c. Cytokinin :
It is another growth hormone that promotes cell division. Letham coined the term cytokinin. The first cytokinin was discovered by Skoog and Miller (1954) during investigation of nutritional requirements of callus tissue culture of Nicotiana tabacum (Tobacco).They observed that the callus proliferated when the nutrient medium was supplemented with coconut milk and degraded sample of DNA (obtained from herring sperm). They named it as kinetin. Chemically kinins are 6-furfuryl amino purine. First natural cytokinin was obtained from unripe maize grains by Letham et al. It is known as Zeatin. 6-benzyl adenine is a synthetic cytokinin hormone. Seven different types of cytokinins are recorded from plants. Natural cytokinins are also reported from plants like Banana flowers, apple and tomato fruits, coconut milk, etc.
Physiological effects and applications of cytokinin :
Besides cell division, it also promotes cell enlargment. High cytokinin promotes shooting. A low ratio of cytokinin to auxin induces root development but a high ratio causes buds and shoot to develop. Cytokinin and auxin ratio and their interactions controls morphogenic differentiation. It promotes the growth of lateral buds and controls apical dominance by cell division. It delays the senescence or ageing and abscission processes in plant organs. This was reported by Richmond and Lang (1957). Formation of interfasicular cambium and expansion of cells are other functions. It also breaks dormancy and promotes the germination of seeds. Cytokinin reverses apical dominance effect. It induces RNA synthesis and formation of interfascicular cambium.
