LAUSR

Hypoxic stress is a major threat to the survival of seagrass but its damage mechanism is unclear. In order to explore the causes of seagrass meadow decline due to hypoxia, the effects of hypoxia in the dark on photosystem II (PSII) of Cymodocea rotundata, a widely distributed seagrass in Indo‐Pacific area, were investigated in this study. The results show that dark hypoxic stress resulted in the inactivation of PSII reaction center, as well as the damage of PSII donor and acceptor sides in C. rotundata. These damages to PSII were not related to leaf senescence, but to the reactive oxygen species (ROS) accumulation. The lower the seawater oxygen content and the longer the hypoxic duration, the harder the recovery of PSII activity under dark normoxia because of dark hypoxia caused D1 protein injury. The interference of hypoxia on the AOX respiratory pathway is an important cause of the PSII damage under dark hypoxic stress. The PSII damage would result in a significant decrease in the photosynthetic activity, thereby affecting the growth and development of C. rotundata. Based on the above results, it is suggested that dark hypoxia may be an important cause of decline of C. rotundata meadow.