Abstract: A water culture experiment with 1/2 Hoagland solution was carried out to study the effects of exogenous 24-epibrassinolide (EBR) on the leaf photosynthetic characteristics and polyamines (PAs) content of cucumber seedlings under hypoxia stress. Under the stress, the leaf net photosynthetic rate (P_n), stomatal conductance (g_s), transpiration rate (T_r), and intercellular CO₂ concentration (C_i) decreased significantly while the chlorophyll content was in adverse, and the seedling growth was inhibited. Hypoxia stress increased the leaf Put, Spd, and Spm contents and Put/PAs ratio significantly, but decreased the (Spd+Spm)/Put. Under the stress, exogenous EBR not only increased the P_n, g_s, T_r, and chlorophyll content significantly, but also increased the contents of free Spm, conjugated Spd and Spm,and bound Put, Spd, and Spm significantly. Meanwhile, the PAs content and (Spd+Spm)/Put ratio were further increased, and the Put/PAs ratio decreased. The results suggested that exogenous EBR could regulate the formation and form transformation of PAs in cucumber seedling leaf, made the leaf keep a higher photosynthetic performance, promote the increase of leaf area and dry mass, and alleviate the damage of hypoxia stress.

Key words: cucumber, hypoxia stress, 24-epibrassinolide, photosynthetic characteristics, polyamine