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Why Do Some Plants Remain Dry Under Water?

Postdoc-stipendium i Danmark | 02/05/16

Plants that remain dry under water could be very relevant in the perspective of a future climate. Postdoc Dennis Konnerup has received a grant from the Carlsberg Foundation to conduct research on how to make plants more ‘waterproof’. This will enhance survival of floods as such extreme events will be more frequent in the future.

Climate change will in many places be accompanied by more frequent and more severe rainfall events, which can flood fields causing loss of crops. As for other organisms, it is also a challenge for plants to stay under water for long periods. Especially ‘breathing’, i.e. exchange of oxygen and carbon dioxide is more difficult in water compared to in air. While some plants do not have any adaptations to surviving under water, others have developed water-repellent leaves on which a layer of air forms, when they are under water. This phenomenon is called a gas film and facilitates exchange of gases and hence survival under water. Studying this mechanism in greater depth could have importance to design of ‘waterproof crops’ and thus agriculture in the future. 

“If we can breed crops that can survive temporary flooding, this will enhance our possibilities of supporting the world’s growing population in the future significantly,” explains postdoc Dennis Konnerup, Freshwater Biological Laboratory, University of Copenhagen. 

Plants With Special Properties Under Water

Some plants have the ability to remain dry, even when under water – an ability similar to water-repellent clothes, where water drops just bounce off. This mechanism is due to a super-hydrophobic surface (see Box) of the leaves, which gives them a silvery look, when they are submerged. This is because of a thin layer of air surrounding the leaves under water, also called a gas film. In 2008, associate professor Ole Pedersen from University of Copenhagen in collaboration with the Australian professor Tim Colmer discovered that the gas film is of advantage to plants that are completely submerged. The reason to this is because it is much easier for the plants with gas films to exchange oxygen and carbon dioxide with the surrounding floodwater, and therefore they have a much better chance of surviving floods. The gas films provide a large gas-water interface, which oxygen and carbon dioxide can diffuse into and then from the gas film into the leaf through apertures called stomata. The stomata remain open when a gas film is present and the plants are therefore still functional even under water. This means that the plant is able to keep an efficient gas exchange with the surroundings although it is submerged. 

Dennis Konnerup explains: 

“We don’t know the foundation for forming a stable and well-functioning gas film in wild plants, which are completely submerged by floods on a regular basis.”

The studies made so far have shown that there are large differences between species in terms of how long they can retain the gas film. Some species lose the gas film after a few days under water whereas others can retain it for several weeks.

“We are not sure why the species with gas films differ so much in their ability to retain it,” Dennis Konnerup says, but claims one reason: “One hypothesis is that it is a certain structure of the leaf surface that determines how well the gas films are retained. However, we still don’t know if these leaf structures change, when the plants are submerged.”

An alternative explanation is that the conditions in the floodwater determine how long the plants can retain the gas film – cool and CO2-enriched water is believed to give optimal conditions for forming and retaining a gas film.

CO2-response curve for rice (Oryza sativa) with photosynthesis as a function of CO2 concentration. The figure shows net photosynthesis under water for leaf segments with intact gas films or with the gas films experimentally removed by brushing the leaf with a dilute detergent that removes the surface tension, so the leaf is not super-hydrophobic anymore. These data demonstrate how efficient the gas film is in facilitating the gas exchange with the surrounding floodwater: in leaves with gas films, photosynthesis is saturated (reaches the maximum) already at 400 mmol m-3, whereas leaves without gas films become saturated at 2000 mmol m-3. (Pedersen et al. (2009)).

Wheat and Flooding

Dennis Konnerup and the rest of the research group at University of Copenhagen are focusing on wheat, as temporary flooding of fields is an increasing problem for farmers nationally as well as internationally. In Denmark, wheat is the most important crop and on a world scale, it is the fourth most produced crop after sugar cane, maize and rice. Flooding of wheat fields often occurs when the plants are small, and therefore, they are very sensitive and can only survive being submerged for a few days. In an experiment, Dennis Konnerup and his colleagues will compare different wheat cultivars in their ability to retain a gas film and survive submergence. 

“We expect to see a pattern where the cultivars that retain a gas film longer, also are the ones that show a better survival when they become submerged,” Dennis Konnerup says. 

Furthermore, the research group compares the wheat cultivars with other members of the grass family such as rice, which is known to be tolerant of submergence. Wheat will also be compared with the wild species, water whorl grass (Catabrosa aquatica), that grows in or near water, is tolerant of submergence and can retain a gas film for several weeks. The researchers will analyse the surface of the leaves (the cuticle) using scanning electron microscopy (SEM) and X-ray tomography to investigate which structures are responsible for gas film formation. Dennis Konnerup explains that these methods can give them an opportunity to follow the cuticle and see how it changes during submergence.

Scanning Electron Microscopy (SEM) photograph of the surface of a leaf from water whorl grass (Catabrosa aquatica). The leaf has grooves, papillae and wax crystals, which all contribute to its super-hydrophobicity. Photo: Dennis Konnerup.

Lack of Oxygen During the Night

An important aspect is to elucidate how the gas film affects the photosynthesis under water during the day, and how the internal oxygen status of the plants decreases during the night when there is no light to drive photosynthesis. The photosynthesis produces oxygen, so during the day there is ample oxygen to support the plant’s respiratory demand. However, during the night the plant may experience very low oxygen concentrations, which is critical for respiration. The leaves can take up oxygen from the surrounding floodwater and in this way keep a certain level of oxygen in the leaves, but the roots are below ground in the anoxic soil. The transport of oxygen occurs via internal air channels from the shoot to the root. In this case, it is also a great advantage for the plant to have a gas film as it enhances the transport of oxygen from the floodwater to the shoot and hence how much oxygen there is available to the roots. 

“Our theory is that the internal oxygen concentrations, especially in the roots, will decrease significantly as the plants lose their gas films during submergence,” Dennis Konnerup says.

Submerged wheat leaf with gas film. The gas film can be seen as a silvery layer when the leaf is under water. The picture also shows a bubble of oxygen produced via photosynthesis. (Photo: Ole Pedersen).

The Carlsberg Foundation’s Contribution

After three years as postdoc in Australia, Dennis Konnerup has received a postdoc grant from the Carlsberg Foundation, which gave him the opportunity to return to Denmark. He is now contributing with his international experience in the research group at University of Copenhagen, where they study submergence tolerance of plants. In Australia, Dennis worked in a group recognised internationally for research in high-level plant physiology. 

“The grant from the Carlsberg Foundation has given me a chance to return to Denmark, where I can contribute with my experience and establish a career as scientist,” Dennis Konnerup explains.

Future Potential

The project is interesting to science as it looks into a mechanism that is newly discovered. Yet it also contains the Scientific Social Responsibility (SSR) approach meaning that it is of relevance to society in general. This is due to the potential applied aspects of the project concerning crops growing in areas prone to flooding, which is likely to be one of the great challenges in the future. In Europe, this could be e.g. wheat and in Africa and Asia it is highly relevant to rice. Although rice is known to be a plant that can grow partially submerged, it suffers when in becomes completely submerged. In Asia, where most of the world’s rice is grown, about 20 million hectares of rice land is prone to flooding. In India and Bangladesh alone, more than 5 million hectares of rice fields are flooded during most of the planting seasons, and the plants risk being completely submerged.

“Therefore, it is of great importance to elucidate how gas films affect the survival of both cultivated and wild rice to potentially breed rice cultivars that could survive longer under water,” Dennis Konnerup concludes.