## 4 Discussion
Despite the large amount of literature on the effect of stress on leaf chlorophyll content, only one study has been conducted on the influence of the tree's water status on pistachio kernels' chlorophylls content, and no significant effect of irrigation on this parameter was found [17]. A possible explanation for this difference may be the lower stress level reached by the plants in [PERSON]'s study [17], in which \(\Psi_{\text{SWP}}\) never reached values below \(-1.5\) MPa. Previous studies have demonstrated that only when \(\Psi_{\text{SWP}}\) decreases below \(-1.5\) MPa pistachio trees can be considered physiologically stressed showing reduction of gas exchanges [26; 43; 44]. In addition, the period when stress was applied may have played a significant role in determining the different response observed in these studies. In fact, [PERSON] et al. [17] applied stress only during stage II of fruit development, when embryos are not yet fully growing [27] and pistachio trees are generally characterized by high drought resistance due to changes in the elastic modulus and a higher degree of osmotic adjustment [45]. In our study, the highest level of water stress was reached in Stage III of fruit development (data not shown) when embryos are rapidly expanding [27]. Furthermore, pistachio trees are more sensitive to water stress in this stage with respect to Stage II [27]. An additional reason for the lower chlorophyll content under water stress observed in our study can be associated to the effect of irrigation on splitting percentage. In our experiment, we did not measure splitting percentage, but [PERSON] [26] found, in the same experimental orchard of this study, a significantly higher splitting percentage (56%) in rainfed trees in comparison with irrigated ones (33%). This effect was consistent among years. It is possible the higher splitting allows light penetration to the kernel that degrades chlorophylls, subsequently leading to the lower chlorophyll content in stressed trees. Volatile compounds were mostly composed by terpenes class and analysis revealed the main presence of \(\alpha\)-Pinene, D-limonene, 2-Carene and \(\beta\)-Myrcene, as shown in Table 3, confirming what previous studies have been reported on pistachio nuts [10; 18; 21]. In details, [PERSON] and [PERSON] [18] in nuts of four Turkish cultivars ([PERSON], Kirnizi, Halebi and SiirtveOhadi) found \(\alpha\)-Pinene (ranging from 15.53 to 48.57%), D-limonene (from 3.15 to 30.04%) and \(\beta\)-Myrcene (from 3.50 to 8.95%) as main components. [PERSON] [10] reported \(\alpha\)-Pinene and D-Limonene as the major compounds in fresh and roasted pistachio nuts in three different Iranian varieties (Ahmad Aghaei, Akbari and Kaleghouchi). Specifically, they reported in fresh pistachio nuts \(\alpha\)-Pinene values ranging from 1.6 to 2.05 mg/kg while in roasted ones from 2.52 to 3.96 mg/kg, and D-Limonene values ranging from 0.34 to 1.63 mg/kg in fresh nuts, while from 3.41 to 6.59 mg/kg in roasted ones. In the cultivar Kerman, [PERSON] [19] reported D-Limonene as the main volatile compound in nut essential oil (80-85%) and in leaf essential oil (78-81%); similar results were reported by [PERSON] [20] (67% in nuts and 53.6% in leaves, respectively). Furthermore, terpenes have been reported as dominant class in the leaves of _P. trebinthus_ and _P. lentiiscus_[46], and in nuts of _P. trebinthus_[47], suggesting that it might be a characteristic related to the _Pistacia_ species. We also found a positive and significant effect of plant water status on two compounds, \(\alpha\)-Pinene and \(\beta\)-Myrcene, as shown in Table 3. As above-mentioned, only few studies have been conducted on the influence of water status on pistachio qualitative characteristics and similar results were found. In fact, [PERSON] et al.