Which hormone is associated with senescence or ageing?

Answer

**Step-by-Step Solution:**

1. **Understanding Senescence in Plants**: Senescence refers to the process of aging in plants. It can be categorized into two types: stress-induced senescence and age-related developmental senescence.

2. **Role of Chlorophyll**: During leaf senescence, chlorophyll degradation occurs. This degradation leads to the exposure of other pigments, such as carotenoids, which can result in the vibrant colors seen in autumn leaves of deciduous trees.

3. **Functions of Senescence**: Senescence plays important roles in plants, particularly in leaves. It helps in recycling nutrients and contributes to the development of storage organs in plants.

4. **Identifying the Hormone**: The hormone primarily associated with plant senescence or aging is abscisic acid (ABA). This hormone is crucial for regulating the aging process in plants.

5. **Stress Response**: Abscisic acid is also known as the "stress hormone" because it helps plants respond to various stress conditions, such as drought and soil salinity.

6. **Conclusion**: Therefore, the hormone associated with senescence or aging in plants is abscisic acid.

**Final Answer**: Abscisic acid is the hormone associated with senescence or aging in plants.

---