Plant Growth and Development
I. Plant Growth
Growth is an irreversible permanent increase in the size of an organ or an individual cell.
Plants show indeterminate growth. Thethree phases of growth in plants are: meristematic division, elongation and maturation.
Depending upon the environmental conditions as well as the phases in the division, the growth is oftwo types: arithmetic and geometrical.
Arithmetic growth can be plotted linearly.
In the geometrical growth, there are three phases: lag phase, log or exponential phase and stationary phase. It can be plotted as S-shaped curve.
Embryonic development shows mixed growth pattern.
The first step in the process of plant growth is seed germination.
Germination can be broadly classified into epigeal and hypogealon the basis of elongation of hypocotyl and epicotyl, respectively.
Mangrove plants germinate via a special type of germination known as viviparousgermination.
All kinds of germination give rise to seedlings.
A seed does not germinate as soon as it is formed. It remains dormant. The growth remains suspended either due to exogenous control or endogenous control.
If the growth is suspended due to exogenous control, then a seed is said to be quiescent, while if the growth is suspended due to endogenous control, then a seed is said to be dormant.
A seed remains dormant until the favourable conditions of its germination arrive.
Seed dormancy can be broken or its duration can be reduced by mechanical scarification of the seed coat, chemical scarification of the seed coat or by changing environmental conditions.
Once the favourable conditions like water, oxygen and appropriate temperature for germination arrive, it starts germinating.
The first step towards the germination of seed is imbibition.The metabolic activities, like respiration, of seeds are resumed after water absorption.
Once it germinates, a plant undergoes changes through growth, maturation, flowering and senescence.
Rate of growth occurs in two ways; namely, absolute and relative.
Differentiationis another sign of growth by which meristematic cells mature into specialised permanent cells.
However, under certain situations, they come back to their original condition and regain the capacity to divide. This is known as dedifferentiation. When the dedifferentiated tissues differentiate again, it is known as redifferentiation.
Under favourable conditions, parenchyma cells can be transformed into another cell type by the process called transdifferentiation.
Growth and differentiation in plants lead to its development.
Sometimes different environmental conditions and other factors drive the plant to develop unusual features. This ability is called plasticity.
Growth and development in plants is regulated by plant growth regulators.
II. Plant Growth Regulators
Plant Growth Regulators are the chemicals which regulate growth.
When they are produced in the plants under natural conditions, they are known as phytohormones.
There are five main plant growth regulators; namely, auxin, gibberellin, cytokinin, ethylene and abscisic acid (ABA).
The response of plants to periods of day/ night is called as photoperiodism. There are three types of plants bases on the duration of sunlight required by them to induce flowering: long day plants, short day plants and day neutral plants.
Vernalisation is the phenomenon in which plants flower in spring by exposure to a prolonged cold winter.