The correct answer is Ethylene. Explanation & Key Takeaways: • Ethylene is a gaseous plant hormone that plays a crucial role in the ripening of fruits. • It accelerates the breakdown of cell walls, conversion of starches to sugars, and the degradation of chlorophyll, leading to fruit ripening. • In climacteric fruits such as bananas, mangoes, and tomatoes, ethylene production increases sharply during ripening. • Ethylene is also responsible for other physiological processes, such as leaf abscission and flower senescence. Important Notes: • Ethylene is commonly used in the agriculture industry to artificially ripen fruits post-harvest for market distribution. • It is synthesized in plants from the amino acid methionine via the intermediate 1-aminocyclopropane-1-carboxylic acid (ACC). • The discovery of ethylene's role in fruit ripening revolutionized the understanding of plant hormones and their applications. Related Concepts: • Auxin: Auxins are plant hormones involved in cell elongation, root initiation, and phototropism. Indole-3-acetic acid (IAA) is a common auxin. • Gibberellin: Gibberellins promote stem elongation, seed germination, and flowering. They are essential for breaking seed dormancy and stimulating growth in certain plants. • Cytokinin: Cytokinins promote cell division and delay leaf senescence. They are also involved in shoot growth and nutrient mobilization within plants.