Growth :
- An irreversible permanent increase in size of an organ or its parts or even of an individual cell.
- Growth is accompanied by metabolic process that occurs at the expense of energy.
Plant growth is generally is indeterminate :
- Plants retain the capacity of unlimited growth throughout their life.
- This ability is due to the presence of meristems at certain locations in their body.
- The cells of such meristems have capacity to divide and self-perpetuate.
- The product eventually looses the capacity to divide and differentiated.
- Apical meristems responsible for primary growth of the plants and principally contribute to the elongation of the plants along their axis.
- The lateral meristem, vascular cambium and cork cambium appears later and responsible for the increase in the girth.
Phases of growth :
- The period of growth is generally divided into three phases
- Meristematic.
- Elongation.
- Maturation.
- Root apex and shoot apex represent the meristematic phase of growth.
- The cells of this region are rich in protoplasm, possesses large conspicuous nuclei.
- Their cell walls are primary in nature, thin and cellulosic with abundant plasmodesmatal connection.
- The cells proximal to that region are the phase of elongation.
- Increased vacuolation, cell enlargement and new cell wall deposition are the characteristic of the cells in this phase.
- Further away from the zone of elongation is the phase of maturation.
- The cells of this zone attain their maximal size in terms of wall thickening and protoplasmic modifications.
Condition of growth :
- Water, oxygen and nutrients as very essential element for growth.
- Turgidity of cells helps in extension growth.
- Water also provides the medium for enzymatic activities needed for growth.
- Oxygen helps in releasing metabolic energy essential for growth activities.
- Nutrients are required by plants for synthesis of protoplasm and act as source of energy.
Differentiation, dedifferentiation and redifferentiation :
- The cells derived from root apical and shoot apical meristems and cambium differentiate and mature to perform specific functions.
- This act of maturation is termed as differentiation.
- During differentiation major changes takes place in their cell wall and protoplasm.
- Differentiated tracheary element cells loose their protoplasm, develop a very strong, elastic lignocellulosic secondary cell walls.
- The living differentiated cells, that by now have lost the capacity to divide can regain the capacity of division under certain condition is dedifferentiation.
- Development of interfascicular cambium and cork cambium from fully differentiated parenchymatous cells is the example of dedifferentiation.
- Cells produced by the dedifferentiated tissues again loose the capacity to divide and mature to perform specific function is called redifferentiation.
PLANT GROWTH REGULATORS :
Characteristics :
- The plant growth regulators are small, simple molecules of diverse chemical composition.
- They could be:
- Indole compounds (indole-3-acetic acid, IAA);
- adenine derivatives (N6-furfurylamino purine, kinetin)
- derivatives of carotenoids (abscisic acid,ABA)
- terpenes (gibberellic acid, GA2)
- Gases (ethylene, C2H4)
- One group of PGRs are involved in growth promoting activities such as cell division, cell enlargement, pattern formation, tropic growth, flowering, fruiting and seed germination. These are called plant growth promoters, e.g. auxin, gibberellins and cytokinin.
- Another group of PGRs play important role in plant responses towards to wounds and stresses of biotic and abiotic origin. They involved in inhibitory responses like dormancy and abscission, e.g. abscisic acid.
