Cytokinin was the third growth-promoting hormone discovered. Its discovery occurred when tissue-culture propagation of plants was in its infancy, in the early 1960s. At that time, scientists could maintain tissue in aseptic culture but were unable to stimulate cell division. Through trial and error, chemicals capable of stimulating cell division were eventually discovered in herring sperm and in coconut milk. These compounds were named cytokinins, a derivative of the word "cytokinesis," which means cell division.
Cytokinin is found in rapidly growing tissues, both vegetative and reproductive. It is produced in shoots and roots. Sachs and Thimann (1967) showed that cytokinins can counter the effects of apical dominance and induce lateral bud outgrowth. The research subsequently showed that these compounds can move isotope-labeled metabolites to their point of application, which was named "hormonally directed transport." Cytokinin can act as a growth factor and as an antisen-escence hormone in leaves and fruits.
Three major uses of cytokinins occur in fruit production. The greatest is in micropropagation, where cytokinin levels are manipulated in culture to induce tissue proliferation. The second is in fruit tree development, where applications of cytokinin sprays or paints are used to stimulate lateral bud outgrowth and improve tree structure. In addition, cytokinins are used in apple development, as chemical thinners, and to alter the length-to-diameter ratio of apple fruit.
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