Study shines new light on how young tree seedlings develop

The first few weeks of a tree’s life can be the worst.

As it pushes thin new roots into the soil, it also touches small new leaves. Water and energy are precious. Most seedlings never make it through their first month on the ground.

But although much is known about the growing process, there remains a layer of mystery surrounding 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 may 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 on 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 essentially different.”

Johnson and a team of researchers used a powerful X-ray called a synchrotron to take extremely detailed images of cross-sections of ponderosa pine seedlings at various stages of hydration. Located at the University of California-Berkeley, the synchrotron electrons accelerate to almost the speed of light, and although they kill a human cell directly, plants can, it turns out, resist the intense force for a short period of time.

That said, X-rays of Johnson and his staff showed the intact stem of the pine seedlings over several days, and took pictures of what was going on inside the plant. The images show extremely detailed black-and-white images detailing socks of hydrated cells in gray. As the images move and the seedlings dry out, black skies can be seen on the images, almost as if the stems are being eaten from the outside.

He and other researchers thought that the xylem of the plant – a central nervous system of the plant, in a sense – would dry up quickly if it went without water. It turned out that they were wrong – and the resulting images provide unprecedented insight into the first few weeks of a tree’s life.

“The way we thought these seedlings failed, hydraulically, when they dried out, was not at all how they failed,” he said. “We thought the fascial tissue – the xylem – would 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 around it. Even in some of ‘ the seedlings that looked like they were torn apart (due to lack of water), the xylem is completely hydrated. “

All plants have xylem tissue; it transports water through the plant. And in older plants, the xylem often dried out when a plant was dried. But the images Johnson made show that the sanitation of seedlings is completely different from that of her 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.

“For me, this is the most vulnerable stage of life. If a settler dies, it will die in the first few weeks of life,” Johnson said. “In the field, we see 99% of natural regeneration plants die – you 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 susceptible tissues outside the xylem are to water loss in the first weeks of a seedling’s life. If a wild-seeded seedling survives, it is often because that particular site had more favorable conditions, such as more moisture when the seed entered a depression where it was more protected from the elements.

In addition to the detailed black-and-white images, the team also made matching color images of the sea dike trunks 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 yellow, red, and blue are striking on the laser-produced images, the real eye-opener for Johnson was the black-and-white reality of the decimated, dried-out steels and their central core, which was the last to give up.

“I was completely shocked. It was not what any of us expected on paper,” Johnson said, pointing to one image of a withered trunk that almost seemed to chew. “That’s at a dehydration level that would kill that plant. That, that xylem is as full as it is 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 the first few weeks of plants are understood. It is almost as if, he said, the first leaves that emerge from a seedling are connected to a completely different set of tissues. “The xylem may not be the sanitary for the first few leaves of the plant, which is bizarre because that is what we learned in plant physiology,” he added.