Physiological responses of mature Persian oak (Quercus brantii L.) under natural conditions to drought stress

Introduction and Objective: Drought significantly impacts plant growth and has become a critical global concern. Understanding how plants respond to drought is essential. Under arid conditions, forests are prone to experiencing various types of stress. Trees protect themselves from drought stress through both physical and chemical defenses. Physical defenses include increasing the density of structures like leaves, thorns, and hairs, while chemical defenses involve producing physiological responses that serve as protective barriers against environmental factors. Oaks are known for their resistance to drought and heat, which is attributed to their physiological responses during dry conditions. The reaction of the dominant species of Persian oak (Quercus brantii) in the Zagros forests to recent droughts suggests that this species is highly sensitive to these conditions. The response to drought stress in mature, multi-year-old trees differs from that of seedlings of the same species. Certain physiological mechanisms influence the internal changes in trees as they react to stress. Therefore, the objective of this study is to examine the effects of drought stress on the physiological traits of mature oak trees under both optimal and stressful natural conditions.
Materials and Methods: This research was conducted on multi-year-old Persian oak trees over two years (2020-2022) in the natural forest environment of the Fajr Safa agro-industrial complex, located 10 kilometers from Khorramabad city. The experiment followed a completely randomized block design with three levels of drought stress. Drought treatments included a severe drought treatment (where the soil was completely covered with white plastic to prevent any moisture from reaching it for two years), moderate soil moisture treatment (with irrigation every 10 days), and a control group. The statistical sample included 120 trees, divided into 30 trees for each treatment group. On September 10, 2022, at the end of the experiment, leaves were collected from each tree at a fixed height of two meters around the trunk, from all four cardinal directions, and were taken to the laboratory. By measuring fresh weight, turgid weight, and dry weight, the relative water content of the leaves was calculated. To further analyze physiological traits, the leaves were dried in an oven at 70°C for 72 hours and ground into a powder. The following were measured in trees across all three treatment groups: proline levels, soluble and insoluble sugars, enzyme activity (peroxidase, superoxide dismutase, and ascorbate oxidase), as well as the amounts of carotenoids, chlorophyll a, chlorophyll b, and total chlorophyll.
Findings: The analysis of variance results indicated that different treatment levels had a significant effect at the 0.05 level on the amounts of proline, relative water content of the leaves, and the activities of enzymes (peroxidase, superoxide dismutase, and ascorbate oxidase). However, no significant difference at the 0.05 level was observed between treatments regarding soluble sugars, insoluble sugars, and photosynthetic pigments (carotenoids, chlorophyll a, chlorophyll b, and total chlorophyll). Among the treatment levels, the relative water content of the leaves showed the most variation. As drought stress increased, the relative water content of the leaves decreased, while proline, soluble sugars, insoluble sugars, enzymes (superoxide dismutase, peroxidase, and ascorbate oxidase), and photosynthetic pigments (chlorophyll a, chlorophyll b, total chlorophyll, and carotenoids) showed an increasing trend. The highest proline levels were found in the control group, with the lowest observed in the irrigated group. Relative water content, peroxidase, and superoxide dismutase activity were highest in the irrigated group. Under severe drought stress, levels of proline, soluble sugars, and insoluble sugars increased compared to the control group.
Conclusion: Based on the results of this study, mature oak trees tend to preserve carbohydrates and sugars under stress. This research highlights the adaptability of mature oak trees to both optimal conditions (comparing control with irrigated trees) and their resilience during dry periods (comparing control with stressed trees). The findings indicate that mature Persian oak trees can resist drought stress to some extent by increasing proline and antioxidant enzyme activity. Compared to previous studies, where seedlings showed greater vulnerability to changes in carbohydrates and soluble sugars under stress, mature trees seem to conserve their carbohydrates and sugars. These findings are highly relevant for forest management strategies and can inform forest specialists and stakeholders. Overall, the results suggest a strong relationship between drought stress responses and plant physiology. It can be concluded that the climatic conditions affecting Persian oak stands are critical and should be a major consideration for forest managers responsible for developing management programs for the western forests of the country.