Question

The hormone which reduces transpiration rate by inducing stomatal closure is (a) \(\mathrm{ABA}\) (b) ethylene (c) cytokinin (d) gibberellin.

Short answer

The hormone that reduces transpiration rate by inducing stomatal closure is (a) ABA.

Step-by-step solution

Understanding the concept of transpiration and stomatal closure

Transpiration is the process of water movement through a plant and its evaporation from aerial parts, like leaves. Stomata are tiny openings on leaves that facilitate gas exchange; they also play a role in transpiration. Certain hormones can induce the closure of these stomata, thus reducing transpiration.

Identifying the hormone responsible for inducing stomatal closure

Among the given options (a) Abscisic Acid (ABA), (b) ethylene, (c) cytokinin, and (d) gibberellin, ABA is known for its role in inducing stomatal closure during times of water stress. This adaptive response helps the plant to conserve water.

Choosing the correct hormone

From the understanding of plant hormones and their functions regarding transpiration and stomatal closure, it is clear that Abscisic Acid (ABA) is the hormone that induces stomatal closure to reduce the transpiration rate.

Key concepts

Stomatal Closure

Understanding how plants regulate water loss is crucial for students studying botany and plant physiology. One key mechanism is stomatal closure, a process that involves tiny openings on leaf surfaces called stomata. These openings are surrounded by guard cells that swell to open and shrink to close the pores. Environmental cues, such as light, carbon dioxide concentration, and especially water availability, trigger these changes.

To enhance comprehension, it’s helpful to visualize stomata like valves on a plant’s surface, controlling the vital exchange between the plant and its environment. When the plant is dehydrated or undergoing water stress, the necessity to shut these valves is crucial to prevent excess water loss through transpiration. Abscisic Acid (ABA) is the hormone that signals the guard cells to promote stomatal closure when the plant is facing drought conditions. By doing so, ABA plays a protective role, aiding in the plant's survival during arid periods.

Transpiration

Plants have a unique process called transpiration, which might be compared to how humans sweat to cool down. It’s a natural consequence of plant respiration, where water is lost through evaporation primarily from the leaves. Notably, this serves to cool the plant but also results in water loss.

Student’s might be surprised to find this process isn’t purely wasteful - it actually has multiple benefits. It helps in the uptake and transport of essential nutrients from the soil and maintains the flow of water from roots to leaves. However, like any process requiring balance, excessive transpiration can threaten a plant's survival by causing dehydration. To understand transpiration thoroughly, it’s important to recognize the environmental factors affecting it, such as temperature, humidity, wind, and light, which typically appear in examination questions or lab experiments on plant transpiration studies.

Plant Hormones

Plant hormones, or phytohormones, are chemical messengers playing pivotal roles in regulating growth, development, and responses to stresses in plants. They are fascinating because, unlike animals, plants cannot move to escape their environment; instead, they adjust biochemically. Abscisic Acid (ABA) is one of these hormones and is often referred to as the 'stress hormone' of plants due to its involvement in response to abiotic stressors.

Other than ABA, several hormones have distinct functions:

• Auxins - promote cell elongation, root formation, and bud growth,
• Gibberellins - stimulate stem elongation, seed germination, and fruit development,
• Cytokinins - promote cell division and shoot formation,
• Ethylene - influences fruit ripening and leaf abscission,
• Brassinosteroids - stimulate cell expansion and pollen tube growth.

Each has a specific role, and they often work in harmony to ensure plant vitality. To help students understand, one might compare these hormones to a team working together to build and maintain a healthy, responsive organism - the plant.