How plants conserve water

When the heads of government of the UN Member States meet in Copenhagen in December for the world climate summit, one topic will be on the agenda again: climate change. The struggle by politicians to find a way of limiting global warming will also be followed with interest by plant researchers and experts in farming. After all, these people are already seeing the effects of the rise in temperature.

Modern cultivated plants have forgotten how to conserve water

Rainer Hedrich, Chairman of the Department of Molecular Plant Physiology and Biophysics at the University of Würzburg, is among those interested in the consequences of protracted periods of drought and rising temperatures for the plant world. “Having been subjected to centuries of cultivation, today's cultivated plants have lost some of their vitality. To put it bluntly, our crops have forgotten how best to conserve water,” says Hedrich. This means they would not be able to withstand a global change in climate with lengthy periods of heat and drought.

Hedrich has researched the water balance of plants. His most recent findings are reported in the latest issue of the journal Proceedings of the National Academy of Sciences (USA).

Dilemma attached to water shortage and water loss

Plants extract water from the soil and carbon dioxide from the air, which they use during photosynthesis to produce carbohydrates and oxygen. They release water into the environment in the form of water vapor.

“The release of water vapor as an unavoidable consequence of photosynthesis does not constitute a problem for the plant as long as it has enough water at its disposal,” explains Hedrich. However, if rain does not materialize, the plant cannot absorb any more water through its roots and, at the same time, it loses more water to the increasingly dry atmosphere.

However, the plant is not completely defenseless against this dilemma. “Its outer skin, the so-called epidermis, is covered with a layer of wax which is impermeable to water and carbon dioxide,” says Hedrich. It is only through microscopically small, controllable pores that the plant can absorb carbon dioxide and release water vapor.

Sensory cells register the water content of the plant

How does this work? “These pores consist of two guard cells. When these expand, the pore opens; when they contract, the pore closes again,” explains Hedrich. This process is controlled by the plant drawing specific salts – the positively charged potassium ion and the negatively charged chloride ion – into and out of the guard cell through special channels.

“The anion channels of the guard cells have a crucial role to play in water conservation,” comments Hedrich. The plant perceives that the soil is drying out and sends a hormone to the guard cells. Once there, this hormone activates a signal chain that causes the anion channels to open and to set a process in motion that ends with the pores closing.

The sensory cells which are able to recognize water stress also have the ability to measure the concentration of carbon dioxide in the leaf as well as the intensity and composition of sunlight. “This means that the plant is able to keep the pores closed and only open them to absorb carbon dioxide when there is sufficient water and light available for the production of carbohydrates,” explains Hedrich.

Consequences for farming

Using precise knowledge of the metabolic processes in plants, Hedrich hopes that it will be possible to make modern cultivated plants able to cope with the requirements of climate change. His interest therefore extends to plants which, like the famous “Rose of Jericho”, have become real experts at surviving water shortage. “These extremophiles can even survive after being dried out completely,” he says. An exact understanding of this ability could help us to optimize useful plants and crops specifically so that they can cope with global warming.

Activity of guard cell anion channel SLAC1 is controlled by drought-stress signaling kinase-phosphatase pair. Dietmar Geiger, Sönke Scherzer, Patrick Mumm, Annette Stange, Irene Marten, Hubert Bauer, Peter Ache, Susanne Matschi, Anja Liese, Khaled A. S. Al-Rasheid, Tina Romeis, and Rainer Hedrich. PNAS, 2009, doi/10.1073/pnas.0912021106

Contact

Prof. Dr. Rainer Hedrich, phone +49 931 3186100, hedrich@botanik.uni-wuerzburg.de