Which one of the following plant hormones is most commonly used to induce parthenocarpy, that is, the development of seedless fruits without fertilisation?

Introduction:
Parthenocarpy is an important concept in plant physiology and agriculture, referring to the formation of fruits without fertilisation, resulting in seedless fruits. Many commercially valuable fruits such as seedless grapes can be produced using this process. Plant growth regulators, also called plant hormones, can be used to induce parthenocarpy artificially. This question asks which hormone is most commonly associated with inducing parthenocarpic fruit formation.

Given Data / Assumptions:
- The key process is parthenocarpy, defined as fruit development without seed formation and without fertilisation. - Hormone options are abscisic acid, gibberellin, ethylene and cytokinin. - We assume standard agricultural practice where growth regulators are sprayed or applied to flowers. - The focus is on the hormone primarily used to induce fruit set and growth in the absence of fertilisation.

Concept / Approach:
Gibberellins are plant hormones that promote stem elongation, germination and fruit growth. They are frequently used to induce parthenocarpy in fruits such as grapes and tomatoes. By applying gibberellins to flowers, fruit set occurs even without pollination and fertilisation, leading to seedless fruits. Abscisic acid generally inhibits growth and promotes dormancy, ethylene is associated with fruit ripening and senescence, and cytokinins primarily promote cell division. Therefore, gibberellin is the hormone most closely linked with induced parthenocarpy.

Step-by-Step Solution:
Step 1: Recall the definition of parthenocarpy as fruit formation without fertilisation, leading to seedless fruits. Step 2: Connect practical examples such as seedless grapes in vineyards, where growth regulators are used to improve fruit quality. Step 3: Remember that gibberellins, especially GA3, are sprayed on flowers to stimulate ovary growth and fruit development irrespective of seed formation. Step 4: Consider the main functions of the other hormones: abscisic acid for dormancy and stress, ethylene for ripening, cytokinins for cell division and delay of senescence. Step 5: Conclude that gibberellin is the correct hormone for inducing parthenocarpic fruits.

Verification / Alternative check:
Plant physiology references and horticulture guides describe experiments where unpollinated flowers treated with gibberellin solutions set fruits that are seedless. These case studies are often cited in textbooks to illustrate hormonal control of fruit development. In addition, multiple competitive exam questions explicitly associate gibberellin with parthenocarpy, which confirms it as the expected answer for this topic.

Why Other Options Are Wrong:
Abscisic acid: Known as a stress hormone that promotes stomatal closure and seed dormancy; it does not normally induce fruit formation without fertilisation. Ethylene: Plays a major role in fruit ripening and leaf abscission but is not mainly used to start fruit development in the absence of pollination. Cytokinin: Promotes cell division and delays leaf senescence, and is used in tissue culture, but is not primarily responsible for parthenocarpic fruit set.

Common Pitfalls:
Because multiple hormones influence fruit development, students may confuse their roles and think of ethylene or cytokinins when they see a fruit related question. Another mistake is to mix up parthenocarpy (seedless fruit) with apomixis (seed formation without fertilisation). To avoid confusion, it helps to remember the classic example of GA3 sprays on grapes to induce seedless large berries, which directly links gibberellin with parthenocarpy.

Final Answer:
The hormone most commonly used to induce parthenocarpic (seedless) fruit development is Gibberellin.