Abstract: With water culture, this paper studied the effects of exogenous spermidine (Spd) on the net photosynthetic rate (P_n), intercellular CO₂ concentrations (C_i), stomatal conductance (G_s), transpiration rate (T_r), apparent quantum yield (Φ_c), and carboxylation efficiency (CE) of cucumber seedlings under hypoxia stress. The results showed that the P_n decreased gradually under hypoxia stress, and reached the minimum 10 days after by 63.33% of the control. Compared with that of hypoxia-stressed plants, the P_n after 10 days application of exogenous Spd increased 1.25 times. A negative correlation (R²=0.4730～0.7118) was found between P_n and C_i. G_s and T_r changed in wider ranges, which decreased under hypoxia-stress, but increased under hypoxia-stress plus exogenous Spd application. There was a significant positive correlation between G_s and T_r (R²=0.7821～0.9458), but these two parameters had no significant correlation with P_n; Hypoxia stress induced a decrease of Φ_c and CE by 63.01% and 72.33%, respectively, while hypoxia stress plus exogenous Spd application made Φ_c and CE increase by 23% and 14%, respectively. The photo-inhibition of cucumber seedlings under hypoxia stress was mainly caused by nonstomatal limitation, while exogenous Spd alleviated the hypoxia stress by repairing photosynthesis system.

Key words: UV-B radiation, Spring wheat, Chemical composition, Fungal colonization, Decomposition