Abstract: A water culture experiment was conducted to study the relationship between photosynthetic thermal dissipation and xanthophyll cycle in cucumber seedling leaves under hypoxia stress (the dissolved oxygen concentration in nutrient solution was 0.9-1.1 mg·L^-1). Under the hypoxia stress, there was a significant decrease in the quantum yield of PSⅡ photochemistry rate (Φ_PSⅡ), net photosynthetic rate (P_n) under saturation light intensity, quanta yield (AQY), and maximal photochemical efficiency (F_v/F_m), suggesting that the photoinhibition of the seedling leaves was induced. Meanwhile, the thermal dissipation (NPQ) and the allocation of dissipation energy (D) by antenna increased, but the photochemical quenching apparent (q_P) decreased, suggesting the enhancement of thermal dissipation in cucumber leaves under hypoxia stress. A positive correlation was observed between NPQ and xanthophyll de-epoxidation state (DEPS), and both of them were promoted by ascorbic acid (AsA) and inhibited by 1,4-dithiothreitol (DTT), suggesting that xanthophyll cycle was the major pathway of photosynthetic thermal dissipation in cucumber seedling leaves under hypoxia stress.

Key words: hypoxia stress, cucumber, photoinhibition, xanthophyll cycle