Plant Growth and Development
PLANT
GROWTH AND ITS PHASES
GROWTH
Growth is an irreversible permanent increase in the size of
an organ, its parts or even an individual cell Growth is a characteristic
feature of a living organism. and it is accompanied by metabolic processes. For
example, the digestion of food produces nutrients that are then given to the
cells of the body, which use these nutrients to produce energy. Now, this
energy will help in normal physiological function, growth. and the development
of the organism.
Indeterminate
Plant Growth
Plants have the capacity for unlimited growth for their
whole life which makes them unique. This ability of plants is due to the presence
of meristem at various locations inside their body. The meristematic cells can
divide continuously throughout the life of the plant. Due to this, new cells
are always being added to the body of the plant, and such growth is called an
open form of growth.
Various types of meristem are present in the plant body like
apical meristem, lateral meristem, and intercalary meristem. Lateral meristem
causes an increase in the girth of the plant, whereas apical meristem helps in
increasing the height of the plant. Vascular cambium and cork cambium appear
later in the life of the plant, which also causes an increase in girth. This
form of growth in which the girth of plants increases is called the secondary
growth.
Important
→Primary growth occurs in the earlier life stage of the
plant whereas secondary growth occurs in the later stage of plant life. The
stem of monocot plants does not have meristems so there is no secondary growth
in them. This is evident by observing the thin or herbaceous stems of monocot plants.
Phases of
Growth
Phases of growth are divided into three phases Meristematic
phase or (Formative Phase), Elongation phase, and Maturation phase or (Phase of
differentiation).
Meristematic
Phase (Formative Phase)
It is also called the
phase of cell division. It occurs in the areas where meristematic cells are
present and it is the first phase of growth in plants. It generally occurs at
the root and shoot tip of the plants and other areas also where meristematic
tissues are present. At this phase of growth, cells have dense protoplasm,
contain a large nucleus, have a high respiration rate, and the cell wall is
thin and made up of cellulose with abundant plasmodesmata connections so that
cells can communicate with each other. Cells of this phase divide fast by
mitosis.
Phase of
Elongation
It is the second phase of growth. The cells formed in the
formative phase undergo enlargement. Cells found in this zone have increased
vacuolation, deposition of materials on the cell wall, and increase in cell size.
Phase of
Maturation (Phase of Differentiation)
This is the third and last phase of growth. After the phase
of elongation, the cells undergo specialisation to perform special functions.
In this phase, there is structural and physiological changes occur in the cells.
The cells are the largest and the walls are thickest in this phase.
Growth
Rates
An increase in growth per unit time is called a growth rate. The growth rate can be measured mathematically. The growth rate can be arithmetic or geometric.
Arithmetic
growth
Arithmetic growth is a type of growth rate in which cell
division occurs by mitosis and only one daughter cell divides continuously
whereas other cells undergo differentiation and become mature and permanent
Here growth occurs at a constant rate from the start and it progresses
arithmetically. It occurs in the case of root elongation where elongation
occurs at a constant rate. The graph for arithmetic growth is plotted by taking
time at the X-axis and height of the plant organ at the Y-axis, and the linear
curve Is obtained. The mathematical expression of arithmetic growth is:
Lt= Lo +rt
Where. Lt = length at time t
Lo
= length at beginning.
r = growth rate or elongation per unit time.
Geometric
growth
In Geometrical growth, growth and division occur in every
cell with all the daughters growing and dividing again. This growth occurs in
three phases:
(1) Lag phase: It
is the first phase of growth where the rate of growth is very slow.
(2) Log phase: In
this phase, growth progress is rapid and reaches its maximum. This growth phase
is called log phase or exponential phase.
(3) Stationary phase:
The last phase is called the stationary phase, which happens due to limited
food, space, and accumulation of toxins which slows down the growth.
The slope of the graph in geometric growth is sigmoidal or
S-shaped. The mathematical expression of geometric growth is:
W1 = Woert
Where, W1 = final size (weight, height, number, etc.)
Wo = initial size at the beginning of
the period
r =
growth rate
t = time
of growth
e =
base of natural logarithms
Absolute and relative
growth rate
The 'r' expressed in the above growth equations is the
relative growth rate. This ability of plants to produce new plant material is
called the efficiency index. The increase in the plant material is deduced by
looking at the initial size of the plant i.e., Wo.
Quantitative comparison between the growths of living system
can be made using two ways:
(1) First is the absolute growth rate, which is the
measurement and comparison of total growth per unit time.
(2) The second is the relative growth rate, which is defined
as the growth per unit time per unit of initial growth or, the relative growth
rate is the growth rate per unit of initial growth.
