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; this cell-level loss of turgor causes the leaf to become limp and wilted, and the plant cannot grow, Sack said.
"Drying soil may cause a plant's cells to reach turgor loss point, and the plant will be faced with the choice of either closing its stomata and risking starvation or photosynthesizing with wilted leaves and risking damaging its cell walls and metabolic proteins," 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.
The team also resolved additional decades-old controversies, overturning the long-held assumptions of many scientists about the traits that determine turgor loss point and drought tolerance. 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. Little had been known about the saltiness of cells for plants around the world.
The UCLA team has now demonstrated conclusively that it is the saltiness of the cell sap that explains drought tolerance across species. Their first approach was mathematical; the team revisited the fundamental equations that govern wilting behavior and solved them for the
|Contact: Stuart Wolpert|
University of California - Los Angeles