explain in detail the differences between hydropassive and hydroactive stomatal closure
Hydropassive and hydroactive stomatal closure are two mechanisms through which plants regulate the opening and closing of their stomata, which are tiny pores located on the surface of leaves and stems. Stomata play a crucial role in the exchange of gases, such as carbon dioxide and oxygen, as well as in the regulation of water loss through transpiration.
1. Hydropassive Stomatal Closure:
Hydropassive stomatal closure is the default response of stomata to changes in the surrounding environmental conditions, particularly with respect to water availability. This closure mechanism is not directly influenced by any metabolic or biochemical signals within the plant. Instead, it is primarily driven by the passive movement of water within the plant cells.
When water availability is sufficient, stomata open to allow the entry of carbon dioxide for photosynthesis and the exit of oxygen and water vapor. However, when water availability decreases, water potential within the plant decreases as well. As a result, water from the guard cells surrounding the stomata moves outwards, leading to a loss of turgidity. Consequently, the guard cells become flaccid and close the stomatal pore, reducing water loss through transpiration.
Hydropassive stomatal closure is mainly influenced by external factors such as the availability of water in the soil, drought conditions, and high temperatures. It is a rapid response, occurring within minutes, and is primarily aimed at conserving water.
2. Hydroactive Stomatal Closure:
Unlike hydropassive stomatal closure, hydroactive stomatal closure is an active process that involves a more complex set of physiological and biochemical responses within the plant. This closure mechanism is regulated by various signaling molecules and hormones, such as abscisic acid (ABA), which is synthesized in response to water stress.
When a plant perceives water stress, it triggers specific signaling pathways that result in the production and release of ABA. This hormone then acts on the guard cells surrounding the stomata. ABA binds to specific receptors on the guard cell membranes, which initiates a signaling cascade within the guard cell.
This signaling cascade ultimately leads to the efflux of potassium ions (K+) from the guard cells. As a consequence, water also moves out of the guard cells, leading to the loss of turgor pressure. The reduction in turgor pressure causes the guard cells to become flaccid and subsequently close the stomatal pore, limiting water loss.
Hydroactive stomatal closure may take longer to occur compared to hydropassive stomatal closure, taking hours or even days. It is a more coordinated response involving various internal signaling mechanisms that work together to regulate stomatal function.
In summary, hydropassive stomatal closure is the quick response of stomata to changes in water availability, driven by the movement of water within the plant cells. On the other hand, hydroactive stomatal closure is a more complex and slower process that involves the production and signaling of hormones like ABA, leading to the active closure of stomata. Both mechanisms are crucial for plants to maintain water balance and respond to environmental cues effectively.