ANS:A - hold back stem growth and halt plants at a desired height.

Plant tranquillisers, also known as growth regulators or plant growth retardants, are chemicals used in agriculture to hold back stem growth and halt plants at a desired height. Explanation:

1. Controlling Growth: Plant tranquillisers are commonly used to control the height of plants, particularly in crops such as wheat, rice, and ornamental plants. By inhibiting the elongation of stems, these chemicals help maintain a more compact and manageable plant structure.
2. Desired Height: In many agricultural settings, maintaining plants at a specific height is desirable for various reasons, such as facilitating mechanical harvesting, reducing lodging (the bending or breaking of stems), and improving overall crop uniformity.
3. Application: Plant tranquillisers are typically applied as sprays or drenches to the foliage or soil around the plants. They work by interfering with the synthesis or action of plant hormones called gibberellins, which are responsible for stem elongation.
4. Effects on Growth: By inhibiting stem growth, plant tranquillisers do not directly affect other aspects of plant development, such as maturation, ripening, or fruit formation. Their primary function is to regulate plant height and architecture.
5. Seedless Fruit Production: While plant growth regulators can influence fruit development in some cases, such as promoting or inhibiting fruit set or abscission, they are not typically used to produce seedless fruit.