High-density nodal diaphragms in stems slow down, but do not obstruct, longitudinal oxygen diffusion during partial submergence

The species Alternanthera philoxeroides is a flood-tolerant plant that has to cope with the hypoxic stress under submergence. However, the pith cavity in stems of this species is interrupted and partitioned by low-porosity diaphragms at the nodes. To date little knowledge is available about whether discontinuous pith cavities are functional for internal gas transport in plants. To disclose the role of stem discontinuous pith cavities in internal gas transport, the diffusive transport capacity of O2, the tissue O2 status of intact plants, and the influence of restricting longitudinal O2 supply on whole-plant growth during partial submergence were assessed. We found that stem pith cavities were the main pathway for diffusional supply of molecular O2; blocking only one internode significantly decreased the O2 flux to lower internodes, and the reduced O2 flux translated into reduced growth in partially submerged plants. A major output component of the study is a model that uses normalized tissue dimensions and concentration gradients to establish a fair foundation for comparison of contrasting species under different experimental conditions. We therefore predict that future studies will use this approach to further broaden the scope and value of resistance and flux measurement in target species.