Morphological and anatomical responses of the common mangrove Rhizophora mucronata seedlings to flooding

Mangrove forests play an important role for both humans and animals especially as a food source and habitat. Rhizophora mucronata is a dominant species in many mangrove areas. When flood occurs, the seedlings which are in the most sensitive stage, might be significantly affected. A purpose of this study was to investigate the morphological and anatomical response of R. mucronata seedlings to the two levels of flooding: waterlogging and submergence. An experiment consisted of three treatments including control (regular drainage), waterlogging (containing 10 cm of water above the soil surface) and complete submergence that lasted for 20 days. Our results show a rapid leaf abscission and an adjustment of leaf angle to more vertical position in the submerged seedlings. Both responses are common in the flooded plants and are related to hormonal regulation. In addition, plant roots in flooded treatment were visibly darker than the controls, possibly due to a damage in highly reducing rhizophere. In terms of anatomical change, we found the enlarged aerenchyma tissues and lenticel formation in root periderm of the flooded plants. These results suggest that the two flooding levels may induce a hypoxia and consequently stimulated an alteration of anatomical characteristics which plays an important role in conveying and storing oxygen within roots. However, there is no significant difference in anatomical features of leaf tissue in this study. Further studies in physiological adaptation are required to better understand the responses of the mangrove seedlings to flooded condition.