Cytokinins are referred to as the cell division hormones, and while cell division will take place only in the presence of one of these hormones, the cytokinins stimulate a number of other plant responses as well. These hormones have been shown to retard senescence in detached leaves and to create metabolic sinks, areas within a tissue where increased metabolism takes place. As a result, there is an increase in transport of metabolites to the area. For example, amino acids will be transported to a site where the presence of cytokinins has increased protein synthesis.
In many plants, these hormones stimulate the production of larger, greener leaves by causing leaf cells to expand and by promoting both chloroplast development and chlorophyll synthesis. A number of naturally occurring substances that exhibit cytokinin activity have been identified. Of these, ribosyl zeatin is one of the most abundant in plants.
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