First step in the process of plant growth is seed germination.

Seed germinates when favourable conditions for growth exist in the environment.
Absence of such favourable conditions the seeds do not germinate and goes into a period of suspended growth or rest.
Development is the sum of two processes: growth and differentiation.
Development of a mature plant from a zygote (fertilised egg) follow a precise and highly ordered succession of events and 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.
Growth is accompanied by metabolic processes (both anabolic and catabolic), that occur at the expense of energy. Example - expansion of a leaf is growth.

Plant Growth Generally is Indeterminate

Plants retain the capacity for unlimited growth throughout their life due to the presence of meristems at certain locations in their body.
Cells loses the capacity to divide and such cells make up the plant body.
Open form of growth - wherein new cells are always being added to the plant body by the activity of the meristem.
Root apical meristem and the shoot apical meristem responsible for primary growth of the plants and principally contribute to the elongation of the plants along their axis.
Secondary growth - In dicotyledonous plants and gymnosperms, the lateral meristems, vascular cambium and cork-cambium appear later in life cause the increase in the girth of the organs in which they are active.

Growth is Measurable

Growth, at a cellular level measure by increase in the amount of protoplasm.

Growth is measured by a variety of parameters some of which are: increase in fresh weight, dry weight, length, area, volume and cell number.
Single maize root apical mersitem can give rise to more than 17,500 new cells per hour.
Cells in a watermelon may increase in size by upto 3,50,000 times.
Growth of a pollen tube is measured in terms of its length, an increase in surface area denotes the growth in a dorsiventral leaf.

Phases of Growth

Period of growth is generally divided into three phases :-

i. Meristematic ii. Elongation iii. Maturation

i. Meristematic - Constantly dividing cells, both at the root apex and the shoot apex, represent the meristematic phase of growth.

Cells in this region are rich in protoplasm, possess large conspicuous nuclei.
Cell walls are primary in nature, thin and cellulosic with abundant plasmodesmatal connections.

ii. Elongation - cells proximal (just next, away from the tip) to the meristematic zone represent the phase of .

Increased vacuolation, cell enlargement and new cell wall deposition are the characteristics of the cells in this phase.

Further

iii. Maturation - away from the apex, i.e., more proximal to the phase of elongation, lies the portion of axis which is undergoing the phase of maturation. The

Cells attain their maximal size in terms of wall thickening and protoplasmic modifications.

Growth Rates

• Increased growth per unit time is termed as growth rate.

- Rate of growth can be expressed mathematically.

• It is of two types :- 1. Arithmetic 2. Geometrical

1. Arithmetic growth - following mitotic cell division, only one daughter cell continues to divide while the other differentiates and matures.

Simplest expression of arithmetic growth is exemplified by a root elongating at a constant rate.
Plotting the length of the organ against time, a linear curve is obtained. Mathematically, it is expressed as

2. Geometrical growth - initial growth is slow (lag phase), and it increases rapidly thereafter – at an exponential rate (log or exponential phase). Here, both the progeny

Cells following mitotic cell division retain the ability to divide and continue to do so with limited nutrient supply, the growth slows down leading to a stationary phase.
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.

r is the relative growth rate and is also the measure of the ability of the plant to produce new

plant material, referred to as efficiency index.

• Quantitative comparisons between the growth of living system can also be made in two ways :

measurement and the comparison of total growth per unit time is called the absolute growth rate.
The growth of the given system per unit time expressed on a common basis, e.g., per unit initial parameter is called the relative growth rate.

Conditions for Growth

Plant cells grow in size by cell enlargement which in turn requires water and Turgidity of cells helps in extension growth.
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.
Optimum temperature range best suited for its growth.

Light and gravity also affect certain phases/stages of growth.

DIFFERENTIATION, DEDIFFERENTIATION AND REDIFFERENTIATION

1. Differentiation - Cells derived from root apical and shoot-apical meristems and cambium differentiate and mature to perform specific function and leading to maturation.

Cells undergo few to major structural changes both in their cell walls and protoplasm.
For example, to form a tracheary element, the cells would lose their protoplasm.

They also develop a very strong, elastic, lignocellulosic secondary cell walls, to carry water to long distances even under extreme tension. Try to correlate the various anatomical features you encounter in plants to the functions they perform. Plants show another interesting phenomenon. The

2. Dedifferentiation - living differentiated cells, that by now have lost the capacity to divide can regain the capacity of division under certain conditions.

For example, formation of meristems – interfascicular cambium and cork cambium from fully differentiated parenchyma cells.

3. Redifferentiated - such meristems/tissues are able to divide and produce cells that once again lose the capacity to divide but mature to perform specific functions.

DEVELOPMENT

Development is a term that includes all changes that an organism goes through during its life cycle from germination of the seed to senescence.

Plants follow different pathways in response to environment or phases of life to form different kinds of structures and this ability is called plasticity.

Ex. heterophylly in cotton, coriander and larkspur.
Leaves of the juvenile plant are different in shape from those in mature plants.
difference in shapes of leaves produced in air and those produced in water in buttercup also represent the heterophyllous development due to environment.
Growth, differentiation and development are very closely related events in the life of a plant.
Development is considered as the sum of growth and differentiation.
Development in plants (i.e., both growth and differentiation) is under the control of

Intrinsic factors - includes both intracellular (genetic) or intercellular factors (chemicals such as plant growth regulators)
Extrinsic factors - includes light, temperature, water, oxygen, nutrition, etc.

Plant Growth Regulators Characteristics

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)
Gases (ethylene, C2 H4 ).
Plant growth regulators are variously described as plant growth substances, plant hormones or phytohormones.
PGRs can be broadly divided into two groups based on their functions in a living plant body.

