Waterlogging adaptation in mangroves: a review of aeration through aerenchyma and its functions

This review focuses on studies related to aerenchymal tissues and their functions in mangrove trees, particularly from the perspective of trees' hypoxic tolerance, which is crucial for understanding ecophysiological mechanisms underlying mangrove tree growth and evaluating functions of mangrove ecosystems. Mangrove trees possess a well-developed aeration system connecting aerial roots to submerged roots. This internal aeration structure connecting organs already exists during the seedling stage before aerial roots fully develop. Root porosity ranges from 6% to 60%, exhibiting species-specific characteristics. This variation likely correlates with the anaerobic conditions of each species' habitat. The internal aeration structure is open to the atmosphere via lenticels on the aerial roots and stem surface and via cork warts on the abaxial leaf surface, allowing the diffusion of oxygen, nitrogen, and methane driven by concentration gradients. Despite this extensive ventilation system, prolonged waterlogging at high tide inevitably leads to root hypoxia, causing anaerobic fermentation and damage induced by reactive oxygen species. As a tolerance mechanism against these stresses, mangrove trees possess antioxidant systems, though tolerance capacity varies among species. Some of the oxygen delivered to the underground part leaks into the soil through the root surface. This creates a thin oxidative layer on the root surface, reducing the uptake of phytotoxic substances and promoting the nitrification process in anaerobic soil. In addition to hypoxia caused by waterlogging, salt stress is a critical factor requiring adaptation on tidal flats. The energy demand required to cope with salt stress may increase oxygen demand through respiration, but the respiration rate of mangrove roots likely decreases under salt stress because the salt excretion process in mangrove roots is entirely physical in nature.