Anatomic prerequisites for internal root aeration of three tree species of the Amazonian inundation forest

Many tree species of the Amazonian inundation forest not only survive long periods of flooding, they are also able to continue shoot and root growth, flowering, and fruiting despite the unfavorable conditions of the oxygen-depleted soil. Internal root aeration is an effective adaptation mechanism that maintains the high-energy status required by root cells - to support growth and development. In the present study, internal root aeration was investigated in three Amazonian species by using oxygen microelectrodes to measure oxygen concentrations in the root cortex and rhizosphere, and light and fluorescence microscopy to analyze lacunar shape, extent, and distribution, as well as suberin incrustations in the exodermis. A highly porous gas transport system from the shoot to the roots is the prerequisite for an effective internal oxygenation. The results showed that the roots of Nectandra amazonum were not oxygenated, although large, regularly distributed intercellular spaces were observed in the root cortex. The roots of Triplaris pyramidalis were moderately supplied with oxygen via irregularly distributed, developing cavities in the cortex. Internal oxygen transport within regularly distributed lacunae (aerenchyma) in the root cortex of Pseudobombax munguba was reflected by the high concentration of oxygen measured in this species. In the roots of Triplaris pyramidalis, internal oxygen was conserved due to the formation of a hypodermal suberin barrier. By contrast, in Pseudobombax munguba, the absence of a suberized layer resulted in the diffusion of oxygen into the rhizosphere. The tree species display three different strategies, although they grow in the same habitat: no internal aeration at all, internal aeration without loss of oxygen to the rhizosphere, and internal aeration with massive loss of oxygen to the rhizosphere.