PLANT GROWTH AND DEVELOPEMENT
Plant Growth and Development Class 11 PDF
INTRODUCTION
·
Development
is the sum of two processes: growth and differentiation. To begin with, it is
essential and sufficient to know that the development of a mature plant from a
zygote (fertilized egg) follow a precise and highly ordered succession of
events.
·
During
this process a complex body organisation is formed that produces roots, leaves,
branches, flowers, fruits, and seeds, and eventually they die.
GROWTH
·
Growth
can be defined as an irreversible permanent increase in size of an organ or its
parts or even of an individual cell.
·
Generally,
growth is accompanied by metabolic processes (both anabolic and catabolic),
that occur at the expense of energy. Therefore, for example, expansion of a
leaf is growth.
Plant growth and development Notes
Plant Growth Generally is
Indeterminate
·
Plant
growth is unique because plants retain the capacity for unlimited growth
throughout their life. This ability of the plants is due to the presence of
meristems at certain locations in their body.
·
The
cells of such meristems have the capacity to divide and self-perpetuate. The
product, however, soon loses the capacity to divide and such cells make up the
plant body.
·
Root
apical meristem and the shoot apical meristem. they are responsible for the
Seed coat Epicotyl hook Cotyledons Soil line Cotyledon Epicotyl Hypocotyl
Hypocotyl .
·
Germination
and seedling development in bean primary growth of the plants and principally
contribute to the elongation of the plants along their axis.
·
in
dicotyledonous plants and gymnosperms, the lateral meristems, vascular cambium
and cork-cambium appear later in life.
·
These
are the meristems that cause the increase in the girth of the organs in which
they are active. This is known as secondary growth of the plant.
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Plant Growth and Development Class 11 PDF
Phases of Growth
·
The
period of growth is generally divided into three phases, namely, meristematic,
elongation and maturation
·
The
constantly dividing cells, both at the root apex and the shoot apex, represent
the meristematic phase of growth.
·
The
cells proximal (just next, away from the tip) to the meristematic zone represent
the phase of elongation. Increased vacuolation, cell enlargement and new cell
wall deposition are the characteristics of the cells in this phase.
Growth Rates
·
The
increased growth per unit time is termed as growth rate. Thus, rate of growth
can be expressed mathematically. An organism, or a part of the organism can
produce more cells in a variety of ways.
·
The
growth rate shows an increase that may be arithmetic or geometrical.
ARITHMETIC GROWTH
·
In
arithmetic growth, following mitotic cell division, only one daughter cell
continues to divide while the other differentiates and matures.
·
The
simplest expression of arithmetic growth is exemplified by a root elongating at
a constant rate.
o
a
linear curve is obtained. Mathematically, it is expressed as Lt = L0 + rt
Lt = length at time ‘t’
L0 = length at time ‘zero’
r = growth rate / elongation per unit time.
GEOMETRICAL GROWTH.
·
In
most systems, the initial growth is slow (lag phase), and it increases rapidly
thereafter – at an exponential rate (log or exponential phase).
·
However,
with limited nutrient supply, the growth slows down leading to a stationary
phase. If we plot the parameter of growth against time, we get a typical
sigmoid or S-curve.
·
A
sigmoid curve is a characteristic of living organism growing in a natural
environment. It is typical for all cells, tissues and organs of a plant.
W1 = final size (weight, height, number etc.)
W0 = initial size at the
beginning of the period
r = growth rate
t = time of growth
e = base of natural
logarithms
Here, r is the relative growth rate
Hence, the
final size of W1 depends on the initial size, W0.
Conditions for Growth
·
The
plant cells grow in size by cell enlargement which in turn requires water. Turgidity
of cells helps in extension growth. Thus, plant growth and further development
is intimately linked to the water status of the plant.
·
Water
also provides the medium for enzymatic activities needed for growth.
·
Oxygen
helps in releasing metabolic energy essential for growth activities.
·
Nutrients
(macro and micro essential elements) are required by plants for the synthesis
of protoplasm and act as source of energy.