Absolute growth rate
= Growth/ time
Relative growth rate
= Growth rate/initial size.
Caution
Students usually consider absolute growth rate and relative
growth rate as the same. On one hand. where absolute growth rate is the growth
per unit time, on the other hand, the relative growth rate Is the growth rate
per unit initial size of the plant.
Conditions
for Growth
The necessary requirements for the growth of plants are
water, oxygen, nutrients, temperature, light, gravity, and plant regulators.
Water maintains the turgidity of growing cells and provides a medium for
enzymatic activities.
Oxygen is essential for aerobic respiration and thus release
of energy. This energy is utilised to perform various biosynthetic activities
and it is essential for the growth and development of plants. The plants cannot
grow in water logging conditions because their growth of roots is inhibited due
to the reduced availability of oxygen to roots.
Nutrients are required for the synthesis of protoplasm and
for the production of energy.
Temperature is also essential because only at optimum
temperature plants show maximum growth.
Light is required for the synthesis of food by photosynthesis,
whereas gravity determines the direction of orientation of the main root stem
and branches.
Important
The factors which affect the growth of plants are very
important and complex topics to study. Since there are a lot of factors that
can affect the growth of a particular plant and there are chances that at one
particular time, more than one factor might be affecting the growth we cannot
consider only one factor to be responsible for the growth of plant at a time.
DIFFERENTIATION,
DEDIFFERENTIATION AND REDIFFERENTIATION
The phenomenon where cells undergo permanent changes in
their structure, biochemistry, size, physiology of cell wall, and protoplasm
contents. thus enabling the cell to perform a specific function is called
differentiation. These differentiated cells form primary permanent tissue which
is formed from the primary meristem. It occurs in cells derived from the root
and shoots apical meristems.
Examples: Formation of tracheary elements, chlorenchyma, etc.
Dedifferentiation is the phenomenon where certain
differentiated cells regain the ability to divide and become meristematic again.
These cells start dividing again and add new cells. Example: Formation of
meristems-interfascicular cambium and cork cambium from fully differentiated
parenchymatous cells. These are formed from primary permanent tissue hence these
are called secondary meristem.
Redifferentiation is the process where dedifferentiated
cells (secondary meristem) again lose their ability to divide and become
permanent cells. These are called secondary permanent Examples: Secondary
phloem, secondary xylem, etc.
Caution
Students usually get confused between differentiation and
re-differentiation. These are different but in both cases cells lose the
ability to divide and produce new cells.
PLANT
DEVELOPMENT AND GROWTH REGULATORS
DEVELOPMENT
All changes that an organism goes through during its life
cycle from seed germination to senescence are called development. It is also
applicable to tissue and organs. Development can be indicated by sequence in
plants as:
Plants generate various structures by following distinct
paths in response to their environment or phases of life. This ability is known
as plasticity, as shown in cotton, coriander, and larkspur heterophylly. In
such plants, the leaves of the juvenile plant differ from those of the adult plant
in shape. It is called developmental heterophylly.
In buttercup, however, the difference in the shape of leaves
generated in the air and those produced in the water reflects heterophyllous
development as a function of the environment. This is called environmental
heterophylly. As a result, growth, differentiation, and development are all
tightly linked events in a plant's existence. Development is defined as the sum
of growth and differentiation in a broad sense. Plant development (both growth
and differentiation) is influenced by both internal and extrinsic influences.
The former covers both intracellular (genetic) and intercellular (chemicals
like plant growth regulators), whereas the latter includes light, temperature,
water, oxygen, nutrition, etc.
Plant
Growth Regulators
Characteristics
PGRS (plant growth regulators) are small, simple molecules
that come in a variety of chemical configurations. They could be
indole-3-acetic acid (IAA), adenine derivatives (N6-furfuryl amino purine,
kinetin), carotenoids (abscisic acid, ABA), terpenes (gibberellic acid, GA3).
or gases (ethylene, C2H4). In the literature, plant
growth regulators are referred to as plant growth chemicals, plant hormones, or
phytohormones. Based on their roles in a living plant body, PGRs can be
separated into two classes, plant growth promoters and plant growth inhibitors.
Plant growth promoters perform growth promoting activities like cell division,
cell expansion, pattern creation, tropic growth, flowering, fruiting, and seed
formation. Auxins, gibberellins, and cytokinins are examples of plant growth
promoters. Plant responses to wounds and biotic and abiotic stresses are
influenced by the PGRs of the other group. Plant growth inhibitors generally
induce dormancy and abscission. Abscisic acid is purely a plant growth
inhibitor. Ethylene, a gaseous PGR, could belong to either of these classes,
but it is primarily a growth inhibitor.
