## 4 Discussion
Thus, the process of acclimation to stress conditions leads the plants to adjustments regarding leaf morphology, chloroplast pigment composition, and the activity of biochemical processes that prevent photosystems from oxidative damage [64]. Moreover, it is accepted that several environmental conditions, such as drought [80] and salinity [81], which engender oxidative stress in plants, are connected with the induction of phenylpropanol metabolism, a finding that mirrors the role of phenolics in protecting the cells under the aforementioned abiotic stress conditions [82]. Additionally, flavonoids are regarded as photosynthetic apparatus protectants as they partly prevent DNA from the harmful effects of UV radiation [83; 84], while carotenoids constitute a factor of the antioxidant mechanism, which coordinates a plant's defense against ROS composed under stress conditions [85]. In the present work, a slight increase in antioxidant capacity estimated with the DPPH method and carotenoid concentration of the control plants grown under salinity conditions was found compared with normal conditions, but no increase in phenolic, flavonoid, and antioxidant capacity estimated with the FRAP method were observed. Salt stress is thought to be one of those important factors which restrict the development of apple in some regions. However, salt tolerance of some apple rootstocks was previously evaluated and showed large differences among species [21]. Regarding the addition of zeolite to the substrate, it was associated with higher values of leaf, stem, and root DW, transpiration rate, stomatal conductance and internal CO\({}_{2}\) concentration, carotenoid concentration, and antioxidant capacity assessed with the DPPH method in the apple plants of zeolite treatment grown in the absence of salinity compared with the corresponding control plants, although there was not in all cases a statistically significant difference. [PERSON] et al. [86] concluded that the application of zeolite in normal soil improved its ability to retain nutrients in the root zone without affecting its ability to drain, while previous research has shown that the use of zeolite in normal conditions contributed to a higher root DW of bean plants [87]. [PERSON] et al. [88], also, observed that zeolite exceeded perlite, since the first increased the growth of crisp-head lettuces. In addition, when plants grown under salinity conditions of the zeolite and the control treatment were compared, it was found that the application of zeolite contributed to greater leaf DW, relative growth rate, transpiration rate, proline concentration, phenolic concentration, and antioxidant capacity when measured by the DPPH or FRAP method, but not always with significant difference. Zeolite, due to its structure rich in micropores and physicochemical properties, can act as a trap for positive cations, such as ammonium and potassium [86]. At the same time, Na\({}^{+}\) and Cl\({}^{-}\) ions can be absorbed in the network of cavities, which helps the improvement of soil properties. Thus, with the use of zeolite, both the improvement of soil fertility and the gradual release of nutrients are achieved [86]. In this direction, [PERSON] et al. [89] reported a positive effect of zeolite application on maize plants grown under saline conditions, as plant nutrient uptake was improved, while [PERSON] et al. [90] found that zeolite led to a substantial increase in barley biomass when plants were exposed to salinity. In addition, [PERSON] et al. [90] noticed that zeolite enhanced the water and salt holding capacity of soil. During the growth of plants in salinity conditions, zeolite on the one hand significantly increases the availability of nitrogen in plants, thereby enhancing the synthesis of chlorophyll, antioxidant enzymes, and other structural components of the plant and, on the other hand, significantly contributes to increasing the relative water content of stressed plants by increasing water availability and its uptake by roots [91].