Solution

Cytokinin causes delay of senescence called Richmond Lang effect. It checks disappearance of chlorophyll and degradation of protein synthesis in leaves.
Senescense of leaves leads to yellowing and finally leads to the fall from the plant. If a young excised leaf is kept in water, it slowly changes its color to yellow and dies. If such leaves are provided with cytokinin, the yellowing is significantly delayed and such an effect is called Richmond and Langs effect; named after the discoverers.