Abstract: [Objective] In this study, we investigated the changes of the phenotype, chloroplast ultrastructure and photosynthetic characteristics of Liriodendron sino-americanum under waterlogging stress, to reveal the response mechanism of L. sino-americanum in waterlogged stress and thereby provide a theoretical basis for promotion of cultivation and application in waterlogged districts.[Method] L.sino-americanum somatic embryo regeneration seedlings with 3-5 leaves were subjected to sterile waterlogging treatment for various durations (1d, 3d, 5d, 7d, and 2d recovery after 7days waterlogging), and the effects of the treatments on phenotype, chloroplast ultrastructure and photosynthetic characteristics were investigated.[Result] The results indicated that:1) The longer waterlogging treatment led to more severe damages to L.sino-americanum seedlings including curled and yellow leaves, or even leaves fell off from the base. However, the apical buds had no obvious changes, which could be due to the defense mechanism initiated by L.sino-americanum under waterlogging stress. 2) Waterlogging stress resulted in swelled chloroplasts and severe damages to the chloroplast ultrastructure of L.sino-americanum including degradation of chloroplast membrane system, loose and disordered arrangement of grana and stoma lamella, accumulation of starch grains, and osmiophilic granules density increased and and gathered together at the late stage of stress. 3) The net photosynthetic rate, stomatal conductance and transpiration rate were decreased whereas intercellular CO₂ concentration was increased slowly. The electron transport rate, photochemical quenching coefficient, photochemical efficiency and photochemical actual quantum yield in photosynthesizerⅡ showed a significant decline. The photosynthetic characteristics were partially restored after 2 days recovery treatment after 7d waterlogging stress. These data suggested that waterlogging stress caused serious damage in photosynthesizerⅡ activity center, and L.sino-americanum reduced gas exchanges by closing the stomata.[Conclusion] Taken together, waterlogging stress can cause the damages of chloroplast ultrastructure of L.sino-americanum, and then damage the photosynthesizerⅡ activity center, resulting in a decrease in leaf photosynthetic capacity and inhibition on growth and development of L.sino-americanum seedlings, but the seedlings may obtain to certain degree resistance under short-term waterlogging stress.

Key words: Liriodendron sino-americanum, embryo regeneration seedlings, waterlogging stress, chloroplastic ultrastructure, photosynthetic characteristics