Stomata are structures that allow plants to exchange gases with the air. Contemporary plants in Florida have fewer stomata than their ancestors did a few decades ago. Credit: Emmy Lammertsma
As carbon dioxide levels have risen during the last 150 years, the
density of pores that allow plants to breathe has dwindled by 34%, restricting
the amount of water vapor the plants release to the atmosphere, report scientists
from Indiana Univ. Bloomington and Utrecht Univ. in the Netherlands in an
upcoming issue of the Proceedings of the
National Academy of Sciences.
In a separate paper, also to be published by PNAS, many of the same scientists
describe a model they devised that predicts doubling today’s carbon dioxide
levels will dramatically reduce the amount of water released by plants.
The scientists gathered their data from a diversity of plant
species in Florida,
including living individuals as well as samples extracted from herbarium collections
and peat formations 100 to 150 years old.
“The increase in carbon dioxide by about 100 parts per
million has had a profound effect on the number of stomata and, to a lesser
extent, the size of the stomata,” said Research Scientist in Biology and
Professor Emeritus in Geology David Dilcher, the two papers’ sole American
coauthor. “Our analysis of that structural change shows there’s been a
huge reduction in the release of water to the atmosphere.”
Most plants use a pore-like structure called stomata (singular:
stoma) on the undersides of leaves to absorb carbon dioxide from the air. The
carbon dioxide is used to build sugars, which can be used by the plant as
energy or for incorporation into the plants’ fibrous cell walls. Stomata also
allow plants to “transpire” water, or release water to the
atmosphere. Transpiration helps drive the absorption of water at the roots, and
also cools the plants in the same way sweating cools mammals.
If there are fewer stomata, or the stomata are closed more of the
day, gas exchange will be limited—transpiration included.
“The carbon cycle is important, but so is the water
cycle,” Dilcher said. “If transpiration decreases, there may be more
moisture in the ground at first, but if there’s less rainfall that may mean
there’s less moisture in ground eventually. This is part of the hyrdrogeologic
cycle. Land plants are a crucially important part of it.”
Dilcher also said less transpiration may mean the shade of an old
oak tree may not be as cool of a respite as it used to be.
Researchers extract stomata-bearing leaves from a peat formation in Florida. At some sites, the peat was estimated to be as much as 150 years old. Credit: Emmy Lammertsma
“When plants transpire they cool,” he said. “So the
air around the plants that are transpirating less could be a bit warmer than
they have been. But the hydrogeologic cycle is complex. It’s hard to predict
how changing one thing will affect other aspects. We would have to see how
these things play out.”
While it is well known that long-lived plants can adjust their
number of stomata each season depending on growing conditions, little is known
about the long-term structural changes in stomata number or size over periods
of decades or centuries.
“Our first paper shows connection between temperature,
transpiration, and stomata density,” Dilcher said. “The second paper
really is about applying what we know to the future.”
That model suggests that a doubling of today’s carbon dioxide
levels — from 390 parts per million to 800 ppm—will halve the amount of water
lost to the air, concluding in the second paper that “plant adaptation to
rising CO2 is currently altering the hydrological cycle and climate and will
continue to do so throughout this century.”
Dilcher and his Dutch colleagues say that a drier atmosphere could
mean less rainfall and therefore less movement of water through Florida’s watersheds.
The Florida Everglades depend heavily on the slow, steady flow of
groundwater from upstate. The siphoning of that water to development has raised
questions about the future of the Everglades
as a national resource.
Dilcher’s Dutch coauthors for the two papers were Emmy Lammertsma,
Hugo de Boer, Stefan Dekker, Andre Lotter, Friederike Wagner-Cremer, and Martin
Wassen, all of Utrecht Univ. in Utrecht,
The project received support from Utrecht
Univ.’s High Potential