→ Growth is one of the most conspicuous events in any living organism. It is an irreversible increase expressed in parameters such as sizes, area, length, height, volume, cell number, etc. It conspicuously involves increased protoplasmic materials.
→ In plants, meristems are the sites of growth. Root and shoot apical meristems sometimes along with intercalary meristem, contribute to the elongation growth of the plant axis.
→ Growth is indeterminate in higher plants. Following cell division in root and shoot apical meristem cells, the growth could be arithmetic or geometrical.
→ Growth may not be and generally is not sustained at a high rate throughout the life of cell/tissue/organ/organism.
→ One can define three principal phases of growth- the lag, log, and senescent phase.
→ When a cell loses the capacity to divide it leads to differentiation. Differentiation results in the development of structures that are commensurate with the function the cells finally have to perform.
→ General principles for differentiation for cells, tissues, and organs are similar.
→ A differentiated cell may differentiate and then redifferentiate.
→ Since differentiation in plants is open, the development could also be flexible, i.e, the development is the sum of growth and differentiation. Plant exhibit plasticity in development.
→ Plant growth and development are under the control of both intrinsic and extrinsic factors.
→ Intercellular intrinsic factors are the chemical substances, called plant growth regulators (PGR).
→ There are diverse groups of PGRs in plants, principally belonging to five groups: auxins, gibberellins, cytokinins, abscisic acid, and ethylene. These PGR’s are synthesized in various parts of the plant; they control different differentiation and developmental events.
→ Any PGR has diverse physiological effects on plants. Diverse PGRs also manifest similar effects. PGRs may act synergistically or antagonistically.
→ Plant growth and development are also affected by light, temperature, nutrition, oxygen status, gravity, and such external factors.
→ Flowering in some plants is induced only when exposed to a certain duration of photoperiod. Depending on the nature of photoperiod requirements, the plants are called short-day plants, long-day plants, and day-neutral plants.
→ Certain plants also need to be exposed to low temperatures so as to hasten to flower later in life. This treatment is known as vernalization
→ Vernalisation: Vernalisation is the low-temperature requirement of some plants for flowering. The cold treatment given to shoot tips or seeds is called vernalization.
→ Photoperiodism: Flowering in certain plants depends not only on a combination of light and dark exposures but also on their relative durations. This is termed photoperiodism.
→ Short-day plants/Long-day plants: The former group of plants is called short-day plants while the later ones are termed long-day plants.
→ Stress hormone: ABA stimulates the closure of stomata in the epidermis and increases the tolerance of plants to various kinds of stresses. Therefore, it is also called the stress hormone.
→ Apical dominance: In most higher plants, the growing apical bud inhibits the growth of the lateral (axillary) buds, a phenomenon called apical dominance.
→ Dedifferentiation: Plants show another interesting phenomenon. The living differentiated cells, that by now have lost the capacity to divide can regain the capacity of division under certain conditions. This phenomenon is termed dedifferentiation.
→ Differentiation: The cells derived from root apical and shoot-apical meristems and cambium differentiate and mature to perform specific functions. This act leading to maturation is termed differentiation.
→ Absolute growth rate: Measurement and the comparison of total growth per unit time is called the absolute growth rate.
→ Relative growth rate: The growth of the given system per unit time expressed on a common basis e.g. per unit initial parameter is called the relative growth rate.
→ The open form of growth: The cell(s) of such meristems have the capacity to divide and self-perpetuate. The product, however, soon loses the capacity to divide and such cells make up the plant body. This form of growth wherein new cells are always being added to the plant body by the activity of the meristem is called the open form of growth.