Usefulness and limits of the crop water stress index obtained from leaf temperature for vine water status monitoring

Aims: This work aimed i) to calibrate the accuracy of estimating vineyard water status by crop water stress index (CWSI) compared to stem water potential; ii) to determine the time interval during the day that best correlates to stem water potential and iii) to understand the its usefulness.

Methods and Results: Four levels of irrigation were set up in 2017 on a Cabernet-Sauvignon vineyard grafted to 110R in Morata de Tajuña (Madrid, Spain). The experimental design was a completely randomized four-block design. During two seasons, 2018 and 2019, stem water potential (SWP) and leaf temperature were measured at three time points during the day (8:00; 12.00 and 16:00 solar time) in five dates during 2018 and three dates in 2019. CWSI was calculated based on leaf temperature as the ratio: (T_{treat leaf} – T_wet)/(T_dry – T_wet). Leaf temperature (T_{treat leaf}) was measured with an infrared camera model FLIR-E60; Four shaded leaves per treatment were sampled at each time of measurement, for a total of 16 leaves per measurement interval. ANOVA for CWSI and stem water potential was also performed to compare the sensitivity of each parameter to vine water status. All statistical analyses were performed with the Statistix10 package.

Results showed that stem water potential was slightly more sensitive than CWSI to estimate vine water status. Different relationships were found during the season between CWSI and SWP. The determination coefficient was higher at midseason than at the beginning or late in the growing season. The highest R² were found at noon and during the evening, being no-significant in the morning.

Conclusions: 

Crop Water Stress Index obtained from leaf temperature could be used to estimate plant water status although assuming that it is less sensitive than Stem Water Potential. The index was more accurate in describing the plant water status in midseason than either early or late in the season and better at midday and evening than in the morning.

Significance and Impact of the Study: The study confirms the use of CWSI as a tool to determine vineyard water status and its limitations. Limitations include its effectiveness being confined to midseason and measurements are recommended to be collected from noon onwards. We propose to keep CWSI lower than 0.6 from budbreak until bloom and to move within 0.6 to 0.8 during maturation to ensure SWP is over -1.0MPa (-10 bar) and within -1.0 and -1.2 MPa during ripening.