MRIs could help save traumatised trees

Cosmos Magazine


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A microCT scan of an intact plant stem shows the internal anatomy of the xylem. One advantage of tomography is that the volume can be sliced in any orientation after the scan is completed. Air filled xylem conduits in black can clearly be differentiated from surrounding water filled conduits in grey.

Just as with blood clots travelling up a human leg, tiny air bubbles drawn up through a tree’s roots can be lethal, even if they can’t warn us. And with drier conditions thanks to climate change the problem is likely to get worse. But never fear, deep-tissue health checks for plants are in sight.

A team of international scientists has used high-resolution CT and MRI scans to visualise water-carrying vessels inside living plants, which the researchers hope will one day help farmers cultivate sturdier crops.

To lug water and nutrients up large distances, trees contain microscopic vessels called xylem. These work like straws – as water evaporates through the leaves, moisture is sucked up from the soil through the roots to replenish stocks.

The more water evaporates, the more water xylem have to draw up. But in dry soil, roots often suck in air bubbles instead, which block and dry up the xylem. These “embolisms” have been reported as one of the major causes of plant death during extreme drought events.

To investigate how plants cope with dehydration, University of Western Sydney’s Brendan Choat and his team imaged the xylem in the stems and roots of living oak, aspen and pine trees, using MRI and CT scanning – techniques common in hospitals.

“If we can understand the specific anatomical traits that drive the resistance to drought and embolism, then that gives us a better chance of targeting it with crop selection,” Choat explained.

The team tracked how the xylem in dehydrated plants dried up over time. The scans, the authors found, were a better indicator of plant health than other methods.

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Imaging of a plant root system from a microCT scan taken at the Australian Synchrotron’s Imaging and Medical Beamline. The image A shows air filled xylem vessels in red. The image B shows the connections of air-filled xylem vessels with external tissue removed. This plant was severely water stress leading to almost all vessels becoming embolised.

“Traditionally, researchers looking to monitor the health of trees were forced to use techniques that were indirect and prone to inaccuracies, such as removing tree branches and spinning them in centrifuges,” said Choat. “This [study] provides a new window into how plants respond to environmental stresses.”

To obtain their high-resolution CT scans, the team imaged plants with X-rays produced by particle accelerators, such as the Australian Synchrotron. But Choat hopes MRI and CT scanning using smaller, more accessible lab-based instruments will become a widespread screening tool for selecting plants durable in dry conditions.

This type of scanning could be done on any woody crop, Choat said, such as grapevines, almonds, citrus and olives.

Of course farmers haven’t been idle, waiting for scientists to find out which crop is sturdier.

Farmers anticipating dry, hot conditions may naturally grow grapevine varieties from Spain or Italy, rather than from northern France of Germany, for example. But “a lot of that is not based on data,” Choat explained. “It’s based on where the plants had been growing – it’s retained knowledge in the field.”

“We hope that we can fit in by quantifying how resistant [plants] are to drought and then using that information to feed back into irrigation decisions,” he said.

Knowing which crop is most resistant to low irrigation, for instance, could allow farmers to prioritise water supplies during heat waves, Choat said.

The study was published in the Journal of Plant Physiology.

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