Droughts are killers: Which plants will survive

No water equals dead plant. Yet some are harder to kill than others. New research by UCLA life scientists could lead to predictions of which plant species will escape extinction from climate change. Droughts are worsening around the world, posing a great challenge to plants in all ecosystems, said Lawren Sack, a UCLA professor of ecology and evolutionary biology and senior author of the research. Scientists have debated for more than a century how to predict which species are most vulnerable and, of course, what to do about it. Sack and two members of his laboratory have made a fundamental discovery that resolves this debate and allows for the prediction of how diverse plant species and vegetation types worldwide will tolerate drought, which is critical given the threats posed by climate change, he said. The research is currently available in the online edition of Ecology Letters. Since there are many mechanisms involved in determining the drought tolerance of plants, there has been vigorous debate among plant scientists over which trait is most important. The UCLA team, funded by the National Science Foundation, focused on a trait called turgor loss point, which had never before been proven to predict drought tolerance across plant species and ecosystems. The state of being turgid is due to the rigid or fullness state of a cell due to high water content as a result of differing solute concentrations between a semipermeable membrane. The loss of turgor means the plant will droop and be less rigid. A fundamental difference between plants and animals is that plant cells are enclosed by cell walls while animal cells are not. To keep their cells functional, plants depend on turgor salty water pressure. When leaves open their pores, or stomata, to capture carbon dioxide for photosynthesis, they lose a considerable amount of this water to evaporation. This dehydrates the cells, inducing a loss of pressure. During drought, the cell's water becomes harder to replace. The turgor loss point is reached when leaf cells get to a point at which their walls become flaccid. Sack said: "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." The biologists showed that within ecosystems and around the world, plants that are more drought-tolerant had lower turgor loss points; they could maintain their turgor despite drier soil. Two traits related to plant cells have been thought to affect plants' turgor loss point and improve drought tolerance: Plants can make their cell walls stiffer or they can make their cells saltier by loading them with dissolved solutes. Many prominent scientists have leaned toward the "stiff cell wall" explanation because plants in dry zones around the globe tend to have small, tough leaves. Stiff cell walls might allow the leaf to avoid wilting and to hold onto its water during dry times, scientists reasoned. The UCLA team has now demonstrated conclusively that it is the saltiness of the cell sap that explains drought tolerance across species. The team also collected for the first time drought-tolerance trait data for species worldwide, which confirmed their result. Across species within geographic areas and across the globe, drought tolerance was correlated with the saltiness of the cell sap and not with the stiffness of cell walls. The role of the stiff cell wall was more elusive. "We were surprised to see that having a stiffer cell wall actually reduced drought tolerance slightly — contrary to received wisdom — but that many drought-tolerant plants with lots of salt also had stiff cell walls," said lead author Megan Bartlett, a UCLA graduate student in the department of ecology and evolutionary biology. This seeming contradiction is explained by the secondary need of drought-tolerant plants to protect their dehydrating cells from shrinking as they lose turgor pressure, the researchers said. Bartlett explained: "So the ideal combination for a plant is to have a high solute concentration to keep turgor pressure and a stiff cell wall to prevent it from losing too much water and shrinking as the leaf water pressure drops. But even drought-sensitive plants often have thick cell walls because the tough leaves are also good protection against herbivores and everyday wear and tear." "We're excited to have such a powerful drought indicator that we can measure easily," Bartlett said. "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." There are many plant adaptations for dry conditions which are are structural, including the following: Adaptations of the stomata to reduce water loss, such as reduced numbers or waxy surfaces. Water storage in succulent above-ground parts or water-filled tubers. Adaptations in the root system to increase water absorption.