Charles Darwin, the father of the theory of evolution, had already seen it: on distant islands, you will find flowering plants with woody stems, which the related species on the mainland do not have. This is the result of competition between individual plants on the islands, Darwin thought. In the battle for sunlight, it would be an advantage to go higher than others, evolving from a herbaceous plant to a woody shrub and finally to a tree, Darwin wrote a century and a half ago.
It’s a logical idea, but there’s no hard evidence for it, says Frédéric Lens, research director at Naturalis, the Leiden Institute for Biodiversity. Lens, who started his career at the Catholic University of Louvain, came to the conclusion after a long research that there are more important factors that encourage plants to form wood. Especially the drought. He recently published this finding with Dutch and German colleagues, led by Alexander Zizka from the University of Marburg, in the professional journal PNAS.
Lens found the first clues about ten years ago in the Canary Islands. “That’s where my quarterback fell. You will find the most lignified flowering plants there in the dry coastal regions, not in the higher, more humid areas,” he says. Lens now has a database of over a thousand island species that turned into wood after their herbaceous ancestors colonized the islands. This change from herbaceous to woody occurred independently no less than 175 times.
So it’s quite common, Lens says, and not just among islanders, but also among flowering plants on the continents. There are about 300,000 flowering plants on earth, the so-called angiosperms or angiosperms. Of these 300,000, 180,000 species are woody. Think especially of flowering shrubs and trees, says Lens.
Seven Hundred Plants That Started Forming Wood Again
The history of flowering plants dates back approximately 145 million years. And their common ancestor was a woody plant. Over the course of evolution, almost half of the species have thus left behind the formation of wood. But the ability has been retained in almost all varieties; they all still have the genetic machinery to make wood, a lignin-rich tissue.
Apparently, it is easy for the factory to turn this device back on. Otherwise, it wouldn’t happen so often, Lens says. “There are seven hundred independent cases worldwide of plants that have started to form wood again when their direct ancestors did not. And of those 175 on the islands.
Drought doesn’t have to be the only cause. What also plays a role on the islands is a generally temperate climate with a long growing season. This gives the plant the advantage of flowering longer and living longer. “Any factor that prolongs the life of a plant can promote wood formation,” Lens explains. This also applies to the fact that there are usually no large grazers on the islands which shorten the lifespan of flowering plants. But the common denominator in his database of species that evolved into lignified shrubs and trees is drought.
Air bubbles in the drums
How does lignification help fight drought? Lens: “A plant draws water from root to leaf, against gravity, creating negative pressure in its vessels. It is a vulnerable system. During drought, air bubbles can form in these barrels, blocking water transport. If the plant cannot solve this problem, it will die. This is a common risk for all plants. But it seems that lignification reduces this risk. We see that flowering plants that have begun to form wood resist drought better than their herbaceous cousins.
Species that return to wood remain a small minority of flowering plants, but their change is important for science, Lens says. “Climate change will cause plants in many regions to experience drought stress more often. It will be important to monitor whether this will encourage flowering plants to form more lignin in the stem. »
A world in which plant growth becomes more difficult anyway
It is not yet known exactly how the genetic apparatus of lignification works. This is the subject of an important research project which will now start under the direction of Naturalis and the University of Leiden. This is not only interesting from a scientific point of view, but also important for society, believes Lens. “By knowing this process, we can make food crops more resistant to drought. Because we are heading towards a world in which plant growth will become more difficult anyway. We have to do something to keep feeding the world population.
“Additionally, knowledge of wood formation can help utilize plant parts that are now discarded, such as the stems of tomato plants and other agricultural crops. With the right knowledge, we could turn it into biofuel.
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