Effects of Dissolved O2 and Fe Availability on Growth, Morphology, Aerenchyma Formation and Radial Oxygen Loss of Canna indica L. and Heliconia psittacorum L.f.

Abstract In constructed wetlands (CWs), plants are usually affected by low O2 levels. Under such conditions, most soluble iron is reduced to ferrous (Fe2+) which is highly soluble, and toxic to plants as well. As a consequence of excessive ferrous iron with low O2 supply, plant growth is reduced, leading to declining nutrient removal efficiency. This study was conducted to determine the effects of different dissolved oxygen levels (normoxia and hypoxia) with Fe supplied on growth, morphology, and root anatomy of two wetland plants (Canna indica and Heliconia psittacorum). The plants were grown on a nutrient solution modified from Smart and Barko (1985) under normoxic and hypoxic conditions. All plants were grown in greenhouse conditions for 42 days. Plant growth rates and biomass accumulation were drastically reduced under hypoxia while leaf number was not affected. Under hypoxia, root diameter and root porosity also increased in C. indica, whereas H. psittacorum had greater aerenchyma formation. Moreover, C. indica showed adaptive traits to cope with hypoxia and Fe stress by increasing radial oxygen loss (ROL), releasing O2 to the rhizosphere to resist toxic effects of ferrous iron under hypoxia. In contrast, H. psittacorum had no ROL under hypoxia. Moreover, the plants showed leaf chlorosis, leaf roll, and root rotting. Hence, it is suggested that C. indica could have better performance than H. psittacorum to treat wastewater in CWs as this species can adapt to hypoxic conditions and releases O2 into rhizosphere which improves dissolved oxygen (DO) in the wastewater. Keywords: Aerenchyma, Dissolved oxygen, Iron, Root porosity, Wetland emergent plant