A 2011 study of drought by Aiguo Dai of the US National Center for Atmospheric Research looked at “characteristics and trends” of the climate indicator, Palmer Drought Severity Index, during 1900–2008. The study found that “all the (main) four forms of the PDSI show widespread drying over Africa, East and South Asia, and other areas from 1950 to 2008, and most of this drying is due to recent warming.” Global warming is expected to prolong droughts, and with the earth’s grass and plant cover rapidly declining, it is vital to know which plant varieties would survive the harsh conditions that are to come.

Lawrence Sack and colleagues at the Department of Ecology and Evolution, University of California, report in the journal, Ecology Letters, that their work has resolved a basic question of how a plant in arid conditions overcomes the conflicting demands of conserving water and still being open for uptake of carbon dioxide from the air.

Turgor pressure

Plants need to hold their leaves stiff and upright, for the twin purposes of receiving sunlight to create food through photosynthesis and also for keeping pores, called stomata, open to the air so that they can take in carbon dioxide as raw material. The leaves and stems are filled with water and stay stiff, like a balloon, because of an effect called osmosis, where water flows in the direction of increasing concentration of salts, so as to dilute the concentrated region and balance things. This is the pressure that allows the level of ground water, as in wells, to be below sea level, as the fresh ground water feels a pressure to flow out and dilute the salt water of the sea! The sap of plants contain salts and plants draw water from the soil and from the air, to balance the dilute and reduce osmotic pressure, also called ‘turgor pressure’. In hot weather, the water content evaporates and if it is not replaced from the soil, the turgor pressure drops and the plant wilts and collapses. This closes the stomata and without intake of CO2, the plant would starve.

“Drying soil may cause a plant’s cells to reach the turgor loss point, and the plant will be faced with the choice of either closing its stomata and risking starvation or photosynthesising with wilted leaves and risking damaging its cell walls and metabolic proteins,” says Lawrence Sack. “To be more drought-tolerant, the plant needs to change its turgor loss point so that its cells will be able to keep their turgor even when soil is dry.”

Plants that do well in arid conditions have small and tough leaves and it was considered that this is what helped them to hold on to water in dry periods and also allowed the stomata to remain open. But there could be another effect that helps hold the cell walls stiff and rigid when water is lost. This is to make the plant draw in water with more force, by making the plant contents more salty and increasing the osmotic pressure. That the saltiness of the insides of plant cells is effective in managing the water level in the cell for different purposes is known, but was not considered to be relevant in maintaining turgor pressure.

Other examples of movement actuated by chemical regulation of turgor pressure are the twining of creepers around supports, closing of leaves on touch or the dramatic Venus Fly Trap, which snaps shut on an insect which it touches the inside. The mechanism is a rapid expelling of potassium and chlorine ions and intake of calcium ions and creating osmotic gradients. There were instances of wilted leaves regaining turgor through similar processes, for instance re-stiffening of wilted cabbage leaf without intake of water (Weiss, Randall and Sinclair, Agronomy Physiology Laboratory, Florida), but salinity regulation was not considered important as response to arid weather.

Survey results

The ULCA team, funded by the National Science Foundation, carried out a survey of drought tolerance across different species and ecosystems, worldwide and looked for reasons of how the most drought resistant ones managed lower turgor loss points – or could maintain turgor for longer in drier soil. The first approach was mathematical – and solving the equations that govern wilting behaviour showed that the reason had to be the level of saltiness of the sap. The thick cell wall alone could not prevent wilting, although it did provide other protection that mattered in arid conditions.

One of these was that it prevented the cell from shrinking, which helped retain water. In fact, thick cell walls were common even in plants that did not resist drought, and the function appears to be general protection.

The second approach was empirical – based on the survey, which showed clearly that across species within geographic areas and across the globe, drought tolerance was correlated with sap saltiness and not cell wall thickness. “The salt concentrated in cells holds on to water more tightly and directly allows plants to maintain turgor during drought,” said research co-author and UCLA doctoral student, Christine Scoffoni. The survey actually revealed that thicker walls seemed to reduce drought tolerance, although drought tolerant plants with salty sap also had thick cell walls. The combination – salty sap to keep up the turgor pressure and thick cell walls to provide mechanical support, seems to be ideal, but it was high concentration of salts in the sap that was critical.

Predict tolerance

The result of the survey is that data of turgor loss point and salty cell sap formed reliable bases to predict a plant’s drought tolerance. These parameters explain where different plants are found and which varieties dominate ecosystems. The pinpointing of cell saltiness as the main driver of drought tolerance cleared away major controversies, and it opens the way to predictions of which species could escape extinction from climate change, says Lawrence Sack. The UCLA team is now working with the Xishuangbanna Tropical Botanical Gardens in Yunnan, China, to develop methods to rapidly measure the turgor loss point of plants, so that thousands of species could be assessed for drought resistance across the world.

“We're excited to have such a powerful drought indicator that we can measure easily,” says lead author and UCLA graduate student, Megan Bartlett. “We can apply this across entire ecosystems or plant families to see how plants have adapted to their environment and to develop better strategies for their conservation in the face of climate change.”.