Thermal data to monitor crop-water status in irrigated Mediterranean viticulture

• Thermal imaging is a feasible technique for remote monitoring of grapevine water status.
• Thermal indices (CWSI, IG, ΔTleaf-air) but also TC offer robust information of grapevine water status.
• The best moment to monitor the crop water status on the basis of thermal data occurs between 11 and 14 h.
• The genotype has an apparently limited effect on performance thermal indices under mild drought stress.

Canopy temperature (TC) can be a robust indicator of grapevine water status. However, the assessment of TC by thermography in field conditions requires optimization, especially under variable environmental conditions (daily and seasonal) just like it occurs in Mediterranean areas. Besides, simplicity and robustness should be the basis of a wider use of thermography in field conditions. Therefore the comparison of simpler and more complex approaches in terms of thermal indicators is wise to test. Our major aims were: i) to assess the performance of four common thermal indicators (canopy temperature − TC, Crop Water Stress Index − CWSI, index of relative stomatal conductance − IG, and the difference between TC and the surrounding air − ΔTcanopy-air) to support irrigation decisions ii) to optimize the timing of thermal measurements for different genotypes and iii) to obtain mathematical functions to estimate leaf gas exchange parameters on the basis of thermal data. The trial was conducted in the summer of 2013, in south Portugal. Two V. vinifera red varieties, Touriga Nacional and Aragonez (syn. Tempranillo) were tested. Vines were subjected to two irrigation regimes: i) sustained-deficit irrigation (based on the farm’s schedule − control) and ii) regulated-deficit irrigation (∼50% of the control). We found that measurements done between 11:00 and 17:00 h provide the most significant correlations between TC, CWSI and IG and leaf stomatal conductance and net photosynthesis for both genotypes. Different linear mathematical functions were obtained to estimate leaf gas exchange based on the best performing thermal indicators under field conditions. Our results emphasize the value of TC as a relevant explanatory variable of vine’s physiological status, in spite of being a simpler and non-normalized thermal indicator. The potential relevance of TC for grapevine modelling and phenotyping is also discussed.