Medicago ruthenica (L.) Sojak. cv. Zhilixing response and adaptation strategy to drought stress and rehydration

The effect of drought stress and rehydration on Medicago ruthenica (L.) Sojak cv. Zhilixing morphological structures, physiological metabolism, and matter distribution were investigated to determine the adaptation strategy in an arid environment. Seedling-stage M. ruthenica (with 6-8 leaves) were subjected to continuous drought stress for 12 days and then re-watered for 4 days; samples were collected from the control group (CK), after 9 and 12 days of drought stress, and 4 days after rehydration. Compared with the CK, the stoma opening rate decreased, while the stoma and epidermis cell densities increased by 48.5% and 36.6%, respectively. Smaller and denser epidermal cells formed. The malondialdehyde (MDA) content increased gradually during stress period. Superoxide dismutase activity (SOD), peroxidase activity (POD), and contents of chlorophyll, soluble sugar, soluble protein, and proline first increased and then decreased with prolonged drought. Total biomass also first increased then decreased, reaching the maximum after 9 days (0.433 7 g; 16.4% increase). The distribution of the above-ground biomass was higher than the under-ground biomass, and the under-ground biomass proportion increased with drought, while the opposite effect was observed with the above-ground biomass. After re-watering, all of the physiological and biochemical indexes recovered or exceeded those of the control, indicating strong rehydration sensitivity and resilience. M. ruthenica adaptation to drought stress and rehydration was divided into three periods:active adaptation, passive adaptation, and re-watering. The physiological parameter plasticity index was 0.16, 1.33 times greater than the morphological parameter index, during the active adaptation period. Drought stress adaptation was achieved by altering the antioxidant and osmotic regulation systems to reduce water loss, improving water retention and absorption efficiency, and maintaining the water absorption and loss balance. The morphological parameter plasticity index was 0.24, 1.31 times greater than the physiological parameter index, during the passive adaptation period. M. ruthenica sacrificed biomass and reduced pigments to aid survival. The root-shoot ratio (25.9%), stomatal opening rate (29.7%), and stomatal and epidermal cell density (24.2% and 16.3%, respectively) were higher than those in the CK during the re-watering period. These characteristics promoted rapid water recovery, absorption capacity, and water transport efficiency. Morphological and physiological changes allowed M. ruthenica to adapt to drought stress and rehydration, contributing to its survival in arid conditions.