·
Every
plant organism has an optimum temperature range best suited for its growth. Any
deviation from this range could be detrimental to its survival. Environmental
signals such as light and gravity also affect certain phases/stages of growth.
Plant Growth and Development Class 11 PDF
RELATED POSTS:
DIFFERENIATION, DEDIFFERENTIATION AND
REDIFFERENTIATION
·
The
cells derived from root apical and shoot-apical meristems and cambium
differentiate and mature to perform specific functions. This act leading to
maturation is termed as differentiation.
o
During
differentiation, cells undergo few to major structural changes both in their
cell walls and protoplasm.
o
For
example, to form a treachery element, the cells would lose their protoplasm.
·
The
living differentiated cells, that by now have lost the capacity to divide can
regain the capacity of division under certain conditions. This phenomenon is
termed as dedifferentiation.
o
For
example, formation of meristems – interfascicular cambium and cork cambium from
fully differentiated parenchyma cells.
·
such
meristems/tissues are able to divide and produce cells that once again lose the
capacity to divide but mature to perform specific functions, i.e., get redifferentiated.
DEVELOPMENT
·
Development
is a term that includes all changes that an organism goes through during its
life cycle.
·
Plants
follow different pathways in response to environment or phases of life to form
different kinds of structures. This ability is called plasticity, e.g.,
heterophylly in cotton, coriander and larkspur.
·
Difference
in shapes of leaves produced in air and those produced in water in buttercup
also represent the heterophyllous development due to environment. This
phenomenon of heterophylly is an example of plasticity.
·
Thus,
growth, differentiation and development are very closely related events in the
life of a plant. Broadly, development is considered as the sum of growth and
differentiation.
Plant growth and development Class 11 Notes
PLANT GROWTH REGULATORS
Characteristics
·
The
plant growth regulators (PGRs) 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, GA3 ) or gases (ethylene, C2H4 ).
·
The
PGRs can be broadly divided into two groups based on their functions in a
living plant body.
·
One
group of PGRs are involved in growth promoting activities, such as cell
division, cell enlargement, tropic growth, flowering, fruiting and seed
formation.
·
These
are also called plant growth promoters, e.g., auxins, gibberellins and
cytokinins.
·
The
PGRs of the other group play an important role in plant responses to wounds and
stresses of biotic and abiotic origin.
·
They
are also involved in various growth inhibiting activities such as dormancy and
abscission.
Plant Growth and Development Class 11 PDF
Physiological Effects of Plant Growth
Regulators
Auxins
·
The
term ‘auxin’ is applied to the indole-3-acetic acid (IAA).
·
They
help to initiate rooting in stem cuttings, an application widely used for plant
propagation. Auxins promote flowering e.g. in pineapples.
·
They
help to prevent fruit and leaf drop at early stages but promote the abscission
of older mature leaves and fruits.
·
In
most higher plants, the growing apical bud inhibits the growth of the lateral
(axillary) buds, a phenomenon called apical dominance.
·
Auxins
also induce parthenocarpy, e.g., in tomatoes. They are widely used as
herbicides. 2, 4-D, widely used to kill dicotyledonous weeds, does not affect
mature monocotyledonous plants.
·
It
is used to prepare weed-free lawns by gardeners. Auxin also controls xylem
differentiation and helps in cell division.
Gibberellins
·
There
are more than 100 gibberellins reported from widely different organisms such as
fungi and higher plants. They are denoted as GA1 , GA2 , GA3 and so on.
·
However,
Gibberellic acid (GA3 ) was one of the first gibberellins to be discovered and
remains the most intensively studied form.
·
All
GAs are acidic. They produce a wide range of physiological responses in the
plants. Their 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.
·
They
also delay senescence. Thus, the fruits can be left on the tree longer so as to
extend the market period.
