vineyards in Portugal [82]. This can be eventually explained by the fact that either the cultivar displayed an anisohydric behavior [51] or the relative conductance was not used. According to Acevedo-Opazo et al. [83] and Lanari et al. [84], Ψleaf or Ψstem are reported to correlate well with both soil water content and net photosynthesis, and they are suitable to perform irrigation scheduling on grapevines under DI. In other studies, a better performance was obtained by using this variable measured at predawn [56, 79, 85]. According to Silvestre (2018, personal communication), there is some experimental evidence that Ψstem is not a good indicator in vineyards under high VPD.

Measurements of vegetative growth, when applied to grapevines, can offer simplicity, sensitivity to water stress over extended periods [86], as tissue expansion underlying vegetative growth responds to water status, and are interrelated with crop yield and quality. The stage development of shoot tips can be used reliably to estimate vineyard water status and manage irrigation, given that moderate water stress is primarily affected by soil water content [86]. An experiment to evaluate the visual assessment of shoot tip stage as a method to estimate the water status of vineyards and its utility in vineyard management showed that calculation based on the tip stage [87] is fast, nondestructive, and does not require special skills or equipment and it is independent of prevailing weather conditions [86].

Brillante et al. [80] observed that canopy temperature was an important predictor in determining the water stress experienced by grapevine, especially at midday. These positive results are not always observed: due to excessive wind and turbulence in SW of Portugal, the significant differences in DI treatments could not be identified using proximal radiative canopy temperature [88]. Bellvert et al. [89] emphasized the influence of VPD in using airborne thermal imagery in vineyards. Canopy temperature and derived parameters such as the empirical CWSI [59] have also been used in vineyards by Grant et al. [90] and King and Shellie [91] to monitor plant water stress.

Sap flow performed satisfactorily in detecting grapevine water stress in Alto Douro [82], and in a study developed by Selles et al. [92], diameter changes proved more sensitive than water potentials. Again, many different results were obtained in South Portugal, where differences in DI could not be distinguished using SDV, but were quite clear regarding sap flow records for different treatments [56]. If a single indicator based on sap flow or SDV did not reflect the grapevine response, according to Oliveira et al. [93], their combination could provide more detailed information.

In general, threshold values for DI in vineyard based on water potential have been abundantly suggested, but in the case of vine production, the quality issues are crucial; therefore, information is quite complex and scattered. Classical recommendations often include the use of leaf water potential [94]; a new water stress index based on a water balance model was proposed and tested by Gaudin et al. [95] as a tool for classifying water stress experienced by grapevines in vineyards.
