Study shines new light on young tree seedlings

Date:
August 25, 2020
Source:
University of Georgia
Summary:
X-ray images show a plant's power source may be different than
thought.



FULL STORY ==========================================================================
The first few weeks of a tree seedling's life can be the most precarious.


==========================================================================
As it pushes thin new roots into the ground it's also reaching up with
tiny new leaves. Water and energy are precious. Most seedlings never
make it past their first month on the ground.

But while much is known about the growing process, there remains a
layer of mystery around the mechanisms within these small plants. Now,
a new study by a University of Georgia researcher sheds some light on
the microscopic tissues that help tree seedlings grow. The results could
change how researchers and growers view the first weeks of a tree's life.

"I've been working on newly germinated seedlings for 20 years, and I
feel this is one of the first breakthroughs for me about how different
they are, even from a 20-week-old seedling," said Dan Johnson, an
assistant professor of tree physiology and forest ecology at the UGA
Warnell School of Forestry and Natural Resources. "It's these first few
weeks of life that seem to be fundamentally different." Johnson and
a team of researchers used a high-powered X-ray called a synchrotron
to take extremely detailed cross-section images of ponderosa pine
seedlings at various stages of hydration. Located at the University of California-Berkeley, the synchrotron accelerates electrons to nearly the
speed of light, and while they will instantly kill a human cell, plants,
it turns out, can withstand the intense power for a short period of time.

So, Johnson and his collaborators X-rayed the intact stem of the pine
seedlings over several days, taking images of what was going on inside
the plant. The pictures show extremely detailed black-and-white images
that detail pockets of hydrated cells in gray. As the images progress and
the seedlings dry out, black pockets of air can be seen on the images,
almost as if the stems are being eaten from the outside in.



==========================================================================
He and other researchers thought the plant's xylem -- a central nervous
system of the plant, in a sense -- would quickly dry up if it went
without water.

Turns out, they were wrong -- and the resulting images offer
never-before-seen insights into the first few weeks of a tree seedling's
life.

"The way we thought these seedlings were going to fail, hydraulically,
as they dried out, was not at all how they failed," he said. "We thought
the vascular tissue -- the xylem -- was going to be filled with air. We
call it embolism in humans. But what we found was, it wasn't the xylem
that dried out, it was all the tissue surrounding it. Even in some of the seedlings that looked like they were ripped apart (for lack of water),
the xylem is fully hydrated." All plants have xylem tissue; it transports water throughout the plant. And in older plants, the xylem often does dry
out as a plant faces drought. But the images that Johnson captured show
that seedlings' plumbing is completely different from their older cousins.

The findings were published in the August issue of the American Journal
of Botany. The study was supported by two grants from the National
Science Foundation.

"To me, this is the most vulnerable life stage. If a seedling is going to
die, it's going to die in the first few weeks of life," said Johnson. "In
the field, we see 99% of natural regeneration seedlings die -- you'll
come back to the field one day and thousands have died. And they die in
places where it just dries out too quickly." Johnson said his findings
point to how sensitive tissues outside the xylem are to water loss in the
first few weeks of a seedling's life. When a wild-sewn seedling survives,
it's often because that particular site had more favorable conditions,
such as more moisture or the seed landed in a depression where it was
more protected from the elements.



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In addition to the detailed black-and-white images, the team also made corresponding color images of the seedling stems with a laser, using a
process called confocal microscopy. Different cells reflect in different colors, creating a rainbow of circles that researchers can use to better identify parts of the stem.

But while the yellows, reds and blues are striking on the laser-produced images, the real eye-opener for Johnson was the black-and-white reality
of the decimated, dried out stems and their central core, which was the
last to give up.

"I was completely shocked. It was not what any of us on the paper
expected," said Johnson, pointing to one image of a withered stem that
looks almost chewed up. "That's at a desiccation level that would
kill that plant. So, to have that xylem so full when it's so dead
is counter-intuitive." While the discovery may bring more questions
than answers, Johnson notes that the survival of the xylem may change
how plants' first few weeks are understood. It's almost as if, he said,
the first leaves to emerge from a seedling are connected to a completely different set of tissues. "The xylem might not be the plumbing to the
first few leaves of the plant, which is bizarre because that's what we
learned in plant physiology," he added.


========================================================================== Story Source: Materials provided by University_of_Georgia. Original
written by Kristen Morales. Note: Content may be edited for style
and length.


========================================================================== Related Multimedia:
* Images_of_seedlings_studied_in_the_research ========================================================================== Journal Reference:
1. Megan L. Miller, Adam B. Roddy, Craig R. Brodersen, Andrew
J. McElrone,
Daniel M. Johnson. Anatomical and hydraulic responses to desiccation
in emergent conifer seedlings. American Journal of Botany, 2020;
DOI: 10.1002/ajb2.1517 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/08/200825142335.htm

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