A young poplar is placed horizontally. Within about ten days the trunk begins to bend upwards, as would be expected from a plant that follows gravity. Then the researchers take away the two most obvious references: the direction of light and a stable gravitational signal. And the poplar continues to correct its posture. It does this by building new wood in the right spot.
This is what a research group from INRAE and Université Clermont Auvergne observed in a study published in New Phytologist. In the young hybrid poplars analyzed, the curvature of the trunk itself was sufficient to guide the formation of the so-called tension wood, a tissue capable of generating forces that pull the trunk until it straightens it.
In short, trees possess a form of proprioception: they perceive the configuration of their own body and use that information to correct themselves. No nervous system and no muscles hidden under the cortex, of course. However, the comparison with muscles, also used by INRAE in presenting the results, helps to understand what happens: one side pulls, then the opposite side can come into action.
How you take away the sense of gravity from a tree
The experiment was designed specifically to separate trunk curvature from other signals that normally guide a plant. The researchers worked with young specimens of Populus tremula × Populus alba. First they tilted them to the horizontal. The poplars reacted by forming tension wood at the top of the trunk and began to bow upwards.
After about ten days, when the curve was now evident, the trees were transferred into spherical chambers illuminated uniformly from all directions. So remove the light reference. At the same time, the pots were placed on a clinostat, a device that slowly rotates the plant around a horizontal axis: gravity obviously continues to exist, but its direction with respect to the stem changes continuously and stops providing a stable orientation.
At that point there was above all one thing left to “feel”: how curved the trunk was. And the trunk began to straighten. The complete recording of the variables was obtained on seven plants subjected to the main protocol and on three control plants, a small number that must be kept in mind when extending the conclusions beyond the experimental system studied. The result observed in the poplars, however, was clear enough to allow the researchers to distinguish a simple elastic return of the wood from an active biological response.
The wood changes sides and begins to pull from the opposite side
Here comes the most interesting part. During the first upward bend, tension wood had formed on the upper side of the log. When the gravitational reference was experimentally neutralized, that production stopped. After a few days, new tension wood appeared under the microscope on the opposite, convex side of the curve. It was as if the poplar had changed hands with the rope with which it was pulling its trunk.
The fabric did not only resemble tension wood: it possessed its anatomical characteristics. In wood colored with astra blue the ratio between fibers and vessels was approximately three times higher than in normal or opposite wood. The fibers also had the typical G-layer, a thick gelatinous wall rich in cellulose involved in the production of mechanical tensions.
That formed during straightening was even thicker: on average 2.70 micrometers versus 1.97 micrometers in the tension wood produced during the first phase, about 37% thicker. The authors warn, however, that this difference could also depend on the longer duration of the straightening phase or on the effort needed to move a stem which, in the meantime, continued to grow and stiffen.
The correction does not start as a click. Approximately two days pass between the start of the rotation on the clinostat and the inversion of the curvature. It makes sense: the cambia must produce new cells, differentiate tension wood, and build the G-layer before that tissue can exert significant force.
A tree combines gravity and the shape of its trunk
Until now, the tension wood of hardwoods was described primarily as the tissue that appears on the upper side of a sloping stem and pulls it upwards. The experiment suggests something more refined: the position in which it appears depends on the balance between the gravitational signal and that linked to the curvature of the stem.
When gravity dominates, the poplar seeks the vertical. As information about one’s shape becomes more important, the tension wood can move to the other side and reduce the curve already produced. The authors speak of a true “sensory-motor loop”, a sensory and motor circuitry that is also active in the woody part of the tree.
It is also an important distinction because it avoids turning trunks into barked humans. Trees don’t “see” that they are crooked and don’t have muscles or neurons. Cells react to physical and biological signals, and growth progressively modifies the forces distributed in the wood.
However, the result helps explain how an organism that spends its entire life in the same place can continually correct its architecture as it grows, is pushed by the wind, loaded by snow or tilted by other disturbances.
INRAE indicates possible future implications also in the study of the quality of wood and in the selection of cultivated plants less prone to lodging. These are perspectives yet to be explored: the experiment concerns young poplars in controlled conditions and does not in itself demonstrate that each tree species uses exactly the same mechanism.
What it shows directly is already quite remarkable. A trunk is bent, loses its external references and still continues to know, biologically speaking, how crooked it is and which way it has to pull to get straight again. For a piece of wood that we imagine to be immobile, that’s a lot of work.
