42. Which of the following reduces the transpiration rate when applied to aerial parts of plants?
(A) Phosphon-D
(B) Paraquat
(C) Phenyl mercuric acetate
(D) Valinomycin

Understanding Transpiration and Antitranspirants in Plants

Transpiration is the process where plants lose water vapor through stomata in their leaves, crucial for cooling and nutrient uptake but sometimes excessive under stress. Antitranspirants are chemicals applied to aerial parts to reduce this rate without harming growth. This article answers the MCQ: “Which of the following reduces the transpiration rate when applied to aerial parts of plants?” with options (A) Phosphon-D, (B) Paraquat, (C) Phenyl mercuric acetate, and (D) Valinomycin.

Correct Answer: (C) Phenyl mercuric acetate

Phenyl mercuric acetate (PMA) is a classic antitranspirant. It forms a thin film over stomata, partially closing them and cutting water loss by 40-60%. Farmers use it on crops like wheat during droughts to conserve water.

Detailed Explanation of All Options

Let’s break down each option based on plant physiology and biochemistry.

• (A) Phosphon-D (Fosamine): This herbicide targets woody plants by inhibiting amino acid synthesis, defoliating them. It increases transpiration indirectly by damaging leaves, not reducing it. Wrong choice for antitranspirant effects.

• (B) Paraquat: A fast-acting contact herbicide that generates reactive oxygen species, destroying chloroplast membranes. It causes rapid wilting and boosts initial transpiration before cell death. Not used to reduce transpiration—it’s a desiccant.

• (C) Phenyl mercuric acetate (PMA): Correct. As a stomatal-closing agent, PMA’s mercury-based structure disrupts guard cell function, reducing stomatal aperture. Studies show it lowers transpiration by 50% in crops like rice, ideal for water-scarce areas. Note: Toxic, so phased out for safer options like ABA analogs.

• (D) Valinomycin: An antibiotic potassium ionophore that disrupts ion gradients across membranes. In plants, it increases stomatal opening by altering K+ transport, raising transpiration. Used in lab studies, not as an antitranspirant.

Why PMA Stands Out in Plant Biology

PMA exemplifies metabolic antitranspirants or film-forming agents in older literature (e.g., Class I antitranspirants). Modern alternatives like pinolene or abscisic acid avoid mercury toxicity. For exams like NEET or ICAR, remember PMA for its direct stomatal closure.

This table summarizes key effects for quick revision.

Applications in Agriculture and Research

Reducing transpiration rate helps in drought-prone regions like Rajasthan, conserving soil moisture. PMA’s role highlights biochemical control of guard cells via ion channels—ties into your interests in plant physiology and molecular biology.