Morpho-physiological responses in Alnus subcordata and Acer velutinum seedlings to drought stress

In this study, the effect of drought stress on morphological and physiological characteristics of two forest species: Alnus subcordata and Acer velutinum, which are used in natural forest restoration and afforestation, was evaluated. A factorial experiment was conducted based on a completely randomized design with two factors (two species and four levels of drought stress: (25% (severe stress), 50% (moderate stress), 75% (mild stress), and 100% (no stress) percentage of field capacity). The results showed that drought stress significantly reduced the height, basal diameter, Leaf, and root biomass in both species, Whereas A. subcordata showed more stem biomass reduction. Root to shoot ratio showed a noticeable and slight increase in A. subcordata in A. velutinum respectively. Both species experienced a significant decrease in leaf area in drought conditions. Meanwhile, the specific leaf area decreased in A. subcordata as the degree of water stress increased, but it increased under mild and moderate treatment in A. velutinum seedlings. Total chlorophyll and chlorophyll a decreased in A. subcordata more than A. velutinum. Also, chlorophyll b decrease was similar in both species, while, carotenoids content decreased significantly in A. subcordata and increased insignificantly in A. velutinum. Drought stress showed a significant effect on the relative water content and proline in both species. There were no differences between the two species in proline content. furthermore,  A. velutinum had more relative water content than A. subcordata seedlings under moderate treatment. These findings suggested that the two species did not show similar responses to drought stress. Also, based on the measured characteristics A. velutinum may maintain stronger drought tolerance. The malondialdehyde content and antioxidant enzymes activities in both species increased with increasing stress. A. subcordata had a more increase in malondialdehyde content in drought treatment compared to the control treatment. The activity of antioxidant enzymes in both species increased in moderate and mild stress treatment and A. velutinum showed higher enzyme levels. Our results highlighted that the two species did not show similar responses to drought stress and A. velutinum maintained stronger drought tolerance based on the measured parameters. According to these findings and based on more ability of A. velutinum to cope with drought, it is recommended that A. velutinum plantation has more priority compared to A. subcordata in water deficit regions.