Comparative study on anatomical traits and gas exchange responses due to belowground hypoxic stress and thermal stress in three tropical seagrasses

Abstract

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 O2level (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 O2loss (ROL) barrier. H. ovalis also showed unique responses to root hypoxia and heat stress. Root hypoxia increased O2release 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 O2release in the belowground tissues and overall photosynthetic capacity (O2release 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 O2release 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 O2loss in hypoxic sediment. At the same time, its root oxidation capacity may be compromised under warming scenarios.