How do plants control water loss during transpiration?

Plants control water loss during transpiration primarily through the opening and closing of stomata.

Transpiration is a vital process in plants that involves the movement of water from the roots to the leaves, where it evaporates into the atmosphere. This process is essential for the transport of nutrients, cooling the plant, and maintaining turgor pressure. However, excessive water loss can be detrimental to the plant, especially in arid conditions. Therefore, plants have developed mechanisms to control the rate of transpiration, primarily through the regulation of stomatal openings.

Stomata are tiny pores found mainly on the underside of leaves. They are surrounded by two specialised cells known as guard cells. These cells can change shape to open or close the stomatal pore, thereby controlling the rate of transpiration. When the guard cells are turgid (swollen with water), the stomata are open, and transpiration occurs. Conversely, when the guard cells are flaccid (not swollen), the stomata close, reducing water loss.

The opening and closing of stomata are regulated by various environmental factors. For instance, in bright light, photosynthesis occurs at a high rate, producing glucose that increases the turgidity of guard cells, thus opening the stomata. On the other hand, in darkness, the rate of photosynthesis decreases, reducing the turgidity of guard cells and closing the stomata. Similarly, when the atmospheric carbon dioxide concentration is low, the stomata open to allow more carbon dioxide in for photosynthesis. However, if the plant is under water stress, abscisic acid (a plant hormone) is produced, which signals the guard cells to close the stomata and reduce water loss.

In addition to stomatal control, some plants also have adaptations such as a thick cuticle, reduced leaf area, or hairy leaves, which help to reduce water loss. These adaptations are particularly common in desert plants, which need to conserve water due to the arid conditions.