In plant physiology, which plant hormone is primarily responsible for helping plants respond to drought and water stress by closing stomata and inducing dormancy?

Introduction / Context:
Plants cannot move away from drought or water stress, so they rely on internal chemical signals to adjust their physiology and survive. Plant hormones, also called phytohormones, coordinate many of these responses. One key hormone is especially important for helping plants conserve water during dry conditions by closing stomata and inducing other protective changes. This question checks your understanding of which hormone plays the leading role in drought response and water stress signalling in plants.

Given Data / Assumptions:

• We are dealing with major plant hormones: ethylene, cytokinin, abscisic acid, and auxin.
• The focus is specifically on drought and water stress responses.
• The response described includes stomatal closure and induction of dormancy or stress tolerance.
• We assume standard textbook roles for each hormone in plant physiology.

Concept / Approach:
Abscisic acid (commonly abbreviated ABA) is widely known as the plant stress hormone. When soil water becomes limited, roots and leaves synthesise more ABA, which is then transported to target tissues. In leaves, ABA causes guard cells around stomata to lose turgor, leading to stomatal closure and reduced water loss through transpiration. ABA also promotes seed dormancy and helps plants tolerate various stresses. Ethylene is mainly associated with fruit ripening and responses to mechanical stress, cytokinin promotes cell division and delays senescence, and auxin regulates processes like cell elongation and phototropism. Therefore, the hormone most directly linked to drought response and stomatal closure is abscisic acid.

Step-by-Step Solution:
Step 1: Identify that drought and water stress require plants to reduce water loss, especially through stomata. Step 2: Recall that abscisic acid levels rise in plants under drought, sending a signal that water is limited. Step 3: Understand that ABA acts on guard cells to trigger stomatal closure, reducing transpiration. Step 4: Remember that ABA is also involved in seed dormancy and broader stress tolerance mechanisms. Step 5: Compare this role with ethylene, cytokinin, and auxin, which have different primary functions. Step 6: Conclude that abscisic acid is the hormone that helps plants respond to drought and water stress in the way described.

Verification / Alternative check:
Plant physiology textbooks often label ABA as the stress hormone and include diagrams showing its synthesis in roots and leaves under drought conditions. Experimental studies demonstrate that applying ABA to leaves causes stomata to close even in the absence of drought, while mutants that cannot produce or respond to ABA have difficulty closing stomata and are more vulnerable to water loss. In contrast, experiments with auxin, cytokinin, or ethylene do not show the same fundamental role in stomatal closure under water stress. This experimental evidence supports ABA as the key hormone in drought response.

Why Other Options Are Wrong:
Ethylene, which promotes fruit ripening and leaf abscission, is not the primary hormone responsible for stomatal closure during drought, even though it is important in other stress and developmental responses.
Cytokinin, which mainly promotes cell division and shoot growth, tends to delay senescence and is not the main signal for water stress-induced stomatal closure.
Auxin, which primarily regulates cell elongation and tropic responses, is central to growth and directional responses but not the chief hormone for drought-induced stomatal regulation.

Common Pitfalls:
Students sometimes confuse abscisic acid with ethylene because both can be involved in leaf abscission and stress-related processes. Another common mistake is to assume that the most famous hormone, auxin, must be responsible for all major plant responses. To avoid confusion, remember the mnemonic that ABA stands for “Abscisic Acid” and can also be remembered as “Always Be Alert” for stress, especially drought. Connecting ABA with stomatal closure and water conservation will help you answer related exam questions correctly.

Final Answer:
The hormone that helps plants respond to drought by closing stomata and inducing stress responses is Abscisic acid (ABA), which triggers stomatal closure and stress responses.