Cytokinin is an adenine derivative that serves as a growth regulator in plants. It plays a crucial role in various physiological processes such as cell division, shoot and root development, and chloroplast development. Here is a detailed explanation of cytokinin as an adenine derivative growth regulator:

Cytokinin as an adenine derivative:
- Cytokinins are a class of plant hormones that are derived from the purine base adenine, which is one of the building blocks of DNA and RNA.
- Adenine is a nitrogenous base that is commonly found in nucleotides, the building blocks of nucleic acids. It is also an essential component of ATP (adenosine triphosphate), the energy currency of cells.
- Cytokinins are synthesized in the root tips, young leaves, and developing seeds of plants. They are then transported to different parts of the plant where they regulate growth and development.

Functions of cytokinin:
1. Cell division: Cytokinins promote cell division in various plant tissues, leading to the formation of new cells. They stimulate the activity of enzymes called cyclins, which regulate the cell cycle.
2. Shoot and root development: Cytokinins promote the growth of lateral buds, resulting in the production of new shoots. They also stimulate root growth and branching.
3. Chloroplast development: Cytokinins play a crucial role in the development and functioning of chloroplasts, which are responsible for photosynthesis. They regulate the synthesis of chlorophyll and other photosynthetic pigments.
4. Delaying senescence: Cytokinins can delay the process of senescence, which is the aging and deterioration of plant tissues. They inhibit the breakdown of chlorophyll and other cellular components, thereby prolonging the lifespan of leaves and other organs.

Importance of cytokinins:
- Cytokinins are essential for overall plant growth and development. They interact with other plant hormones such as auxins to regulate various physiological processes.
- The balance between cytokinins and other hormones is crucial for maintaining proper plant growth. For example, a higher concentration of cytokinins relative to auxins promotes shoot development, while a higher concentration of auxins relative to cytokinins promotes root development.
- Cytokinins are also used in plant tissue culture techniques to promote the growth and regeneration of plant cells and tissues in a laboratory setting.

In conclusion, cytokinin is an adenine derivative growth regulator in plants. It is derived from adenine and plays a vital role in cell division, shoot and root development, chloroplast development, and delaying senescence. Cytokinins are essential for overall plant growth and are used in plant tissue culture techniques.