Abstract: The effects of the dithiothretiol（DTT）on activate oxygen metabolism and chlorophyll fluorescences under seawater stress and methyl viologen（MV）induced oxidative stressin leaves of‘Helan 3’（seawater tolerant cultivar）were investigated by hydroponics. The results shown that production rate of superoxide radical，content of hydrogen peroxide（H₂O₂）and malonaldehyde（MDA）were remarkably increased by seawater and MV which could induce oxidative stress on spinach leaves，whereas contents of chlorophyll a（Chl.a），chlorophyll b（Chl.b），total chlorophyll and carotenoid（Car.） remarkably decreased. Under the stress of seawater and treated with MV，maxmium quantum efficiency of photosystem Ⅱ photochemistry（F_v/F_m），the effective PSⅡ quantum yield（Yield），apparent electron transport rate（ETR）and coefficient of photochemical quenching（q_P）were decrease significantly，coefficient of nonphotochemical quenching（NPQ/4）was increased significantly. Accumulation of reactive oxygen species（ROS），degradation of photosynthetic pigments，as well as decrease of F_v/F_m，Yield，ETR，NPQ，q_Pwere aggravated by introduction of dithiothreitol. Our results suggested that the inhibition of xanthophyll cycle under seawater stress decreased the dissipation of non-radiative energy，enhanced the accumulation of ROS，which were leading the degradation of photosynthetic pigments，decreation of activity of PSⅡ and ETR，reduction of energy for photochemical reactions，and photosynthetic performances were affected significantly. Higher activity of xanthophyll cycle in seawater-tolerant spinach plays a major role in maintaining the stability of photosynthetic pigments and photosynthetic activity under seawater stress.

Key words: spinach, Spinacia oleracea, seawater, dithiothretiol, xanthophyll cycle, reactive oxygen species, chlorophyll fluorescence