OPEN ACCESS Yamauchi T, Colmer TD, Pedersen O & Nakazono M (2018) Regulation of root traits for internal aeration and tolerance to soil waterlogging-flooding stress

Root acquisition of water and nutrients is essential for plant growth and crop productivity (Lynch, 2015). An improved understanding of root system development and functioning, to identify root traits contributing to crop yields in various scenarios, is a research frontier that might enable a second Green Revolution needed to sustain world food security (Lynch, 2007). Roots are challenged by various abiotic and biotic constraints in soils, with water status of too little or too much being a major factor resulting in plant stress. Changing rainfall patterns have resulted in increased flood events in many regions, so that the development of flood-tolerant crops is a priority (Bailey-Serres et al., 2012). Water-saturated soils (i.e. waterlogged soils) are often anoxic, so that roots of poorly adapted species suffer oxygen deficiency that reduces respiration and results in a severe energy crisis, whereas welladapted wetland species can thrive (Bailey-Serres and Voesenek, 2008). The detrimental impact on upland crops of soil waterlogging can be substantial (e.g. wheat [Triticum aestivum]; Setter and Waters, 2003; Herzog et al., 2016).
Konnerup D & Pedersen O (2017) Flood tolerance of Glyceria fluitans: the importance of cuticle hydrophobicity, permeability and leaf gas films for underwater gas exchange

 Background and Aims Floating sweet-grass (Glyceria fluitans) can form aerial as well as floating leaves, and these both possess superhydrophobic cuticles, so that gas films are retained when submerged. However, only the adaxial side of the floating leaves is superhydrophobic, so the abaxial side is directly in contact with the water. The aim of this study was to assess the effect of these different gas films on underwater net photosynthesis (PN) and dark respiration (RD).
 Methods Evolution of O2 was used to measure underwater PN in relation to dissolved CO2 on leaf segments with or without gas films, and O2 microelectrodes were used to assess cuticle resistance of floating leaves to O2 uptake in the dark.
 Key Results The adaxial side of aerial leaves was more hydrophobic than the abaxial side and also initially retained a thicker gas film when submerged. Underwater PN vs. dissolved CO2 of aerial leaf segments with gas films had a Km of 172mmol CO2 m–3 and a Pmax of 7.1 µmol O2 m–2 s–1, and the leaf gas films reduced the apparent resistance to CO2 uptake 12-fold. Underwater PN of floating leaves measured at 700mmol CO2 m–3 was 1.5-fold higher than PN of aerial leaves. The floating leaves had significantly lower cuticle resistance to dark O2 uptake on the wettable abaxial side compared with the superhydrophobic adaxial side.
 Conclusions Glyceria fluitans showed high rates of underwater PN and these were obtained at environmentally relevant CO2 concentrations. It appears that the floating leaves possess both aquatic and terrestrial properties and thus have ‘the best of both worlds’ so that floating leaves are particularly adapted to situations where the plant is partially submerged and occasionally experiences complete submergence.

The cover of Annals of Botany where the study on Glyceria fluitans and its superhydrophobic leaves where published.
The cover of New Phytologist where the discovery of leaf gas films was published

Underwater photosynthesis and respiration in leaves of submerged wetland plants: gas films improve CO2 and O2 exchange

Many wetland plants have gas films on submerged leaf surfaces. We tested the hypotheses that leaf gas films enhance CO2 uptake for net photosynthesis (PN) during light periods, and enhance O2 uptake for respiration during dark periods. Leaves of four wetland species that form gas films, and two species that do not, were used. Gas films were also experimentally removed by brushing with 0.05% (v/v) Triton X. Net O2 production in light, or O2 consumption in darkness, was measured at various CO2 and O2 concentrations. When gas films were removed, O2 uptake in darkness was already diffusion-limited at 20.6 kPa (critical O2 pressure for respiration, COPR ≥ 284 mmol O2 m−3), whereas for some leaves with gas films, O2 uptake declined only at approx. 4 kPa (COPR 54 mmol O2 m−3). Gas films also improved CO2 uptake so that, during light periods, underwater PN was enhanced up to sixfold. Gas films on submerged leaves enable continued gas exchange via stomata and thus bypassing of cuticle resistance, enhancing exchange of O2 and CO2 with the surrounding water, and therefore underwater PN and respiration.

The study is published in: 
 Colmer TD, Pedersen O (2008) Underwater photosynthesis and respiration in leaves of submerged wetland plants: gas films improve CO2 and O2 exchange. New Phytologist 177: 918-926.

Underwater photosynthesis of submerged plants – recent advances and methods

We describe the general background and the recent advances in research on underwater photosynthesis of leaf segments, whole communities, and plant dominated aquatic ecosystems and present contemporary methods tailor made to quantify photosynthesis and carbon fixation under water. The majority of studies of aquatic photosynthesis have been carried out with detached leaves or thalli and this selectiveness influences the perception of the regulation of aquatic photosynthesis. We thus recommend assessing the influence of inorganic carbon and temperature on natural aquatic communities of variable density in addition to studying detached leaves in the scenarios of rising CO2 and temperature. Moreover, a growing number of researchers are interested in tolerance of terrestrial plants during flooding as torrential rains sometimes result in overland floods that inundate terrestrial plants. We propose to undertake studies to elucidate the importance of leaf acclimation of terrestrial plants to facilitate gas exchange and light utilization under water as these acclimations influence underwater photosynthesis as well as internal aeration of plant tissues during submergence.

The study is published in: 

Pedersen O, Colmer TD, Sand-Jensen K (2013) Underwater photosynthesis of submerged plants – recent advances and methods. Frontiers in Plant Science  4: doi: 10.3389/fpls.2013.00140.

The cover of FRONTIERS where methods on underwater photosynthesis and respiration were published