Wilted tree leaves in a Hawaiian forest during the extreme drought of 2010-11, which was the worst in at least 11 years and was federally designated a natural disaster. The tree is an alahee (Psydrax odorata). Image: Faith Inman-Narahari |
New research by University
of California, Los Angeles (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.
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 in Ecology Letters.
Why does a sunflower wilt and dessicate
quickly when the soil dries, while the native chaparral shrubs of California survive long
dry seasons with their evergreen leaves? 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.
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
pressure”—pressure produced in cells by internal salty water pushing
against and holding up the cell walls. 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; 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 first time. Their mathematical solution pointed to the importance
of saltier cell sap. Saltier cell sap in each plant cell allows the plant to
maintain turgor pressure during dry times and to continue photosynthesizing and
growing as drought ensues. The equation showed that thick cell walls do not
contribute directly to preventing wilting, although they provide indirect
benefits that can be important in some cases—protection from excessive cell
shrinking and from damage due to the elements or insects and mammals.
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. In fact, species with stiff cell walls were found not
only in arid zones but also in wet systems like rainforests, because here too,
evolution favors long-lived leaves protected from damage.
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, Sack said.
“The salt concentrated in cells holds
on to water more tightly and directly allows plants to maintain turgor during
drought,” said research co-author Christine Scoffoni, a UCLA doctoral
student in the Department of Ecology and Evolutionary Biology.
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.
“While a stiff wall doesn’t maintain
the cell turgor, it prevents the cells from shrinking as the turgor decreases
and holds in water so that cells are still large and hydrated, even at turgor
loss point,” 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.”
Even though the team showed that turgor loss
point and salty cell sap have exceptional power to predict a plant’s drought
tolerance, some of the most famous and diverse desert plants—including cacti,
yuccas, and agaves—exhibit the opposite design, with many flexible-walled cells
that hold dilute sap and would lose turgor rapidly, Sack said.
“These succulents are actually terrible
at tolerating drought, and instead they avoid it,” he said. “Because
much of their tissue is water storage cells, they can open their stomata
minimally during the day or at night and survive with their stored water until
it rains. Flexible cell walls help them release water to the rest of the
plant.”
This new study showed that the saltiness of
cells in plant leaves can explain where plants live and the kinds of plants
that dominate ecosystems around the world. The team is working with
collaborators at the Xishuangbanna Tropical Botanical Gardens in Yunnan, China,
to develop a new method for rapidly measuring turgor loss point across a large
number of species and make possible the critical assessment of drought
tolerance for thousands of species for the first time.
“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.”