In plant physiology and agronomy, “plant tranquillisers” (growth retardants) are chemicals that primarily ________ to manage canopy architecture and lodging risk.

Introduction / Context:
Plant tranquillisers, more commonly called plant growth retardants or anti-gibberellins, are applied to limit excessive vegetative growth without severely affecting photosynthesis or reproductive development. They are widely used to manage crop stature, reduce lodging, and improve harvest index. Understanding their primary effect helps agronomists select the right regulator for canopy management and yield stability.

Given Data / Assumptions:

• The term refers to growth-retarding regulators such as chlormequat chloride (CCC), mepiquat chloride, daminozide, and maleic hydrazide.
• Typical target tissues are rapidly elongating stems and internodes.
• The question seeks the chief agronomic outcome rather than all secondary effects.

Concept / Approach:
Many tranquillisers act by interfering with the biosynthesis or action of gibberellins, hormones that promote stem elongation and cell expansion. By moderating gibberellin pathways, they reduce internode length, producing shorter, sturdier plants that resist lodging in cereals and fiber crops. This height control is the signature purpose of “plant tranquillisers.”

Step-by-Step Solution:
Identify the hormone pathway targeted: gibberellin synthesis/action.Link mechanism to morphology: reduced gibberellin effect → reduced internode elongation.Translate to agronomic outcome: shorter, compact canopy; improved standability; easier harvest.Therefore, the primary role is holding back stem growth at a desired height.

Verification / Alternative check:
Standard agronomy texts show that CCC in wheat or mepiquat in cotton reduces plant height and lodging. While some regulators can secondarily influence maturation timing, the hallmark agronomic reason to apply tranquillisers is height control and lodging reduction.

Why Other Options Are Wrong:
“Cause early maturation” is not the principal and consistent outcome across crops. “Accelerate ripening” is typically linked to ethylene-releasing compounds, not growth retardants. “Produce seedless fruit” describes parthenocarpy regulated by other hormones. “Stimulate apical dominance” contradicts the height-reduction intent.

Common Pitfalls:
Confusing growth retardants with ripening regulators or auxin/ethylene products. Also assuming that any regulator “boosts yield”; tranquillisers primarily reshape canopy and reduce lodging risk, which may only indirectly benefit yield.

Final Answer:
hold back stem elongation and maintain plants at a desired height