NITROGEN FIXATION BY LEGUMES IN FLOODED REGIONS

Abstract

Wetlands are often subjected to annual net losses of N from the system via leaching of the soil, which are not balanced by inputs of N via the mineralization of organic matter. These oligotrophic ecosystems are largely dependent on N inputs from biological nitrogen fixation (BNF), and legume-rhizobial symbioses are some of the main contributors. However, legumes are often sensitive to flooding, and soil inundation can have a serious effect on their growth. This is because legume nodules have a requirement for O₂ to maintain the aerobic respiration necessary to supply the high amounts of ATP needed for nitrogenase activity. Despite having adaptations such as the O₂-carrying protein leghaemoglobin (Lb) to optimize O₂ supply, most terrestrial legumes are not capable of increasing the O₂ supply and within their nodules sufficiently whilst flooded, and hence BNF is reduced or completely curtailed. By contrast, most legumes that fix N₂ in flooded regions have evolved additional mechanisms to optimize the O₂ supply to their flooded nodules, as well as the morphological, structural and biochemical adaptations that allow these legumes to fix N₂ whilst flooded.