Biology Question 108 – NEET-UG 2025
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Leaf senescence is the process of aging in leaves, characterized by the breakdown of macromolecules and remobilization of nutrients (like nitrogen, phosphorus, and potassium) from the senescing leaves to younger, growing parts of the plant or storage organs. This process is regulated by various internal and external factors, including plant hormones.
Let's review the roles of the given phytohormones:
1. **Ethylene:** Promotes senescence and abscission (shedding of leaves, flowers, fruits).
2. **Abscisic acid (ABA):** Promotes senescence, dormancy, and abscission, and is involved in stress responses.
3. **Gibberellin:** Primarily promotes stem elongation, seed germination, and flowering. While it can have complex interactions, it is not primarily known for delaying senescence through nutrient mobilization.
4. **Cytokinin:** Known to delay senescence (the Richmond-Lang effect) by promoting cell division, nutrient mobilization, and maintaining protein and nucleic acid synthesis in leaves.
Remember that ethylene and abscisic acid are generally considered 'stress' or 'aging' hormones, while cytokinins and auxins are 'growth' hormones.
Based on their known functions, Cytokinin is the phytohormone that promotes nutrient mobilization and helps in the delay of leaf senescence in plants. It actively draws nutrients to the treated area, thus preventing its degradation and delaying aging.
The ability of cytokinins to delay senescence is often demonstrated by applying them to detached leaves, which remain green longer than untreated leaves.