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These
are deciduous maple leaves, recently senesced as winter approaches in Rock
Creek Park, Washington, DC.
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Dec.
22, 2013 — Researchers have found new clues to how plants evolved to withstand
wintry weather. In a study to appear in the December 22 issue of the journal
Nature, the team constructed an evolutionary tree of more than 32,000 species
of flowering plants -- the largest time-scaled evolutionary tree to date. By
combining their tree with freezing exposure records and leaf and stem data for
thousands of species, the researchers were able to reconstruct how plants
evolved to cope with cold as they spread across the globe.
The results suggest
that many plants acquired characteristics that helped them thrive in colder
climates -- such as dying back to the roots in winter -- long before they first
encountered freezing. Fossil evidence and reconstructions of past climatic
conditions suggest that early flowering plants lived in warm tropical
environments, explained co-author Jeremy Beaulieu at the National Institute for
Mathematical & Biological Synthesis (NIMBioS) at the University of
Tennessee.
As
plants spread to higher latitudes and elevations, they evolved in ways that
helped them deal with cold conditions. Plants that live in the tundra, such as
Arctic cinquefoil and three-toothed saxifrage, can withstand winter
temperatures below minus 15 degrees Celsius.
Unlike
animals, most plants can't move to escape the cold or generate heat to keep
them warm. It's not so much the cold but the ice that poses problems for
plants. For instance, freezing and thawing cause air bubbles to form in the
plant's internal water transport system.
"Think
about the air bubbles you see suspended in the ice cubes," said co-author
Amy Zanne of the George Washington University. "If enough of these air
bubbles come together as water thaws they can block the flow of water from the
roots to the leaves and kill the plant."
The
researchers identified three traits that help plants get around these problems.
Some plants, such as hickories and oaks, avoid freezing damage by dropping
their leaves before the winter chill sets in -- effectively shutting off the
flow of water between roots and leaves -- and growing new leaves and water
transport cells when warmer weather returns.
Other
plants, such as birches and poplars, also protect themselves by having narrower
water transport cells, which makes the parts of the plant that deliver water
less susceptible to blockage during freezing and thawing.
Still
others die back to the ground in winter and re-sprout from their roots, or
start growing as new plants from seeds when conditions are right.
To
compile the plant trait data for their study, the researchers spent hundreds of
hours scouring and merging multiple large plant databases containing tens of
thousands of species, largely with the support of the National Evolutionary
Synthesis Center in North Carolina and Macquarie University in Australia.
When
they mapped their collected leaf and stem data onto their evolutionary tree for
flowering plants, they found that many plants were well equipped for icy
climates even before cold conditions hit.
Plants
that die back to the ground in winter, for example, acquired the ability to die
and come back when conditions improve long before they first experienced
freezing. Similarly, species with narrow water transport cells acquired a finer
circulatory system well before they confronted cold climates.
"This
suggests that some other environmental pressure -- possibly drought -- caused
these plants to evolve this way, and it happened to work really well for
freezing tolerance too," said Zanne.
The
only exceptions were plants that shed and replace their leaves seasonally --
these plant groups didn't gain the ability to drop their leaves during winter
until after they encountered freezing, Beaulieu added.
As
a next step, the researchers plan to use their evolutionary tree to find out
how plants evolved to withs
tand other environmental stresses in addition to
freezing, such as drought and heat.
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