## 3 Results and Discussion
### Crop Water Stress Index and Degrees above Non-Stress
According to the results obtained, \(\Delta\)T was less dependent than Tc on weather conditions, clearly identified control and stressed trees, and did not show any inflexion point in its relationship with \(\Psi\)stem. Consequently, these advantages of \(\Delta\)T over Tc highlight its utility as a water stress indicator. Similar to \(\Delta\)T, CWSI showed a strong linear relationship with \(\Psi\)stem (Figure 8). Young and mature trees resulted in high correlation coefficients (r = 0.89 and 0.88, respectively). These results are similar to those reported in sweet cherry trees by [PERSON] et al. [53] on the relationship between \(\Psi\)steam and CWSI was identified as \(\text{CWSI}=-0.44\ \Psi\)stem = \(-0.17\) in young sweet cherry trees and as \(\text{CWSI}=-0.86\ \Psi\)stem = \(-0.36\) in mature sweet cherry trees. Regarding the different regression lines found in young and mature trees, [PERSON] et al. [54] reported that young trees have a greater capacity to extract water from water reserves in their organs  
Figure 7: (a) Relationship between midday stem water potential (\(\Psi\)stem) and canopy temperature (Tc). (b) Relationship between \(\Psi\)stem and the difference between canopy and air temperature (\(\Delta\)T). Each point corresponds to the mean \(\pm\) standard error of the mean of six measurements per treatment. CTL and S correspond to control and deficit irrigation treatment for young (\(\gamma\)) and mature (M) sweet cherry trees, respectively. (water storage tissues) than mature trees, and quickly transport it through the plant with the aim of sustaining leaf transpiration. Mature trees require a larger amount of water for their transpiration process because they have a greater leaf area, release more water to the atmosphere and have a proportionally smaller water reserve. Consequently, the mechanism used by mature trees to face water stress does not only consist of recruiting water from the water storage tissues but to promote stomatal closure. Stomatal closure avoids plant water release, decreases tree transpiration, and leads to an increase in leaf temperature [8]. These increments in leaf temperature of mature sweet cherry trees are referred to against the same baselines for young sweet cherry trees (Figure 4). Therefore, for a similar value of \(\Psi\)stem, mature trees exhibit higher CWSI values. However, despite the difference in results for young and mature trees, it can be stated for both of them that CWSI values lower than 0.2 match with \(\Psi\)stem values of well-watered trees. Similar to CWSI, when DAC and DANS indices were compared to \(\Psi\)stem, young and mature trees, they showed significantly different linear regressions (Figure 9a,b). In the case of the DAC index, young trees were more closely related to \(\Psi\)stem than mature trees (r = 0.9 and 0.76, respectively), with maximum values of 4.5 \({}^{\circ}\)C. In the case of the DANS index, mature and young trees were closely related (r = 0.84), and the maximum value (8.6 \({}^{\circ}\)C) was found in mature trees at \(-\)0.95 MPa. As expected according to the results reported by [PERSON] et al. [15], DAC and DANS were strongly related to CWSI, especially DANS (Figure 9c,d). In general, water stress indicators derived from thermal imaging evaluated in the present work were not sensitive enough to detect slight plant water stress in sweet cherry trees, due to Tc strongly depending on both stomatal conductance and transpiration rate, which are physiological processes that are less sensitive than other plant water indicators such as micrometric fluctuation of the different plant organs (trunk, branch, fruit, etc.) [8; 11; 55]. This limitation has been observed in all indices and has been reported in several fruit trees such as apple, citrus and nectarine [56; 57; 58]. This is because water status indicators based on leaf temperature when the soil water deficit is not moderate or severe are highly dependent on weather conditions.