Trees capture water and nutrients through the roots and have to transport it against gravity to the aerial part. This, of course, is a problem, but they have capillary and evapotranspiration mechanisms, and other factors that contribute to the water potential, to solve it. The leaves have stomata, which are modified cells of the epidermis that open or close depending on the concentration of gases that the plant needs to expel or capture from the atmosphere at all times. For example, during the day, plants perform two of their most important physiological functions, photosynthesis and respiration. In respiration, like animals, they capture oxygen and release CO₂. In photosynthesis a reverse gas exchange takes place: CO₂ is fixed and oxygen is released. As the exchanges of photosynthesis predominate over those of respiration, together they release more oxygen and capture more CO₂ during the day. Through the stomata there is a continuous exchange of gases: during the day both processes take place, and at night photosynthesis stops and only respiration continues.
But in addition to these two gases, there is another gas that is very important because it allows plants to carry water from the roots to the aerial part, and that is water vapor. As they fix CO₂ and expel oxygen through open stomata, they also expel water as vapor. And that mechanism is called evapotranspiration. Thanks to this breather mechanism, which works like a plunger, a tension is generated that pulls the liquid water upwards; it rises through waterproof conducting vessels called xylem from the roots to the leaves. In these conductive vessels there is a capillarity effect that allows the water to rise. A force called water potential is created, the result of the balance between the potentials of osmosis, capillarity, evapotransporation and gravity, which measures the capacity of this plant to transport water with nutrients from the soil to the leaves.
At the same time that this occurs, the plants have other conductive vessels called phloem that carry out the opposite transport. As the leaves are photosynthesizing during the day, the excess of assimilated compounds are transformed into sucrose, which is transported from the leaves to the roots. This transport, in addition to going in favor of the gravity gradient, takes place thanks to a source-sink effect, since the cells of the root, not photosintering, consume the sucrose that reaches them from the aerial part in order to obtain the carbohydrates necessary and accumulate the excess in the form of polysaccharides.
As the tree grows, the xylem cells die and their waterproofed cell walls become conductive vessels. These vessels extend vertically in the plant and are communicated laterally with each other through bridges in the wall, and through these bridges the water also circulates. When conditions are normal and up to a certain height of the plant, the water potential that is maximum at the root and minimum at the evapotranspiration surface of the leaves, transports water and nutrients against gravity without problems. But in very large woody species, any of those huge trees that we know, a limit to growth is reached. There comes a time when the tree can no longer grow, not because it lacks nutrients but because physically the difference in water potential is not enough to pump that water higher. The tallest known trees reach just over 110 meters, which seems to be the limit of their ability to pump water from the ground to those heights.
There is a very interesting aspect and it is that trees also suffer embolism. They have embolisms, as if it were a human body, although they are totally different. In this case, this transport of water from the roots to the leaves through the xylem vessels is sometimes interrupted. This is mainly due to the appearance of air bubbles. These bubbles prevent fluidity, the water does not reach certain tissues or certain parts of the tree and those areas are atrophied. Sometimes this can result in the death of the entire tree. Strokes often occur related to adverse circumstances such as drought stress.
Pilar Catalán Rodríguez is a doctor in botany, professor at the University of Zaragoza.
Question sent via email by Lucio Fernandez
Coordination and writing: Victoria Toro
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