Key work is been done here in the
relationship of vein density and climate conditions that will assist in also
mapping local climate in the fossil record.
I suspect that it will become possible to properly place a fossil site
in its relation to its local climate a a matter of course which is surely
valuable information from the fossil record.
It also suggests another variable
to pay attention to in terms of plant breeding and the preparation of plant
lines for arid conditions.
It is all good stuff and a little
surprising inasmuch no one thought to check for this type of variation before.
Hacking code of leaf vein architecture solves mysteries, allows
predictions of past climate
by Staff Writers
How does the structure of leaf vein systems depend on leaf size? Sack
and members of his laboratory observed striking patterns in several studies of
just a few species. Leaf vein systems are made up of major veins (the first
three branching "orders," which are large and visible to the naked
eye) and minor veins, (the mesh embedded within the leaf, which makes up most
of the vein length).
UCLA life scientists have discovered new laws that determine the
construction of leaf vein systems as leaves grow and evolve. These
easy-to-apply mathematical rules can now be used to better predict the climates
of the past using the fossil record.
The research, published in the journal Nature Communications, has a
range of fundamental implications for global ecology and allows researchers to
estimate original leaf sizes from just a fragment of a leaf. This will improve
scientists' prediction and interpretation of climate in the deep past from leaf
fossils.
Leaf veins are of tremendous importance in a plant's life,
providing the nutrients and water that leaves need to conduct photosynthesis
and supporting them in capturing sunlight. Leaf size is also of great
importance for plants' adaptation to their environment, with smaller leaves
being found in drier, sunnier places.
However, little has been known about what determines the architecture
of leaf veins. Mathematical linkages between leaf vein systems and leaf size
have the potential to explain important natural patterns. The new UCLA research
focused on these linkages for plant species distributed
around the globe.
"We found extremely strong, developmentally based scaling of leaf
size and venation that has remained unnoticed until now," said Lawren
Sack, a UCLA professor of ecology and evolutionary biology and lead author of
the research.
How does the structure of leaf vein systems depend on leaf size? Sack
and members of his laboratory observed striking patterns in several studies of
just a few species. Leaf vein systems are made up of major veins (the first
three branching "orders," which are large and visible to the naked
eye) and minor veins, (the mesh embedded within the leaf,
which makes up most of the vein length).
Federally funded by the National Science Foundation, the team of Sack,
UCLA graduate student Christine Scoffoni, three UCLA undergraduate researchers
and colleagues at other U.S. institutions measured hundreds of plant species
worldwide using computer tools to focus on high-resolution images of leaves
that were chemically treated and stained to allow sharp visualization of the
veins.
The team discovered predictable relationships that hold across
different leaves throughout the globe. Larger leaves had their major veins
spaced further apart according to a clear mathematical equation, regardless of
other variations in their structure (like cell size and surface hairiness)
or physiological activities (like photosynthesis and respiration), Sack said.
"This scaling of leaf size and major veins has strong implications
and can potentially explain many observed patterns, such as why leaves tend to
be smaller in drier habitats, why flowering plants have
evolved to dominate the world today, and how to best predict climates of the
past," he said.
These leaf vein relationships can explain, at a global scale, the most famous
biogeographical trend in plant form: the predomination of small leaves in drier
and more exposed habitats. This global pattern was noted as far back as the
ancient Greeks (by Theophrastus of Lesbos ) and
by explorers and scientists ever since. The classical explanation for why small
leaves are more common in dry areas was that smaller leaves are coated by a
thinner layer of still air and can therefore cool faster and prevent
overheating. This would certainly be an advantage when leaves are in hot, dry
environments, but it doesn't explain why smaller leaves are found in cool, dry
places too, Sack noted.
Last year, Scoffoni and Sack proposed that small leaves tend to have
their major veins packed closely together, providing drought tolerance. That
research, published in the journal Plant Physiology,
pointed to an advantage for improving water transport during drought. To
survive, leaves must open the stomatal pores on their surfaces to capture
carbon dioxide, but this causes water to evaporate out of the leaves. The water
must be replaced through the leaf veins, which pull up water through the stem
and root from the soil. This drives a tension in the leaf vein "xylem
pipes," and if the soil becomes too dry, air can be sucked into the pipes,
causing blockage.