Discovery
of Plant Growth Regulators
Interestingly, each of the five major classes of PGRs was
discovered by chance. All of this began with Charles Darwin and his son Francis
Darwin's discovery that the coleoptiles of canary grass bend towards the light
when exposed to unilateral illumination (phototropism). Following a series of
trials, it was determined that the coleoptile's tip was the source of
transmittable effect that caused the entire coleoptile to bend. F.W. Went
isolated auxin from the tips of coleoptiles of oat seedlings.
The fungal pathogen Gibberella fujikuroi causes the
'bakanae' (foolish seedling) disease of rice seedlings. When rice seedlings
were treated with sterile fungal filtrates, symptoms of the disease appeared.
according to E. Kurosawa (1926). That active substance was later identified as
gibberellic acid.
F. Skoog and his co-workers found that callus from
internodal segments of tobacco proliferate only when in addition to auxin, the
nutritive medium is provided with vascular tissue extracts, yeast extract, coconut
milk, or DNA. Miller et al later discovered and crystallised the active molecule
that promotes cytokinesis which he named kinetin.
During the mid-1960s. three independent researchers reported
the purification and chemical characterisation of three different kinds of
inhibitors: inhibitor-B, abscission II and dormin Later all three were proved
to be chemically identical It was named Abscisic Acid (ABA).
Cousins confirmed the release of a volatile substance from
ripened oranges that hastened the ripening of stored unripened bananas. Later
this volatile substance was identified as ethylene, a gaseous plant growth
regulator/hormone.
Important
Phytohormones or plant hormones are examples of plant growth
regulators. Technically a plant hormone is a chemical substance other than a
nutrient produced naturally in plants which may be translocated to another
region for regulating one or more physiological reactions when present in low
concentration.
Physiological
Effects of Plant Growth Regulators
Auxins
Auxins (from the Greek 'auxein' to grow) were discovered in
human urine for the first time. The term 'auxin' refers to indole-3-acetic acid
(AA) as well as other natural and synthetic chemicals with growth regulating
capabilities. They are produced primarily by the developing apices of stems and
roots, from which they travel to their action zones.
Plant auxins such as IAA and indole butyric acid (IBA) have
been identified. These are natural auxins. Synthetic auxins include NAA
(naphthalene acetic acid) and 2,4-D (2,4-dichlorophenoxyacetic acid). All of
these auxins have a long history of application in agricultural and
horticultural practices.
Functions:
(1) They aid in the rooting of stem cuttings, which is a
common method of plant multiplication.
(2) Auxins encourage flowering in plants, such as
pineapples. They help to prevent early fruit and leaf drop but encourage the
abscission of older mature leaves and fruits.
(3) In most higher plants, apical dominance occurs when the
developing apical bud controls the growth of the lateral (axillary) buds.
(4) The growth of lateral buds is common after decapitation
(removal of the shoot tips). It is commonly used in tea plantations and
hedge-making.
(5) Auxins can also induce parthenocarpy in plants. such as
tomatoes.
(6) Herbicides are commonly used with them. The herbicide
2,4-D, which is commonly used to control dicotyledonous weeds, has no effect on
mature monocotyledonous plants. Gardeners use it to prepare weed-free lawns.
(7) Auxin also aids cell division and governs xylem
differentiation.
Important
Auxins are weakly acidic growth hormones having an
unsaturated ring structure and capable of promoting cell elongation, especially
of shoots (more pronounced in decapitated shoot 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.
Gibberellins
Gibberellins are another type of PGR that promotes growth.
Gibberellins have been found in a wide variety of species, including fungi and
higher plants. GA1. GA2. GA3. and so on are
the designations. Gibberellic acid (GA3). on the other hand, was one
of the first gibberellins to be discovered. It (GA3) is one of the
most intensively studied gibberellins. All GAs are acidic in nature. Plants
have a wide range of physiological reactions when we treat them with
gibberellins.
Functions:
(1) Gibberellins are used to increase the length of stalk of
grape stems because of their potential to promote an increase in axis length.
(2) Gibberellins cause fruits to elongate and improve their
shape, such as apples.
(3) Gibberellins also delay senescence. As a result. the
fruits can be left on the tree for a longer period, to extend the market
period.
(4) In the brewing industries, GA3 Is used to
speed up the malting process.
(5) Sugarcane stems store carbohydrates in the form of
sugar. Spraying gibberellins on sugarcane crops improves stem internode length,
resulting in the yield by as much as 20 tonnes per acre. Spraying GAs on juvenile
conifers shortens the maturation process, resulting in earlier seed production.
