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
o Meristematic.
o Elongation.
o 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:
o Indole compounds
(indole-3-acetic acid, IAA);
o adenine derivatives
(N6-furfurylamino purine, kinetin)
o derivatives of
carotenoids (abscisic acid,ABA)
o terpenes (gibberellic
acid, GA2)
o 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.
Discovery of plant
growth regulators :
• Auxin was isolated by
F.W. Went from tips of oat seedlings.
• The ‘bakane’ (foolish
seedling) a disease of rice seedlings, was caused by a fungal pathogen Gibberalla
fujikuroi.
• E. Kurosawa
reported the appearance of the symptom of the disease in uninfected rice
seedlings when treated with sterile filtrate of the fungus. The active
substance was later identified as Gibberellic acid.
• Skoog and Miller
identified and crystallized the cytokinesis promoting active substance that
they termed as kinetin.
• During mid 1960s three
different kinds of inhibitors purified, i.e. inhibitor-B abscission II and
dormin. Later all the three proved to be chemically identical and named as
Abscisic acid (ABA).
• Cousinsdiscovered a
gaseous PGR called ethylene from ripened orange.
Physiological effect of
plant growth regulators :
Auxin :
• The term auxin is
applied to indole-3-acetic acid
• Generally produced by
growing apices of the stems and roots.
• IAA and IBA have been
isolated from plants.
• NAA and 2, 4-D (2,
4-dichlorophenoxyacetic acid) are synthetic auxin.
• Promote rooting in stem
cutting.
• Promote flowering.
• Inhibit fruit and leaf
drop at early stages.
• Promote abscission of
older mature leaves and fruits.
• The growing apical bud
inhibit the growth of lateral bud, the phenomenon is called apical dominance.
• Auxin induces
parthenocarpy.
• Used as herbicides.
• Controls xylem
differentiation.
• Promote cell division.
Gibberellins
:
• Ability to cause an
increase in length of axis is used to increase the length of grapes stalks.
• Gibberellins cause
fruits like apple to elongate and improve its shape.
• Delay senescence
• GA3 is used to speed up
the malting process in brewing industry.
• Gibberellins promote to
increase length of stem in sugar cane.
• Promote early seed
production.
• Promote bolting
(internodes elongation) in beet, cabbages.
Cytokinins
:
• Cytokinins have specific
effects on cytokinesis.
• Zeatin isolated from
corn-kernels and coconut milk.
• Promote cell division.
• Help to produce new
leaves, chloroplast in leaves, lateral shoot growth
• Promote formation of adventitious
shoot.
• Cytokinins help to
overcome apical dominance.
• Promote nutrient
mobilization.
• Delay senescence.
Ethylene :
• Ethylene is a simple
gaseous PGR.
• Synthesized in the
tissue undergoing senescence and ripening fruits.
• Promote horizontal
growth of seedling.
• Promote swelling of axis
and apical hook formation in dicot seedlings.
• Promote senescence and
abscission of plant organs like leaf and flower.
• Increase rate of
respiration during ripening of fruits, called respiratory climactic.
• Breaks seed and bud
dormancy.
• Initiate germination.
• Promote rapid internodes
elongation.
• Promote root growth and
root hair formation.
• Used to initiate
flowering and for synchronizing fruit-set.
• Induce flowering in mango.
• The source of ethylene
is ethephon.
• Promote female flower in
cucumbers thereby increasing the yield.
Abscisic
acid :
• Regulates abscission and
dormancy.
• Acts as general plant
growth inhibitor and an inhibitor of plant metabolism.
• Inhibit seed
germination.
• Stimulates the closure
of stomata and increases the tolerance of plants to various kinds of stresses,
hence called as stress hormone.
• Important role in seed
development, maturation and dormancy.
• Inducing dormancy, ABA
helps seeds to withstand desiccation and other factors unfavourable for growth.
• Acts as antagonist to
Gas.
PHOTOPERIODISM
:
• Some plants require a
periodic exposure to light to induce flowering.
• Response of plants in
terms of day/night in relation to flowering is called photoperiodism.
• Long day plant:
plant requires the exposure to light for a period exceeding critical period.
• Short day plant:
plant requires the exposure to light for a period less than critical period.
• Day neutral plant:
there is no such correlation between exposure to light duration and induction
of flowering response.
• The site of perception
of light/dark duration is the leaves.
VERNALISATION :
• Vernalisation: There are
plants for which flowering is either quantitatively or qualitatively dependent
on exposure to low temperature.
• It prevents precocious
reproductive development late in the growing season.
• Vernalisation refers to
the promotion of flowering by a period of low temperature.
