Plant Growth and Development
⭐️(1) The analysis of growth curve shows that it can be differentiated into three phases:
🔻(i) Lag phase:
It represents initial stages of growth. The rate of growth is very slow in lag phase. More time is needed for little growth in this phase.
🔻(ii) Log phase (Exponential phase):
The growth rate becomes maximum and more rapid. Physiological activities of cells are at their maximum. The log phase is also referred to as grand period of growth.
🔻(iii) Final steady state (Stationary phase) or Adult phase: When the nutrients become limiting, growth slows down, so physiological activities of cells also slows down. This phase is indicated by the maturity of growth system. The rate of growth can be measured by an increase in size or area of an organ of plant like leaf, flower, fruit etc. The rate of growth is called efficiency index.
🔻(i) Growth hormones also called phytohormones
🔻(ii) Term given by Thimann (1948),
🔻(iii) It can be defined as ‘the organic substances which are synthesized in minute quantities in one part of the plant body and transported to another part where they influence specific physiological processes’.
Growth Hormones and Growth Regulators
(i) Auxins (Gk. auxein = to grow) are weakly acidic growth hormones having an unsaturated ring structure and capable of promoting cell elongation, especially of shoots (more pronounced in decapitated shoots and shoot segments) at a concentration of less than 100 ppm which is inhibitory to the roots. Among the growth regulators, auxins were the first to be discovered.
(ii) Types of auxins: There are two major categories of auxins natural auxins and synthetic auxins:
(a) Natural auxins:
These are naturally occurring auxins in plants and therefore, regarded as phytohormones. Indole 3-acetic acid (IAA) is the best known and universal auxin. It is found in all plants and fungi.
(b) Synthetic auxins:
These are synthetic compounds which cause various physiological responses common to IAA. Some of the important synthetic auxins are 2, 4-D (2, 4-dichlorophenoxy acetic acid) is the weedicide. IBA is both natural and synthetic auxin.
(iii) Functions of auxins: Auxins control several kinds of plant growth processes. These are as follows:
(a) Cell elongation:
Auxins promote elongations and growth of stems and roots and enlargement of many fruits by stimulating elongation of cells in all directions.
(b) Apical dominance:
In many plants, the apical bud grows and the lower axillary buds are suppressed. Removal of apical bud results in the growth of lower buds. The auxin (IAA) of the terminal bud inhibits the growth of lateral buds. This phenomenon is known as apical dominance.
(c) Weed control:
Weeds are undesirable in a field with a crop. By the spray of 2, 4-D, broad-leaved weeds can be destroyed but 2, 4-D does not affect mature monocotyledonous plants.
(d) Root differentiation
(e) Control of lodging
Parthenocarpy can be induced by application of IAA in a paste form to the stigma of a flower or by spraying the flowers with a dilute solution of IAA.
(i) Gibberellins are weakly acidic hormones having gibbane ring structure which cause cell elongation of intact plants in general and increased internodal length of genetically dwarfed plants (i.e., corn, pea) in particular.
(ii) Functions of gibberellin
🔻(a) Stem elongation: The gibberellins induce elongation of the internodes.
🔻(b) Leaf expansion: In many plants leaves become broader and elongated when treated with gibberellic acid.
🔻(c) Reversal of dwarfism: One of the most striking effects of gibberellins is the elongation of genetic dwarf (mutant) varieties of plants like corn and pea.
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