In this blog post, we will explain the mechanism of water movement in plants in simple terms using the principles of transpiration, tension, cohesion, and water potential.
Generally, the maximum height of a water column that can be created in the atmosphere is about 10 meters. However, some trees on Earth are over 110 meters tall. How can water absorbed by the roots be transported to the top of a tree 110 meters high?
Since the moisture concentration in the atmosphere is lower than that in the leaves, water diffuses into the atmosphere through the stomata on the leaf surface. This process is called transpiration. When water escapes through the stomata, tension is created inside the xylem, which is the tissue that serves as a water passage, pulling water upward. The water column is able to rise without breaking due to the strong cohesive force of water. The cohesive force of water is greater than the tension generated in the xylem, so the water column rises from the roots to the leaves without breaking, as if connected by a string. This process of water transport in the xylem is called the “transpiration → tension → cohesion” mechanism.
This mechanism can be explained by water potential. Water potential is the amount of water contained in soil or plants converted into energy, and it indicates the ability of water to move. The unit used is pascal (Pa, 1 MPa = 10^6 Pa). Water moves from areas of high water potential to areas of low water potential without consuming energy. The water potential of pure water is 0 MPa, but when the pressure decreases or a solute is added and the ion concentration increases, the water potential decreases. The water potential of soil is -0.01 to -3 MPa, and that of the atmosphere is -95 MPa. Generally, the water potential decreases from the soil to the roots, stems, and leaves, and accordingly, water diffuses from the roots to the stems and then to the leaves, where it is released into the atmosphere through stomata.
These changes in water potential are one of the main driving forces behind the movement of water within plants. In particular, the reason why water can be pulled upward in tall trees is due to the gradient of water potential. Water always moves from a high water potential to a low water potential, so water in the soil moves through the roots to the stems and from the stems to the leaves. This allows water to reach the highest point of the tree.
The opening and closing of stomata is controlled by a pair of guard cells on the surface of the leaves. When the concentration of ions inside the guard cells increases due to the action of light, the water potential decreases, causing water to enter the guard cells and open the stomata. The plant then absorbs carbon dioxide from the atmosphere and produces glucose through photosynthesis. The problem is that when plants open their stomata to absorb carbon dioxide, they lose water, and conversely, when they close their stomata to prevent water loss, they must give up carbon dioxide. Plants, which need both water and glucose, have developed a system to solve this dilemma by opening their stomata during the day when sunlight is available for photosynthesis and closing them at night when it is not, thereby regulating the absorption of carbon dioxide and the release of water. As a result, the opening and closing of stomata occurs in a regular cycle.
Plants have also developed various mechanisms to adapt to environmental changes and survive. For example, plants living in areas with extreme climate conditions have developed the ability to regulate the opening and closing cycle of stomata to maximize water conservation. These adaptations enable plants to overcome stress caused by drought or excessive moisture, allowing them to survive in various environments.
In conclusion, the process by which water reaches the tips of leaves in tall trees is achieved by the mechanisms of transpiration, turgor, and cohesion, as well as the gradient of water potential. This process plays an important role in the survival of plants in response to environmental changes. This amazing ability of plants is another example of the complexity and sophistication of nature.
