Responses of Nodulated Legumes to Oxygen Deficiency

Legumes are intolerant of flooding and symbiotic nitrogen fixation is particularly sensitive to waterlogging. Morphologically, the most obvious features of nodules subjected to flooding are the enhancement of lenticel- like structures on their surfaces and increased nodule cortication. These anatomical features increase the supply of oxygen to the bacteroid- containing tissue. Nevertheless, nitrogen fixation is severely impaired under waterlogged conditions. Exposure to subambient oxygen concentrations decreases the nitrogenase activity of detached root nodules. However, some recovery may occur in intact nodulated legumes after exposure to short-term moderate anoxygenic conditions. Tolerance of long-term moderate oxygen deficiency by grain legumes with determinate nodules is achieved by several structural adaptive mechanisms (e.g., the nodule surface is covered with loosely packed cells or the nodules have an increased number of unoccluded intercellular spaces, enhancing gaseous permeability) and biochemical adaptations (increased expression of ureide-forming enzymes and leghemoglobin content). Nevertheless, the tolerance mechanisms displayed by these plants under conditions of moderate hypoxia fail to provide an adequate environment for nitrogen fixation under severe hypoxia. This adaptation involves changes in photosynthate allocation to underground organs, aerenchyma formation in both roots and nodules to help maintain high rates of respiration (and hence mineral uptake), a less compact cortex with an increased nodule gas permeability, increased recycling of CO2, a less demanding nitrogenase activity and a more efficient nodule structure.