Summary
Growth occurs by cell division, cellelongation
and cell maturation. The first phase is lag phase, the second is log phase and
the final phase is steady state phase. The log phase is otherwise known as exponential phase. The three phases are
collectively called Grand period of growth. Plant exhibits plasticity in
development. Plant growth and development are controlled by both internal and
external factors. The internal factors are chemical substances called Plant
Growth Regulators (PGRs). The hormones are classified into five groups: Auxins,
gibberellins, cytokinins, abscisic acid and ethylene. These PGRs are
synthesized in various parts of the plant. PGRs may act synergistically or
antagonistically. The external factors affecting growth includes water,
nutrition, temperature, oxygen and light. Mechanism of flowering is controlled
by light period (photoperiodism) and temperature (vernalization). The
physiological changes on flowering with effect from relative length of light
and darkness (photoperiodism) are called photoperiodism. A bluish biliprotein
responsible for the perception of light in photophysiological process
(induction and inhibition of flowering) is called Phytochrome. Besides photoperiod certain plants require a low temperature in the earlier stages for
flowering. Many biennial and perennial plants are induced to flower by low
temperature (0oC to 5oC). This process is called vernalization and the reversal effect
of vernalization is called devernalization.
The condition of a seed when it fails to germinate even in suitable
environmental condition is called seed
dormancy. Thus, dormancy can be overcome by following methods such as
scarification, impaction, stratification, alternating temperatures and light.
Senescence refers to all collective, progressive and deteriorative processes
which ultimately lead to complete loss of organization and function. Senescence
is of four types and they are overall, top, deciduous and progressive.
Senescence is controlled by plant’s own genetic programme. Death of the plant
or its parts consequent to senescence is called Programmed Cell Death (PCD). The final stage of senescence is
abscission. Abscission is a physiological process of shedding of organs from
the parent plant body. The study of functioning of plants under adverse
environmental conditions is called stress
physiology. The environmental stress may broadly be divided into biotic and
abiotic stress. The adverse effect on plants caused by other living organisms
such as viruses, bacteria, fungi, parasites, insects, weeds is competing plants
are called biotic stress. Abiotic stress may occur due to an atmospheric
condition or soil condition.
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