Background The ability to maintain sufficient oxygen levels in the belowground tissues and the rhizosphere is crucial for the growth and survival of seagrasses in habitats with highly reduced sediment. Such ability varies depending on plant anatomical features and environmental conditions. Methods In the present study, we compared anatomical structures of roots, rhizomes and leaves of the tropical intertidal seagrasses, Cymodocea rotundata, Thalassia hemprichii and Halophila ovalis, followed by an investigation of their gas exchange both in the belowground and aboveground tissues and photosynthetic electron transport rates (ETR) in response to experimental manipulations of O2 level (normoxia and root hypoxia) and temperature (30 °C and 40 °C). Results We found that C. rotundata and T. hemprichii displayed mostly comparable anatomical structures, whereas H. ovalis displayed various distinctive features, including leaf porosity, number and size of lacunae in roots and rhizomes and structure of radial O2 loss (ROL) barrier. H. ovalis also showed unique responses to root hypoxia and heat stress. Root hypoxia increased O2 release from belowground tissues and overall photosynthetic activity of H. ovalis but did not affect the other two seagrasses. More pronounced warming effects were detected in H. ovalis, measured as lower O2 release in the belowground tissues and overall photosynthetic capacity (O2 release and dissolved inorganic carbon uptake in the light and ETR). High temperature inhibited photosynthesis of C. rotundata and T. hemprichii but did not affect their O2 release in belowground tissues. Our data show that seagrasses inhabiting the same area respond differently to root hypoxia and temperature, possibly due to their differences in anatomical and physiological attributes. Halophila ovalis is highly dependent on photosynthesis and appears to be the most sensitive species with the highest tendency of O2 loss in hypoxic sediment. At the same time, its root oxidation capacity may be compromised under warming scenarios.

During midday low tides, tropical intertidal seagrasses are challenged by high irradiance and high temperature. This study assessed photosynthetic and oxidative stress responses of Thalassia hemprichii and Halophila ovalis exposed to 150 and 1000 μmol photons m−2 s−1 and 30 and 40 °C for 3 h. High temperature (40 °C) significantly decreased the maximum quantum yield of both seagrasses and this heat-induced photoinhibition was exacerbated by high irradiance (1000 μmol photons m−2 s−1). High irradiance also aggravated the effects of high temperature on the effective quantum yield of T. hemprichii. Non-photochemical quenching (NPQ) of T. hemprichii was induced by both stressors with no additive effects. In contrast, NPQ of H. ovalis was induced under high irradiance at 30 °C but was inhibited at 40 °C. Nevertheless, antioxidant enzyme activity and reactive oxygen species content did not differ among treatments in either seagrass. Monitoring chloroplast distribution in H. ovalis revealed a partial inhibitory effect of high temperature on chloroplast avoidance movement under high irradiance. Our results suggest that warming events may cause detrimental impacts on shallow water seagrasses. Halophila ovalis may be more vulnerable than T. hemprichii as its photoprotection, i.e. NPQ and chloroplast avoidance movement, was hindered at high temperature.