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Seagrasses demonstrate complex ecological functions, and their morphological and physiological indicators are affected by various environmental factors. In this study, we conducted six sampling surveys of a Thalassia hemprichii seagrass bed in Xincun Bay, Lingshui, Hainan in 2019. The morphological and physiological characteristics of seagrasses in January, March, May, July, September, and November of a single year were studied, and the effects of various environmental factors on the morphological and physiological characteristics of T. hemprichii were analyzed. The results demonstrated markedly differences in leaf length and width, rhizome length and thickness, and root length across different months. The highest value [(15.05±6.13) cm] of leaf length occurred in May, and the lowest value [(7.19±2.55) cm] in September. The highest [(11.93±1.68) mm] and the lowest [(8.73±1.96) mm] values for leaf width were recorded in November and September, respectively. The highest value [(5.22± 1.71) mm] for rhizome diameter occurred in November, and the lowest value [(4.06±0.74) mm] in March. The C content of the underground seagrass tissues was significantly different across months, with the highest value (31.23%±0.94%) in November and the lowest value (24.90%±3.48%) in March. Seawater temperature had a significantly positive correlation with leaf width and C content of the underground tissues. Salinity was significantly positively correlated with the leaf and rhizome lengths of T. hemprichii and had a negative relationship with the C content of the underground tissues. There was a significant positive correlation between pH and leaf length, leaf width, and C and N content of the underground tissues. The ammonium level in seawater was significantly positively correlated with the N content of aboveground tissues. Nitrate level of seawater was negatively correlated with the C/N ratio of the aboveground parts. There was a significant positive correlation between the dissolved inorganic nitrogen (DIN) in seawater and the N content of the underground parts. Sediment organic carbon had a negative relationship with leaf length and width, and rhizome growth. The results demonstrated that the growth of T. hemprichii in Xincun Bay was greatly affected by temperature, salinity, and pH. A high nutrient level in Xincun Bay may have a negative impact on T. hemprichii. In order to protect the T. hemprichii meadow, measures should be taken to limit the input of nutrients in Xincun Bay. The results of this study may provide data support and serve as a scientific basis for the restoration of declining seagrass.

The distribution of seagrass meadows in intertidal zone is determined by the intensity of desiccation stress. The physiological characteristics of plants are the important basis to reflect their environmental stress effects. In order to explore the effects of the duration of photothermal desiccation stress on the physiological characteristics of seagrass during the tidal cycle, taking the tropical dominant seagrass Thalassia hemprichii as the research object, a simulation experiment was carried out in a solar greenhouse with high temperature (41.0-44.5℃) and strong light (633-1 682 μmol·m-2·s-1) environmental conditions. The effects of different desiccation time (0, 10, 20, 40, 60, 90 min·d-1, continuous for 7 d) on the growth rate, photosynthetic pigment content and photosynthetic fluorescence parameters of the leaves, as well as on the activities of antioxidant enzymes and the contents of nonstructural carbohydrates and secondary metabolites of leaves and roots were studied. The results were as follows: (1) The diurnal periodic photothermal desiccation of ≤10 min·d-1 had no significant effect on the growth rate and physiological characteristics of T. hemprichii (P>0.05). (2) Under the photothermal desiccation conditions, the damage of T. hemprichii leaves increased, the color of leaves changed from green to brown, and the growth rate of leaves decreased with the extension of desiccation time. Meanwhile, the photosynthetic pigment content, chlorophyll a/chlorophyll b and the chlorophyll fluorescence parameters of leaves showed a downward trend; the contents of MDA and soluble sugar and the activities of POD, SOD and CAT of leaves and roots showed an upward trend; whereas the contents of starch, total phenol and total flavonoids in leaves increased first and then decreased. (3) The diurnal periodic photothermal desiccation of 90 min·d-1 was the upper limit of incomplete inactivation for T. hemprichii chloroplast PS Ⅱ photosynthetic reaction center. In summary, a long term photothermal desiccation stress during tidal cycle has negative impacts on the growth and physiological characteristics for T. hemprichii. The results provide the reference for understanding the distribution of tropical seagrasses in the intertidal zone and the location selection of seagrass meadows for ecological restoration.