The correct option is Root apices.
Explanation
Cytokinins are plant growth regulators (phytohormones) that play a crucial role in promoting cell division (cytokinesis). Their synthesis is strictly localized to regions exhibiting active growth and rapid cell division.
Analysis:
- Root Apices (Correct): Natural cytokinins are synthesized primarily in the root apices (root tips). From these sites, they are translocated upwards to the shoot and other plant parts via the xylem vessels. This upward transport allows them to regulate growth and development in the aerial parts of the plant.
- Other Options (Incorrect):
- Leaf blades: Mature leaves are generally sinks for cytokinins rather than sources. Cytokinins help delay leaf senescence (aging) but are not primarily synthesized there.
- Guard cells: These cells regulate stomatal opening and closing. While they respond to hormones like Abscisic Acid (ABA), they are not sites of cytokinin synthesis.
- Pollen sacs: While developing seeds and young fruits are sites of cytokinin activity due to cell division, the root apex remains the primary and most consistent site of synthesis for the plant's systemic needs.
Key Takeaway:
Natural cytokinins are primarily synthesized in root apices and transported acropetally (upwards) through the xylem to promote cell division and delay senescence.