Does mangrove stem bark have an internal pathway for gas flow?

Key Message The aerenchyma in the stem bark of mangroves allows a gas flow pathway outside the xylem, mainly in the lower stem, as the aerenchyma is similar to aerial roots. Abstract Mangroves have developed a network of gas spaces in aerenchyma tissue to allow an internal pathway for gas flow into the roots. The presence of aerenchyma in the stem bark of Rhizophora mangle and Avicennia germinans suggests that air might flow through this tissue external to the xylem. The structure of R. mangle and A. germinans stem bark aerenchyma and the air volumetric flow rate ( Q ) among aerial roots and two different levels in the stem were compared. Samples of the bark and aerenchyma traits were analyzed using light microscopy, scanning electron microscopy (SEM), and resin casts. Hypertrophy in lenticels widened their opening. In A. germinans , there were interconnected canal segments with a straight and parallel pattern in the upper part of the stem and curvatures in the lower level. In R. mangle , the canals were more extended. Both species contained honeycomb-like aerenchyma in the lower level of the stem, contributing to canal structures' resistance to internal pressure. Both species had phi thickening in the radial walls, and R. mangle H-trichosclereids supporting the canal structures may resist internal pressure. The lysis of cortical cells contributed to canal formation in the lower stem of A. germinans . The shape and length of the aerenchyma longitudinal canals were determined mainly by the species. There was a more significant effect of stem level and roots on traits related to intercellular spaces in aerenchyma and lenticel openings. Mangrove species share common functional adaptations and have developed distinctive structural features that allow them to live in a temporary anaerobic root environment.