Plant Growth
* Growth is a permanent increase in the size of an organ or individual cell in plants.
* The presence of meristems is plants in plants helps them retain the capacity to grow throughout their life.
* Apical meristems are localised to shoot and root apex and help in the primary growth of a plant.
* Lateral meristems are present in woody plants and help in the secondary growth of a plant.
* Growth is measured y different parameters including size, area, length and number.
* The different phases of growth are zone of division, elongation and maturation.
Growth Rates
* Plant growth comprises cell division, cell elongation and cell differentiation.
* Growth is measured per unit time and it is called growth rate.
* Arithmetic growth refers to the division of one daughter cell obtained during mitosis, while the other daughter cell enlarges and differentiates.
* Geometric growth refers to the division of both daughter cells obtained during mitosis.
* The growth rate of geometric growth is called relative growth rate or ‘efficiency index’.
* Plants require specific conditions for their growth, which include water, oxygen, nutrients , temperature, light and gravity.
Plant Development
* Plant development includes both an increase in the number of cells as well as cell differentiation.
* Differentiation is the maturation of cells through structural changes, for specific functions.
* The life cycle of plants is controlled by several intrinsic and extrinsic factors.
* Dedifferentiation is the ability of plant cells to regain the capacity of division after differentiation.
* Meristems formed by dedifferentiation give rise to cells that lose the ability to divide, which is called redifferentiation.
* Plasticity is the development of different structures in plants in response to different phases of life or the environment.
Plant Growth Regulators
* Plant growth regulators (PGRs) are chemical compounds and are also called plant hormones or phytochromes.
* They can be categorised into plant growth promoters and inhibitors.
* Plant growth promoters enhance cell division, cell enlargement, flowering, fruiting and seed formation.
* Plant growth inhibitors respond to wounds and stress from biotic and abiotic factors by inhibiting growth.
Effects of Plant Growth Regulators
* Plant growth regulators are of two types- promoters and inhibitors.
* Plant growth promoters include auxins, gibberellins, cytokinins and ethylene.
* Auxins promote root formation, induce flowering, prevent premature falling of leaves and fruit and promote abscission in mature leaves and fruit.
* They are also used in parthenocarpy which is the formation of seedless fruit.
* Gibberellin promote plant growth by increasing the length of the plant body, enhancing fruit shape, delaying senescence and enhancing the maturity period.
* Cytokinins promote the growth of lateral and adventitious shoots and delay senescence.
* Ethylene promotes ripening of fruit, initiates flowering, breaks seed and bud dormancy, lengthens plant internodes and promotes root and root-hair formation.
* Plant growth inhibitors include abscisic acid. It helps plants cope with environmental stress by regulating abscission and dormancy.
Effect of Light and Temperature on Plants
* The response of plants to the relative period of day or night is called photoperiodism.
* Long-day plants need to be exposed to more than 12 hours of light to induce flowering.
* Short-day plants to be exposed to less than 12 hours of light to induce flowering.
* Day-neutral plants are independent of the duration of light exposure for flowering.
* The development of flowers in response to a period of low temperature is called vernalisation.