`star` Cytokinin
`star` Ethylene
`star` Absisic Acid


● Cytokinins have `color{violet}"specific effects on cytokinesis"`, and were discovered as `color{violet}"kinetin"` (a modified form of adenine, a purine) from the `color{violet}"autoclaved herring sperm DNA"`.

● `color{Brown}"Kinetin"` does not occur `color{violet}"naturally in plants"`.

● Search for natural substances with `color{violet}"cytokinin-like activities"` led to the isolation of `color{violet}"zeatin"` from corn-kernels and coconut milk.

● Since the `color{violet}"discovery of zeatin"`, several naturally occurring cytokinins, and some `color{violet}"synthetic compounds"` with cell division promoting activity, have been identified.

● `color{violet}"Natural cytokinins"` are synthesised in regions where `color{violet}"rapid cell division occurs"`, for example, `color{violet}"root apices"`, `color{violet}"developing shoot buds"`, `color{violet}"young fruits"` etc.

● It helps to `color{violet}"produce new leaves"`, `color{violet}"chloroplasts in leaves"`, `color{violet}"lateral shoot growth"` and `color{violet}"adventitious shoot formation"`.

● Cytokinins help overcome the `color{violet}"apical dominance"`.

● They `color{violet}"promote nutrient mobilisation"` which helps in the `color{violet}"delay of leaf senescence"`.


● Ethylene is a `color{violet}"simple gaseous PGR"`.

● It is synthesised in large amounts by `color{violet}"tissues undergoing senescence"` and ripening fruits.

● Influences of ethylene on plants include `color{violet}"horizontal growth of seedlings,"` `color{violet}"swelling of the axis"` and `color{violet}"apical hook formation"` in dicot seedlings.

● Ethylene promotes `color{violet}"senescence and abscission"` of plant organs especially of leaves and flowers.

● Ethylene is highly effective in `color{violet}"fruit ripening"`.

● It enhances the `color{violet}"respiration rate during ripening"` of the fruits.

● This rise in `color{violet}"rate of respiration"` is called `color{Brown}"respiratory climactic."`

● Ethylene breaks `color{violet}"seed and bud dormancy"`, initiates `color{violet}"germination in peanut seeds"`, sprouting of potato tubers.

● Ethylene promotes `color{violet}"rapid internode/petiole elongation"` in deep water rice plants.

● It helps leaves/ upper parts of the shoot to `color{violet}"remain above water."`

● Ethylene also `color{violet}"promotes root growth"` and `color{violet}"root hair formation"`, thus helping the plants to increase their absorption surface.

● Ethylene is used to `color{violet}"initiate flowering"` and for synchronising fruit-set in pineapples.

● It also induces `color{violet}"flowering in mango"`.

● Since ethylene regulates so many `color{violet}"physiological processes"`, it is one of the `color{violet}"most widely used PGR"` in agriculture.

● The most widely used compound as `color{violet}"source of ethylene"` is `color{Brown}"ethephon"`.

● Ethephon in an `color{violet}"aqueous solution"` is readily absorbed and transported within the plant and `color{violet}"releases ethylene slowly"`.

● Ethephon `color{violet}"hastens fruit ripening"` in tomatoes and apples and `color{violet}"accelerates abscission"` in flowers and fruits (thinning of cotton, cherry, walnut).

● It promotes `color{violet}"female flowers in cucumbers"` thereby increasing the yield.


● `color{Brown}"Abscisic acid (ABA)"` was discovered for its role in `color{violet}"regulating abscission and dormancy"`.

● But like other PGRs, it also has other `color{violet}"wide ranging effects"` on plant growth and development.

● It acts as a general `color{violet}"plant growth inhibitor"` and an `color{violet}"inhibitor of plant metabolism"`.

● ABA `color{violet}"inhibits seed germination"`.

● ABA stimulates the `color{violet}"closure of stomata"` in the epidermis and increases the tolerance of plants to `color{violet}"various kinds of stresses"`.

● Therefore, it is also called the `color{violet}"stress hormone"`.

● ABA plays an important role in `color{violet}"seed development, maturation"` and `color{violet}"dormancy"`.

● By inducing dormancy, ABA helps seeds to `color{violet}"withstand desiccation"` and other factors `color{violet}"unfavourable for growth"`.

● In most situations, ABA acts as an `color{violet}"antagonist to GAs"`.

● For any and every phase of `color{violet}"growth, differentiation"` and `color{violet}"development of plants"`, one or the other PGR has some role to play.

● Such roles could be `color{violet}"complimentary or antagonistic"`.

● These could be `color{violet}"individualistic or synergistic"`.

● Similarly, there are a `color{violet}"number of events"` in the life of a plant where more than one `color{violet}"PGR interact"` to affect that event, e.g., `color{violet}"dormancy in seeds"`/`color{violet}" buds, abscission, senescence"`, `color{violet}"apical dominance"`, etc.

● The role of PGR is of only one kind of `color{violet}"intrinsic control"`.

● Along with `color{violet}"genomic control"` and `color{violet}"extrinsic factors,"` they play an important role in plant growth and development.

● Many of the extrinsic factors such as `color{violet}"temperature and light"`, control plant growth and development via PGR.

● Some of such events could be: `color{violet}"vernalisation, flowering"`, `color{violet}"dormancy, seed germination"`, `color{violet}"plant movements"`, etc.