Trees are suffering from global warming. Due to droughts that are too intense and too frequent, entire forest massifs are dying. Many coniferous trees could be under the spotlight. To identify the most endangered species, scientists from INRA came at SOLEIL to detect air bubbles within the vessels transporting water in the plant.
This research was carried out on the X-ray beamline PSICHE in 2016. As part of our series 'Next Chapter,' we reconnected with the scientists filmed in 2016 and asked them how their research has progressed since then.
Sylvain Delzon (Biogeco, INRAE), during his filmed experiments in 2016, comments on one of the images obtained by X-ray tomography on the PSICHE beamline.
"Is the oak tree really that vulnerable to drought?" It was to answer this question that Sylvain Delzon, an ecophysiologist at INRAE Bordeaux, went to the SOLEIL synchrotron in 2016. Thanks to X-ray microtomography on the PSICHE beamline, the researcher was able to observe the formation of small air bubbles in the tree's vessels and assess its threshold of resistance to drought. Indeed, the thirstier the plant becomes, the more eagerly it draws water from the soil, increasing the number of air bubbles in the sap, which eventually threatens the plant's survival. "The great advantage of the PSICHE beamline is that it's possible to scan an entire living plant without having to cut it down," notes Sylvain Delzon. "There’s no measurement artifact, no air bubbles artificially entering the vessels during sample preparation, which would otherwise skew the measurements."
"And, to my knowledge, only at the SOLEIL synchrotron do tomographs have hollow marble stages where large plants up to two meters tall can be placed," adds the researcher, who has been returning regularly to the SOLEIL synchrotron since then. After a week of measurements, and the study of a dozen young plants subjected to varying levels of water stress, the scientists confirmed that the temperate oak is far more resistant than previous studies had suggested (1). Similar work on the PSICHE beamline was later carried out on ash trees and vines, with the same conclusion: these species are more resilient to drought than expected (2).
Sylvain Delzon and his colleagues regularly collect new data at the SOLEIL synchrotron to better understand and anticipate the effects of climate change on plant life. In 2018, for example, they showed that the accumulation of air bubbles in the vascular system—corresponding to the phenomenon of embolism—is irreversible. In other words, air builds up in the vessels during each drought episode until the vascular system is 90% filled, ultimately causing the plant's death. "One might have thought that plants reset their system after each winter, that they expelled the accumulated air bubbles, but that’s not the case," concludes Sylvain Delzon (3).
Then, in 2020, the researchers tackled a widely accepted theory in biology that trees can grow to great heights thanks to their vascular system. To test this hypothesis, the researchers decided to analyze the world’s largest moss, which stands at 25 cm tall. "We discovered from the images obtained through PSICHE that this moss has channels called hydroids, which function exactly like the vessels of trees, such as the oak, and are capable of transporting sap upwards." It is therefore not the absence of a vascular system that prevents some plants from growing to great heights; the limiting factor in growth must be found elsewhere. The results of this study, which challenge a long-standing biological dogma, were published in Nature Plants (4).
Recently, tree roots have become the focus of the researchers’ attention. Thanks to an ingenious cultivation system that allows each root branch to grow in small cylinders that can be scanned in 3D on the PSICHE beamline, the scientists have demonstrated a significant effect of drought on root size (5). These measurements complement those routinely taken at INRAE Bordeaux's Biogeco laboratory, notably with centrifuges called "cavitons," which can simulate drought episodes and monitor their effects over time. "The limitation with the synchrotron is that we can't conduct long-term measurements over several months. Ideally, we'd have a mini-synchrotron available all the time!" concludes Sylvain Delzon, who, until that invention arrives, returns each year to take measurements at the SOLEIL synchrotron.
