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.

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

Limited effect of radial oxygen loss on ammonia oxidizers in Typha angustifolia root hairs

The benefits of plant–microbe interactions have been exploited extensively for nutrient removal. Radial oxygen loss in aquatic macrophytes potentially promotes nitrification and accelerates nitrogen removal through coupled nitrification–denitrification process. Nitrification is likely the limiting activity for an effective nitrogen removal in wetlands. In this work, we have quantified the effect of radial oxygen losses in Typha angustifolia plants in environments of contrasting salinities, including a temporary lagoon, a constructed wetland, and a river estuary. In all sites, radial oxygen diffusion occurred mainly at a narrow band, from 1 to 5 cm from the root tip, and were almost absent at the tip and basal sections of the root (> 5 cm). Root sections with active oxygen diffusion tended to show higher bacterial and archaeal densities in the rhizoplane according to 16S rRNA gene abundance data, except at higher salinities. Archaeal amoA /bacterial amoA gene ratios were highly variable among sites. Archaeal nitrifiers were only favoured over bacteria on the root surface of Typha collected from the constructed wetland. Collectively, radial oxygen loss had little effect on the nitrifying microbial community at the smaller scale (differences according to root-section), and observed differences were more likely related to prevailing physicochemical conditions of the studied environments or to long-term effects of the root microenvironment (root vs sediment comparisons).

Radial oxygen loss is correlated with nitrogen nutrition in mangroves

The present study aimed to explore the possible functions of radial oxygen loss (ROL) on mangrove nutrition. A field survey was conducted to explore the relations among ROL, root anatomy and leaf N in different mangrove species along a continuous tidal gradient. Three mangroves with different ROL (Avicennia marina [A. marina], Kandelia obovata and Rhizophora stylosa) were then selected to further explore the dynamics of N at the root-soil interface. The results showed that seaward pioneer mangrove species such as A. marina appeared to exhibit higher leaf N despite growing under poorer nutrient conditions. Greater leaf N in pioneer mangroves coincided with their special root structure (e.g., high porosity together with a thin lignified/suberized exodermis) and powerful ROL. An interesting positive relation was observed between ROL and leaf N in mangroves. Moreover, rhizo-box data further showed that soil nitrification was also strongly correlated with ROL. A. marina, which had the highest ROL among the three mangrove species studied, consistently possessed the highest levels of NO3−, nitrification and ammonia-oxidizing bacteria and archaea gene copies in the rhizosphere. Besides, both NO3− and NH4+ influxes were found to be higher in the roots of A. marina when compared to those of K. obovata and R. stylosa. In summary, greater N acquisition by pioneer mangroves such as A. marina was strongly correlated with ROL which would regulate N transformation and translocation at the root-soil interface. The implications of this study may be significant in mangrove nutrition and the mechanisms involved in mangrove zonation.

Some Accessions of Amazonian Wild Rice (Oryza glumaepatula) Constitutively Form a Barrier to Radial Oxygen Loss along Adventitious Roots under Aerated Conditions

A barrier to radial oxygen loss (ROL), which reduces the loss of oxygen transported via the aerenchyma to the root tips, enables the roots of wetland plants to grow into anoxic/hypoxic waterlogged soil. However, little is known about its genetic regulation. Quantitative trait loci (QTLs) mapping can help to understand the factors that regulate barrier formation. Rice (Oryza sativa) inducibly forms an ROL barrier under stagnant conditions, while a few wetland plants constitutively form one under aerated conditions. Here, we evaluated the formation of a constitutive ROL barrier in a total of four accessions from two wild rice species. Three of the accessions were wetland accessions of O. glumaepatula, and the fourth was a non-wetland species of O. rufipogon. These species have an AA type genome, which allows them to be crossed with cultivated rice. The three O. glumaepatula accessions (W2165, W2149, and W1183) formed an ROL barrier under aerated conditions. The O. rufipogon accession (W1962) did not form a constitutive ROL barrier, but it formed an inducible ROL barrier under stagnant conditions. The three O. glumaepatula accessions should be useful for QTL mapping to understand how a constitutive ROL barrier forms. The constitutive barrier of W2165 was closely associated with suberization and resistance to penetration by an apoplastic tracer (periodic acid) at the exodermis but did not include lignin at the sclerenchyma.