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The following points highlight the eight major physiological effects of Kinetin. The physiological effects are: 1. Cell Division 2. Cell Enlargement 3. Initiation of Inter-fascicular Cambium 4. Morphogenesis 5. Counteraction of Apical Dominance 6. Dormancy of Seeds 7. Delay of senescence: The Richmond-Lang Effect and 8. Promotion of Chloroplast Development.
Physiological Effect # 1. Cell Division:
One of the most important biological effects of kinetin on plants is to induce cell division in the presence of sufficient amount of auxin (IAA), especially in tobacco pith callus, carrot root tissue, soybean cotyledon, pea callus etc.
Physiological Effect # 2. Cell Enlargement:
Like auxins and gibberellins, the kinetin may also induce cell enlargement. Significant cell enlargement has been observed after kinetin treatment in leaf discs cut from etiolated leaves of Phaseolus vulgaris, pumpkin cotyledons, tobacco pith cultures, cortical cells of tobacco roots, excised Jerusalem artichoke tissue etc.
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While cytokinins have been shown to promote cell expansion (although without proton extrusion) in leafy cotyledons of some plants such as mustard, sunflower, cucumber, radish etc. but this effect is not exhibited by auxin or gibberellin i.e., they do not promote cell expansion in cotyledons
Physiological Effect # 3. Initiation of Inter-fascicular Cambium:
Kinetin can induce formation of inter-fascicular cambium. This has in fact been shown by Sorokin et al (1962) in pea stem sections.
Physiological Effect # 4. Morphogenesis:
Kinetin also has ability to cause morphogenetic changes in an otherwise undifferentiated callus.
For instance, the tobacco pith callus can be made to develop either buds or roots by changing the concentration of kinetin and auxin:
The effect of kinetin in stimulating or initiating the formation of buds has also been observed in leaf cuttings of Saintipaulia ionantha, Begonia, Bryophyllum and in mosses e.g., Tortella etc. A positive effect of kinetin on the regeneration of shoots from cultured root segments has also been reported in Isatis tinctoria and Convolvulus arvensis.
Physiological Effect # 5. Counteraction of Apical Dominance:
Wickson and Thimann (1958) in one of their experiments found that the growth of the lateral buds of pea stem section (second internode) in culture solutions containing IAA was inhibited. But the growth of lateral buds could continue if IAA was not included.
On the other hand, the addition of kinetin along with IAA stimulated the growth of these buds. They obtained similar results with entire shoots and concluded that the apical dominance might be under the control of a balance of concentrations between endogenous kinetin like substances (cytokinins) and IAA.
That cytokinins play a role in initiating the growth of lateral buds has also been proved by physiological studies made on cytokinin overproducing mutants of tobacco. The wild type tobacco plants show strong apical dominance, but in cytokinin-overproduce mutants the lateral buds grow vigorously and the plants tend to be bush-like.
Physiological Effect # 6. Dormancy of Seeds:
Like gibberellins, the dormancy of certain light sensitive seeds such as lettuce and tobacco can also be broken by kinetin treatment in dark. Furthermore, the inhibitory effect of inferred light treatment on the germination of the above seeds is also overcome by kinetin treatment. Seeds of parasitic plants e.g., Striga asiatica which require the presence of their host for germination can also be induced to germinate by treating them with kinetin even in the absence of their host.
Physiological Effect # 7. Delay of senescence: The Richmond-Lang Effect:
The ageing process of the leaves usually accompanies with loss of chlorophyll (i.e., yellowing) and rapid breakdown of proteins. This is called as senescence. In 1957, Richmond and Lang showed that this senescence could be postponed to several days in detached Xanthium leaves by kinetin treatment. This effect of kinetin in delaying the senescence is called as Richmond-Lang effect.
Mothes (1960) and other workers have shown mobilization of nutrients and other substances – including auxins to the kinetin treated areas. In intact plants, the delay of senescence at some part due to kinetin treatment may result in senescence in other part of the plant.
The observation of Osborne (1962) and other workers suggest that the high protein content in kinetin treated tissue is probably due to more synthesis of proteins than their degradation, and this in turn may be due to the regulatory action of kinetin on RNA synthesis.
One of the important factors in delay of senescence in kinetin treated leaves is their physiological age. For instance, mature leaves of Nicotiana rustica have been found to be more responsive to kinetin treatment in delaying senescence than the younger leaves.
Physiological Effect # 8. Promotion of Chloroplast Development:
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Cytokinins are also known to greatly enhance conversion of etioplasts into chloroplasts when etiolated seedlings after treatment with cytokinins are exposed to light. In such cases, the chloroplasts develop extensive grana and chlorophylls and the rate of synthesis of photo- synthetic enzymes is much greater in comparison to those etiolated seedlings which are illuminated without cytokinin treatment.
