Growth performance and defense response of Fraxinus excelsior L. seedlings to drought stress

Drought is predicted to increase drastically in the coming decades, making it crucial to understand the adaptive responses of tree species to such conditions. This study evaluates the effect of drought stress on the morphological, physiological, and biochemical parameters of Fraxinus excelsior, a native species from the Hyrcanian forest. The experiment was conducted using a completely randomized design, where one-year-old seedlings were subjected to four water stress conditions: 25% (severe stress), 50% (moderate stress), 75% (mild stress), and 100% field capacity (no stress). Results showed that drought reduced the growth rate and biomass of seedlings. Compared to the control treatment, the rate of decline in seedling height, biomass, leaf area, and specific leaf area index was 47.6%, 43.2%, 77.1%, and 38.1%, respectively, in the severe stress treatment. The root-to-shoot ratio increased by 44.8%. As water stress increased, the content of chlorophyll, chlorophyll a, chlorophyll b, and carotenoids showed a slight and insignificant decrease. The relative water content of seedlings decreased by 34.20% and 23.18% in severe and moderate stress treatments compared to the control treatment. Proline and malondialdehyde increased by 93.8% and 165.06%, respectively, in the severe stress treatment. Superoxide dismutase enzyme activity increased by 5%, while peroxidase enzyme activity increased by 273% and 240% in mild and moderate stress treatments, respectively. The data from this study suggests that F. excelsior seedlings cope with drought through avoidance and tolerance strategies such as reducing growth and biomass, increasing root-to-shoot ratio, retaining photosynthetic pigments and osmotic potential, and increasing enzymatic activities up to 50% FC. This species is recommended for afforestation and reforestation in drought-prone areas.