·
Sugarcane
stores carbohydrate as sugar in their stems. Spraying sugarcane crop with
gibberellins increases the length of the stem.
plant growth and development class 11 notes for neet
Cytokinins
·
Cytokinins
have specific effects on cytokinesis, and were discovered as kinetin (a
modified form of adenine, a purine) from the autoclaved herring sperm DNA.
·
Natural
cytokinins are synthesised in regions where rapid cell division occurs, for
example, root apices, developing shoot buds, young fruits etc.
·
It
helps to produce new leaves, chloroplasts in leaves, lateral shoot growth and
adventitious shoot formation.
·
Cytokinins
help overcome the apical dominance. They promote nutrient mobilisation which
helps in the delay of leaf senescence.
Ethylene
·
Ethylene
is a simple gaseous PGR. It is synthesized in large amounts by tissues
undergoing senescence and ripening fruits. Influences of ethylene on plants
include horizontal growth of seedlings.
·
Swelling
of the axis and apical hook formation in dicot seedlings. Ethylene promotes
senescence and abscission of plant organs especially of leaves and flowers.
·
Ethylene
is highly effective in fruit ripening. It enhances the respiration rate during
ripening of the fruits. This rise in rate of respiration is called respiratory
climactic.
·
Ethylene
breaks seed and bud dormancy, initiates germination in peanut seeds, sprouting
of potato tubers.
·
Ethylene
promotes rapid internode/petiole elongation in deep water rice plants. It helps
leaves/ upper parts of the shoot to remain above water.
·
Ethylene
also promotes root growth and root hair formation, thus helping the plants to
increase their absorption surface.
·
The
most widely used compound as source of ethylene is ethephon. Ethephon hastens
fruit ripening in tomatoes and apples and accelerates abscission in flowers and
fruits (thinning of cotton, cherry, walnut). It promotes female flowers in
cucumbers thereby increasing the yield.
Abscisic acid
·
Abscisic
acid (ABA) was discovered for its role in regulating abscission and dormancy.
But like other PGRs, it also has other wide ranging effects on plant growth and
development.
·
ABA
inhibits seed germination. ABA stimulates the closure of stomata in the
epidermis and increases the tolerance of plants to various kinds of stresses.
·
Therefore,
it is also called the stress hormone. ABA plays an important role in seed
development, maturation and dormancy.
·
There
are a number of events in the life of a plant where more than one PGR interact
to affect that event, e.g., dormancy in seeds/ buds, abscission, senescence,
apical dominance, etc.
·
Many
of the extrinsic factors such as temperature and light, control plant growth
and development via PGR. Some of such events could be: vernalisation,
flowering, dormancy, seed germination, plant movements, etc.
Plant growth and development PDF
PHOTOPERIODISM
·
Some
plants require the exposure to light for a period exceeding a well defined
critical duration, while others must be exposed to light for a period less than
this critical duration before the flowering is initiated in them. The former
group of plants are called long day plants while the latter ones are termed
short day plants.
·
There
are many plants, however, where there is no such correlation between exposure
to light duration and induction of flowering response; such plants are called
day-neutral plants.
·
It
can be said that flowering in certain plants depends not only on a combination
of light and dark exposures but also their relative durations. This response of
plants to periods of day/night is termed photoperiodism.
VERNALISATION
·
There
are plants for which flowering is either quantitatively or qualitatively
dependent on exposure to low temperature. This phenomenon is termed vernalisation.
·
It
prevents precocious reproductive development late in the growing season, and
enables the plant to have sufficient time to reach maturity.
·
Vernalisation
refers specially to the promotion of flowering by a period of low temperature.
Some important food plants, wheat, barley, They have two kinds of varieties:
winter and spring varieties.
·
The
‘spring’ variety are normally planted in the spring and come to flower and
produce grain before the end of the growing season.
·
Winter
varieties, however, if planted in spring would normally fail to flower or
produce mature grain within a span of a flowering season. Hence, they are
planted in autumn.
·
Another
example of vernalisation is seen in biennial plants. Biennials are monocarpic
plants that normally flower and die in the second season. Sugarbeet, cabbages,
carrots are some of the common biennials.