## 3 Results and Discussion
### Crop Water Stress Index and Degrees above Non-Stress
Regarding the DAC index, in young trees the seasonal evolution was barely higher than results obtained by \(\Delta\)T; on the other hand, in mature trees, the DAC index resulted in values which achieved a 4.4 \({}^{\circ}\)C difference between control and water-stressed trees, while on the same dates \(\Delta\)T did not achieve values higher than 2.0 \({}^{\circ}\)C (Figure 6a,b). A linear relationship between the thermal indicators and \(\Psi\)stem was calculated. The Tc showed a non-linear relationship with \(\Psi\)stem (Figure 7a). As expected, higher Tc values were related to trees submitted to water stress (MS and YS). Although the coefficient of correlation obtained between \(\Psi\)stem and Tc for all the trees exhibited a strong relationship (r = 0.73), Tc as a water stress indicator showed important limitations. Thus, the second-grade polynomial relationship obtained showed  
Figure 6: Seasonal evolution of degrees above control (**a**,**b**) and non-stressed (**c**,**d**) in young (**a**,**c**) and mature (**b**,**d**) sweet cherry trees during the study period. Each point corresponds to the mean \(\pm\) standard error of the mean for six images per treatment. Asterisks indicate statistically significant differences between treatments by ANOVA (\(p<0.05\)). CTL and S correspond to control and deficit irrigation treatment for young (Y) and mature (M) sweet cherry trees, respectively. FI is full irrigation period, D is drought period and R is recovery period in young sweet cherry trees (Plot 1), and 100% and 55% are the percentages of ETc applied in mature sweet cherry trees (Plot 2). two different relationships. At first, Tc increased linearly as \(\Psi\)stem fell from \(-0.5\) MPa to a threshold value close to \(-1.0\) MPa, which corresponded to \(33\ ^{\circ}\)C. From that value onwards, \(\Psi\)stem values below \(-1.0\) MPa were not related to higher values of Tc. It was observed that Tc did not exceed values above \(36\ ^{\circ}\)C regardless of the intensity of the water deficit applied. Consequently, within the Tc range between \(33\) and \(36\ ^{\circ}\)C, similar values were measured in CTL trees on a hot day of high evaporative demand (\(\Psi\)stem = \(-0.8\) MPa) and in sweet cherry trees under severe water stress (\(\Psi\)stem = \(-2.0\) MPa). Therefore, while it is known that in sweet cherry trees water deficit induces stomatal closure and increases leaf temperature [6; 8], it is also well known that Tc is highly dependent on tree density, canopy architecture, tree phenological stage and environmental conditions [14; 51]. In light of this, the use of absolute values of Tc cannot be recommended as a water stress indicator. The \(\Delta\)T exhibited a linear relationship with \(\Psi\)stem (Figure 7b). The negative \(\Delta\)T values obtained by CTL trees (young and mature) were related to \(\Psi\)stem values below \(-0.8\) MPa, which corresponded to trees under non-limiting soil water conditions. In sweet cherry trees under post-harvest deficit irrigation, \(-1.5\) MPa is generally considered a threshold value for irrigation management and higher values have been reported not to negatively affect the yield in the following year and reduce excessive vegetative growth [5]. In this sense, \(1.6\ ^{\circ}\)C has been suggested as the \(\Delta\)T corresponding value to \(-1.5\) MPa. The relationship between \(\Psi\)steam and \(\Delta\)T was significantly different in young and mature trees. The weaker relationship found in mature trees is due to the fact that MS trees did not reach \(\Psi\)stem values below \(-1.3\) MPa (Figure 2b). The consistent relationship found in the young sweet cherry trees (r = 0.91) was similar to that reported in peach trees by [PERSON] and [PERSON] [42] and higher than that reported in almond trees by [PERSON] et al. [52] with similar \(\Delta\)T maximum values at \(3.6\ ^{\circ}\)C and \(\Psi\)stem values below \(-2.0\) MPa.