Research makes possible rapid assessment of plant drought tolerance

UCLA life scientists, working with colleagues in China, have discovered a new method to quickly assess plants’ drought tolerance. The method works for many diverse species growing around the world. The research, published in the journal Methods in Ecology and Evolution, may revolutionize the ability to survey plant species for their ability to withstand drought, said senior author Lawren Sack, a UCLA professor of ecology and evolutionary biology. "This method can be applied rapidly and reliably for diverse species across ecosystems worldwide," he said of the federally funded research by the National Science Foundation. Droughts, which are a major threat to plants worldwide, are expected to become both more frequent and more severe with climate change, Sack said. Assessing species’ vulnerabilities to drought is essential to predict their responses to climate change and plan their conservation, he said. Earlier this year, Sack and his research team resolved a decades-old debate about what leaf traits best predict drought tolerance for diverse plant species worldwide. However, these leaf traits are too difficult and time-consuming to measure, often taking up to two days for one species, Sack said. The UCLA team worked with collaborators at the Xishuangbanna Tropical Botanical Gardens (XTBG) in Yunnan, China, to develop a method for measuring leaf drought tolerance that is 30 times faster. It is based on an important trait known as "turgor loss point." During drought, the leaf cells’ water becomes harder to replace. The turgor loss point is reached when leaf cells become so dehydrated their walls become flaccid. This cell-level loss of turgor — or swollenness — causes the leaf to become limp and wilted, and the plant cannot grow, Sack said. The new method, based on a technique called "osmometry," requires only about 10 minutes per leaf, sufficient to make a fast estimate for a given species. Plant growth depends on the ability to withstand enormous losses of water to evaporation when they open their pores, or stomata, to take in carbon dioxide for photosynthesis. The amount of evaporation a plant can tolerate depends on the water pressure inside of its cells, which in turn depends on its turgor potential; the pushing force of water against the inside of the cell walls; and the osmotic potential inside the cell, which is to say, the pulling force of dissolved salt molecules on the water molecules — the same force that makes water with salt added boil at a higher temperature. Plant cells need to maintain their turgor pressure to hold up their cell walls, but as evaporation dries out the cells, they lose turgor pressure, said co-author Christine Scoffoni, a UCLA graduate student in the Department of Ecology and Evolutionary Biology. At the turgor loss point, saltier cells have a stronger pulling force holding the water molecules inside the cell. Plants with saltier cells can keep their stomata open in drier conditions, Sack said. The turgor loss point, which varies widely among species, is a powerful determinant of the plant’s drought tolerance.