Validation of thermal indices for water status identification in grapevine

• We examine the applicability of thermography for the assessment of vineyard water status.
• Evaluation of variation in stomatal conductance through the use of IG and CWSI calculated indexes has been done.
• Strongest relationship between thermal indexes and measured stomatal conductance at midday, on the shaded side of the canopy.
• This relationship was obtained independently of the plant water status.
• Berry emissivity was determined to be 0.96.

The use of thermal imaging represents a substantial progress in monitoring plant water status and therefore drought stress in field conditions. However, the effective use of thermal imaging requires consistent methods for data acquisition and image analysis. We determined the temperature variation of grapevine canopies by the use of thermal imaging in a proximal manner, and calculated stomatal conductance index (IG) and crop water stress index (CWSI), aiming to assess the plant water status that was measured as variations in stomatal conductance. The study was conducted in a hillside commercial vineyard with Graciano (Vitis vinifera L.) vines grown under two different water statuses. Leaf stomatal conductance was measured to determine plant water status and indices derived from individual grapevine leaves, clusters and canopies were assessed by thermal imaging. Measurements were carried out under different light conditions (sunlit and shaded part of the canopy) and at different times of the day (morning, midday and afternoon) to analyze the robustness and sensitivity of thermal imaging for detecting changes in a range of plant water status and experimental conditions.Highly significant correlations were found between the calculated indices (IG and CWSI) and the measured stomatal conductance. The strongest relationships between IG and CWSI and the measured stomatal conductance were obtained at midday, on the shaded side of the grapevine canopy. Therefore, those sampling conditions were the most appropriate to estimate variation in stomatal conductance in a non-contact manner through the use of thermal imaging.Furthermore, the sensitivity of berry temperature to changes in grapevine water status was quantified. The acquired thermal images for vine clusters were corrected for berry emissivity, which was estimated to be 0.96. For water stressed grapevines, berry temperature was increased as much as 1–2 °C above clusters from non-water stressed grapevines, thus potentially affecting berry composition.