How Trees Stand Up Straight
AAnyone who has walked through a forest after a strong wind will have seen trees leaning at odd angles. Some never recover, but many young trees slowly lift themselves back towards the vertical. For a long time, botanists explained this recovery mainly in terms of two outside signals: gravity, which tells a plant which way is down, and light, which tells it where the sky is. A new study by researchers at INRAE, France's national research institute for agriculture, food and the environment, suggests that this explanation is incomplete. Trees, it appears, also rely on an inner sense of their own shape.
BIn animals, this inner sense is called proprioception. It is the reason a person can touch their nose with their eyes closed: sensors in the muscles and joints constantly report the position of the body to the brain. For many years, proprioception was regarded as something that only animals possessed. However, in 2012 a team that included INRAE scientists published evidence that plants are also able to sense how they are curved. The latest work, which appears in the journal New Phytologist, goes further by showing how trees actually use this information to correct their posture.
CThe main difficulty in studying this sense in plants is that gravity and light are always present, so it is hard to tell which signal a plant is responding to. To solve this problem, the researchers built an unusual piece of equipment. Young poplar trees were placed on a horizontal platform that rotated inside a large sphere. The inside of the sphere was lit equally from every direction, so the plants received no steady clue about where the light was coming from. Because the platform kept turning, the pull of gravity on the stems kept changing direction, which meant that it could not guide their growth either. In effect, the trees were left with only one reliable source of information: their own shape.
DBefore the experiment began, the poplars were grown lying on their sides for about ten days. During this period, their stems bent upwards as the trees tried to grow towards the vertical, which left each stem with a clear curve. Once the trees were placed in the rotating sphere, something remarkable happened. Over several weeks, the curved stems gradually straightened, even though neither gravity nor light could have been telling them to do so. The only reasonable explanation was that the trees were detecting their own curvature and responding to it.
EWhen the scientists examined the wood inside the stems, they discovered how the straightening had been achieved. Trees produce a special material called tension wood, which pulls on the stem as it forms, rather like a muscle. Until now, it was generally believed that tension wood develops only on the upper side of a leaning stem, where it helps to lift the tree. In the poplars, however, the tension wood on the original upper side stopped forming once the trees were inside the sphere. Instead, the same kind of wood began to develop on the opposite side, pulling the stem in the other direction until it was almost straight. The two sides therefore behaved like a pair of opposing muscles, similar to those that bend and straighten a human arm. Bruno Moulia, a research director at INRAE, described the process as "a genuine sensorimotor loop" at work in the woody parts of trees.
FThe discovery has several possible practical uses. Storms, landslides and a changing climate can all leave trees bent or displaced, so knowing how trees regain their posture could become increasingly important. The findings may also be useful in agriculture, where cereal crops sometimes fall over before they can be harvested, a problem known as lodging. If breeders understood the mechanism better, they might be able to develop crop varieties that are less likely to collapse. Finally, the research could benefit the timber industry. When tension wood forms in a poorly coordinated way, it can leave excessive stresses inside the wood, which can reduce the quality of the timber. A better understanding of how trees control tension wood could therefore help foresters and breeders to produce better wood.