Extended Abstract
Introduction: Citrus mostly belong to the Citrus genus and the Rutaceae family, which are among the most commercially important fruits in the world. Tropical and subtropical regions with arid and semi-arid climates are the main areas for citrus production. Citrus fruits are important and popular crops that contribute to the human diet. Despite these beneficial compounds, there are still factors that affect the quality and production of citrus fruits. Citrus production is affected by biotic and abiotic stresses such as salinity. Soil salinity is a major global issue that has negative effects on agricultural production. Iron deficiency and salinity are among the most important factors affecting plant growth and the occurrence of stresses that may occur in saline and alkaline soils. One of the methods of resistance to abiotic stresses is the selection of appropriate Rootstock. Therefore, the aim of this study was to evaluate the activity of some antioxidant enzymes and osmolytes in two rootstocks of sour orange and volcameriana in relation to combined salinity and alkalinity stress.
Materials and Methods: This experiment included investigating the tolerance of two citrus rootstocks, including Sour orange and Volkameriana, to salinity and alkaline soils. For this purpose, one-year-old seedlings of these rootstocks were obtained from a commercial nursery in Dezful in 2019 and transferred to the greenhouse of the Faculty of Agriculture, Shahid Chamran University of Ahvaz, and then cultivated in 7-liter pots (containing a sand substrate). Hoagland nutrient solution was used to nourish and better establish the seedlings. After establishing the seedlings in the new conditions in June, they were subjected to salinity (sodium chloride) and alkali stress conditions. Salinity stress included concentrations of 0, 30, 60, and 90 mM sodium chloride. To prevent osmotic shock to the plants, the 90 mM salinity treatment was gradually applied in three steps. Alkaline stress was applied by increasing the pH of the nutrient solution from 6.5 to 8.2 using sodium carbonate. Salinity treatment was continued until the plants showed signs of stress. This experiment was a factorial experiment in a completely randomized design with 4 replications. At the end of the experiment, attributes such as phenol content, antioxidant capacity, hydrogen peroxide, malondialdehyde, and the activity of antioxidant enzymes such as peroxidase, catalase, ascorbate peroxidase, and superoxidase dismutase were measured.
Results and Discussion: The results showed that increasing the intensity of salinity and alkalinity stress significantly increased the amount of hydrogen peroxide, malondialdehyde and antioxidant capacity in both sour orange and Volkameriana rootstocks, but the amount of phenol increased in sour orange rootstock and decreased in Volkameriana 
rootstock in most cases. The highest amount of malondialdehyde (54.63 mmol/g FW) was obtained in Volkameriana rootstock in the presence of 120 mM NaCl and pH 8.2. At pH 8.2 and in the presence of 90 mM NaCl, the antioxidant capacity increased by 5.01% in sour orange rootstock and by 3.26% in Volkameriana rootstock, compared to the control. Increasing the salinity intensity significantly increased the activity of antioxidant enzymes such as peroxidase, catalase, and superoxide dismutase in both sour orange and Volkameriana rootstocks, which was higher at pH 8.2 than pH 6.5. Among all treatments, the highest catalase activity (2.47 units/mg protein) was obtained in the sour orange rootstock exposed to pH 2.8 and 90 mM NaCl and the lowest catalase activity (0.70 units/mg protein) was observed in the Volkameriana rootstock grown in the presence of pH 6.5 and 30 mM NaCl. The salt-induced increase in the activity of the superoxide dismutase enzyme was 52.69% in the sour orange rootstock and 34.60% in the Volkameriana rootstock. The activity of antioxidant enzymes increased with increasing salinity levels compared to the control. Under environmental stresses, which often induce oxidative stress, plants produce reactive oxygen species (ROS), which are harmful to plant growth due to their destructive effects on intracellular components and plant metabolism, leading to oxidative damage to cells. The amount of ROS is regulated by the activity of enzymes. During stress, the activity of peroxidase, catalase, and ascorbate peroxidase enzymes often increases. The activity of antioxidant enzymes can increase under saline and alkaline stress conditions and reduce the ROS.
Conclusion: In general, the results showed that the sour orange rootstock had greater tolerance to salt and alkali stress conditions than the Volkameriana rootstock, due mainly to possessing an increased antioxidant enzyme activity and phenol content.