The team had found, using computer simulations and detailed experiments
on a range of plant species, that because smaller leaves have major veins that
are packed closer together - a higher major vein length per leaf area - they
had more "superhighways" for water transport. The greater number of
major veins in smaller leaves provides drought tolerance by routing water
around blockages during drought.
This explanation is strongly supported by the team's new discovery of a
striking global trend: higher major vein length per leaf area in smaller
leaves.
The Nature Communications research provides a new ability to estimate
leaf size from a leaf fragment and to better estimate past climate from fossil
deposits that are rich in leaf fragments. Because of the very strong tendency
for smaller leaves to have higher major vein length per leaf area, one can use
a simple equation to estimate leaf size from fragments.
Major vein length per leaf area can be measured by anyone willing to
look closely at the large and small leaves around them.
"We encourage anyone to grab a big and a small leaf from trees on
the street and see for yourself that the major veins are larger and spaced
further apart in the larger leaf," Scoffoni said.
Because leaf size is used by paleobiologists to "hindcast"
the rainfall experienced when those fossil plants were
alive and to determine the type of ecosystem in which they existed, the ability
to estimate intact leaf size from fragmentary remains will be very useful for
estimates of climate and biodiversity in the fossil record, Sack said.
The research also points to a new explanation for why leaf vein
evolution allowed flowering plants to take over tens of millions of years ago
from earlier evolved groups, such as cycads, conifers and
ferns. Because, with few exceptions, only flowering plants have densely packed
minor veins, and these allow a high photosynthetic rate - providing water to
keep the leaf cells hydrated and nutrients to fuel photosynthesis - flowering
plants can achieve much higher photosynthetic rates than earlier evolved
groups, Sack said.
The UCLA team's new research also showed that the major and minor vein
systems in the leaf evolve independently and that the relationship between
these systems differs depending on life size.
"While the major veins show close relationships with leaf size,
becoming more spaced apart and larger in diameter in larger leaves, the minor
veins are independent of leaf size and their numbers can be high in small
leaves or large leaves," Sack said.
"This uniquely gives flowering plants the ability to make large or
small leaves with a wide range of photosynthetic rates. The ability of the
flowering plants to achieve high minor-vein length per area across a wide range
of leaf sizes allows them to adapt to a much wider range of habitats - from
shade to sun, from wet to dry, from warm to cold - than any other plant group,
helping them to become the dominant plants today."
The strength of the mathematical linkage of leaf veins with leaf size
across diverse species raises the question of cause.
The UCLA team explains that these patterns arise from the fact of a
shared script or "program" for leaf expansion and the formation of
leaf veins. The team reviewed the past 50 years of studies of isolated plant
species and found striking commonalities across species in their leaf
development.
"Leaves develop in two stages," Sack said. "First, the
tiny budding leaf expands slightly and slowly, and then it starts a distinct,
rapid growth stage and expands to its final size."
The major veins form during the first, slow phase of leaf growth,
and their numbers are complete before the rapid expansion phase, he said.
During the rapid expansion phase, those major veins are pushed apart, and can
simply extend and thicken to match the leaf expansion. Minor veins can continue
to be initiated in between the major veins during the rapid phase, as the
growing leaf can continue to lay down new branching strands of minor veins.
In the final, mature leaf, it is possible for minor veins to be spaced
closely, even in a large leaf where the major veins would be spaced apart.
"The generality of the development program is striking," Sack
said, "It's consistent with the fact that different plant species share
important vein development genes - and the global scaling patterns of leaf vein
structure with leaf size emerge in consequence."
These vein trends, confirmed with high-resolution measurements, are
"obvious everywhere under our noses," Sack and Scoffoni said.
Why had these trends escaped notice until now?
"This is the time for plants," Sack said. "It's amazing
what is waiting to be discovered in plant biology. It seems limitless right
now. The previous century is known for exciting discoveries in physics and
molecular biology, but this century belongs to plant biology. Especially given
the centrality of plants for food and biosphere sustainability, more attention
is being focused, and the more people look, the more fundamental discoveries
will be made."
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