Abstract: To clarify the alleviative effect of exogenous abscisic acid (ABA) on Medicago sativa under drought stress, we conducted an experiment with the drought-tolerant cultivar Dryland and drought-sensitive cultivar WL354HQ. With four treatments including water spray control, exogenous ABA treatment, drought treatment, and drought + ABA treatment, we analyzed the effects of exogenous ABA on the physiological characteristics of M. sativa seedlings with different drought tolerance. The results showed that drought stress significantly inhibited seedling growth, induced severe oxidative stress and osmotic adjustment responses, and suppressed photosynthetic capacity. Compared with drought treatment, the drought + ABA treatment significantly alleviated the inhibition of drought on seedling growth. Specifically, root length of drought-tolerant and drought-sensitive cultivars increased significantly by 34.6% and 9.8%, respectively. Meanwhile, membrane lipid peroxidation in plant cells was effectively alleviated. Malondialdehyde (MDA) content in leaves and roots of the drought-tolerant cultivar significantly decreased by 20.0% and 54.9%, respectively. Excessive proline accumulation was inhibited. Proline content in leaves and roots decreased by 19.8% and 64.0% in the drought-tolerant cultivar, and by 3.5% and 20.2% in the drought-sensitive cultivar. The photosynthetic stability of the drought-tolerant cultivar was superior to that of the drought-sensitive cultivar. Specifically, leaf transpiration rate was significantly decreased by 62.5% and 49.1%, and net photosynthetic rate was significantly reduced by 58.9% and 32.7% in drought-tolerant and drought-sensitive cultivars, respecti-vely. In conclusion, ABA mitigated drought stress in M. sativa by coordinating osmotic adjustment and oxidative defense systems, and the alleviatory effect was stronger in the drought-tolerant cultivar.

Key words: abscisic acid, Medicago sativa, drought stress, physiological characteristics, photosynthetic trait