(6) Gibberellins also promote bolting process (internode
elongation just prior to flowering) in beets, cabbages, and many other plants
with rosette habits. It can replace cold treatment.
Important
All GAs are acidic and GA3 was the first gibberellin to be
discovered. Gibberellins are weakly acidic growth hormones having gibbane ring
structure which causes cell elongation of intact plants in general and
increased internodal length of genetically dwarfed plants. For example: Pea,
Corn.
Cytokinins
Cytokinins were identified as kinetin (a modified form of
adenine, a purine) in autoclaved herring sperm DNA and have unique effects on
cytokinesis. Kinetin is not found in plants naturally. Zeatin was isolated from
corn kernels and coconut milk during a search for natural compounds with
cytokinin-like properties.
Several naturally occurring cytokinins, as well as several
synthesised compounds with cell division promoting action, have been discovered
since the discovery of zeatin.
Natural cytokinins are produced in areas with fast cell
division, such as root spices, growing shoot buds, and immature fruits. It
helps with the formation of new leaves, chloroplasts in leaves, lateral shoot
growth, and adventitious shoot formation. Cytokinins overcome the apical
dominance. It promotes nutrient mobilisation, which helps to delay leaf
senescence. Thus, its function is antagonistic to auxin which promotes apical
dominance.
Ethylene
Ethylene is a gaseous PGR It is produced in huge quantities
by senescent tissues and ripening fruits. The effects of ethylene on plants
include horizontal seedling growth, axis swelling, and apical hook development
in dicot seedlings. Ethylene causes senescence and abscission in plant organs,
especially leaves and flowers. Ethylene is highly effective in fruit ripening.
It hastens the ripening of the fruit by increasing the rate of respiration.
Ethylene breaks seed and bud dormancy, initiates germination
in peanut seed, potato tuber sprouting. Ethylene promotes rapid
internode/petiole elongation in deep water rice plants. It aids in keeping the
leaves and upper sections of the shoot above water. Ethylene also encourages
root growth and formation of root hairs, allowing plants to expand their
absorption surface. In pineapples, ethylene is used to start flowering and
synchronize the fruit set It also induces flowering in mango.
Ethylene is one of the most extensively used PGRS in
agriculture since it governs so many physiological processes. The compound
ethephon is the most commonly used source of ethylene. In an aqueous solution,
ethephon is quickly absorbed and carried throughout the plant, where it slowly
releases ethylene. Ethephon stimulates abscission in flowers and fruits and
hastens fruit ripening in tomatoes and apples (thinning of cotton, cherry,
walnut). It promotes female flowers in cucumbers and thus increases yield.
Functions:
(1) Ethylene helps in horizontal seedling growth, axis
swelling, and apical hook development in dicot seedlings.
(2) This hormone is highly effective in fruit ripening. It
hastens the ripening of the fruit by increasing the rate of respiration.
(3) Ethylene breaks seed and bud dormancy.
(4) It promotes rapid internode /petiole elongation in deep
water rice plants.
(5) Ethylene also encourages root growth and formation of
root hairs, allowing plants to expand their absorption surface.
Abscisic
acid
Abscisic acid (ABA) is known for its role in controlling abscission
and dormancy, as previously stated. However, like other PGRs, it has a variety
of additional effects on plant growth and development It serves as a general
plant growth inhibitor as well as a metabolic inhibitor. Seed germination is
inhibited by ABA. ABA enhances plant tolerance to environmental stress by
stimulating the closure of stomata in the epidermis. As a result, it is also
known as the 'stress hormone. Seed development, maturity, and dormancy are all
aided by ABA.
ABA induces dormancy in seeds, allowing them to tolerate
desiccation and other growth-inhibiting conditions. In the majority of cases,
ABA is antagonistic to GA. To summarise, one or more PGRs play a role in every
phase of plant growth, differentiation, and development Complementary or
antagonistic functions are possible. These could be synergistic or
individualistic. Similarly, there are a variety of events in a plant's life
where many PGRs interact to influence that event, such as dormancy in
seeds/buds, abscission, senescence, apical dominance, and so on.
Keep in mind that PGR is simply one type of intrinsic
control Along with genomic control and extrinsic factors, they play a key role
in plant growth and development. Many of the extrinsic factors like temperature
and light control plant growth and development via PGR. Some examples of such
incidents include vernalisation flowering, dormancy, seed germination, plant
movements, etc.
Important
ABA is known as the stress hormone as it enhances plant
tolerance to diverse stresses like drought flood, salinity. climate stresses,
etc.