1. = PGRs are involved in growth promoting activities, such as cell division, cell enlargement, pattern formation, tropic growth, flowering, fruiting and seed formation.

These are also called plant growth promoters, e.g., auxins, gibberellins and cytokinins.

2. = PGRs involved in various growth inhibiting activities such as dormancy and abscission. The PGR abscisic acid belongs to this group.

☆ Gaseous PGR, ethylene, could fit either of the groups, but it is largely an inhibitor of growth activities.

▪ The Discovery of Plant Growth Regulators

☆ Interestingly, the discovery of each of the five major groups of PGRs have been accidental.

• Auxin Discovery

Started with the observation of Charles Darwin and his son Francis Darwin when they observed that the coleoptiles of canary grass responded to unilateral illumination by growing towards the light source (phototropism).
After a series of experiments, it was concluded that the tip of coleoptile was the site of transmittable influence that caused the bending of the entire coleoptile.
Auxin was isolated by F.W. Went from tips of coleoptiles of oat seedlings.(NEET 2014,2015)

• Gibberlin Discovery

‘bakanae’ (foolish seedling) disease of rice seedlings, was caused by a fungal pathogen Gibberella fujikuroi. E. Kurosawa (1926) reported the appearance of symptoms of the disease in rice seedlings when they were treated with sterile filtrates of the fungus.
Active substances were later identified as gibberellic acid.

• Cytokines Discovery

F. Skoog and his co-workers observed that from the internodal segments of tobacco stems the callus (a mass of undifferentiated cells) proliferated only if, in addition to auxins the nutrients medium was supplemented with one of the following: extracts of vascular tissues, yeast extract, coconut milk or DNA.
Miller et al. (1955), later identified and crystallised the cytokinesis promoting active substance that they termed kinetin.

• Abscisic Acid Discovery

During mid-1960s, three independent researches reported the purification and chemical characterisation of three different kinds of inhibitors: inhibitor-B, abscission II and dormin.
Later all the three were proved to be chemically identical. It was named abscisic acid (ABA).

• Ethylene Discovery

H.H. Cousins (1910) 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 PGR.

Physiological Effects of Plant Growth Regulators

1. Auxin

Auxins (from Greek ‘auxein’ : to grow) was first isolated from human urine.
Term ‘auxin’ is applied to the indole-3-acetic acid (IAA), and to other natural and synthetic compounds having certain growth regulating properties.
Produced by the growing apices of the stems and roots, from where they migrate to the regions of their action.
Auxins like IAA and indole butyric acid (IBA) have been isolated from plants.
NAA (naphthalene acetic acid) and 2, 4-D (2, 4- dichlorophenoxyacetic) are synthetic auxins. (Aipmt 2009)
Used extensively in agricultural and horticultural practices.
Initiate rooting in stem cuttings, an application widely used for plant propagation.

Auxins promote flowering e.g. in pineapples. (NEET 2019)

Help to prevent fruit and leaf drop at early stages but promote the abscission of older mature leaves and fruits. (NEET 2017)

In most higher plants, the growing apical bud inhibits the growth of the lateral (axillary) buds, a phenomenon called apical dominance and removal of shoot tips (decapitation) usually results in the growth of lateral buds.
Auxins also induce parthenocarpy, e.g., in tomatoes.
Widely used as herbicides. 2, 4-D, widely used to kill dicotyledonous weeds, does not affect mature monocotyledonous plants.
Used to prepare weed-free lawns by gardeners.

Auxin also controls xylem differentiation and helps in cell division.

2. Gibberellins

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.
Gibberellic acid (GA3) was one of the first gibberellins to be discovered and remains the most intensively studied form.
All GAs are acidic.
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 and thus fruits can be left on the tree longer so as to extend the market period.
GA³ is used to speed up the malting process in brewing industry.

Sugarcane stores carbohydrate as sugar in their stem gibberellins increases the length of the stem, thus increasing the yield by as much as 20 tonnes per acre.
Spraying juvenile conifers with GAs hastens the maturity period, thus leading to early seed production.
Gibberellins also promotes bolting (internode elongation just prior to flowering) in beet, cabbages and many plants with rosette habit.

3. Cytokinins

Discovered as kinetin (a modified form of adenine, a purine) from the autoclaved herring sperm DNA.
Kinetin does not occur naturally in plants.
Natural substances with cytokinin-like activities led to the isolation of zeatin from corn-kernels and coconut milk.
Discovery of zeatin, several naturally occurring cytokinins, and some synthetic compounds with cell division promoting activity, have been identified.
Natural cytokinins are synthesised in regions where rapid cell division occurs, for example, root apices, developing shoot buds, young fruits etc.
It produce leaves, chloroplasts in leaves, lateral shoot growth and adventitious shoot formation. Cytokinins help overcome the apical dominance.
Promote nutrient mobilisation which helps in the delay of leaf senescence.

4. Ethylene

Gaseous PGR synthesised in large amounts by tissues undergoing senescence and ripening fruits.
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.
Enhances the respiration rate during ripening of the fruits and rise in rate of respiration is called respiratory climactic.
Ethylene breaks seed and bud dormancy, initiates germination in peanut seeds, sprouting of potato tubers.
Promotes rapid internode/petiole elongation in deep water rice plants.
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.
Ethylene is used to initiate flowering and for synchronising fruit-set in pineapples. (NEET 2019) Induces flowering in mango.
One of the most widely used PGR in agriculture.
Most widely used compound as source of ethylene is ethephon.
Ethephon in an aqueous solution is readily absorbed and transported within the plant and releases ethylene slowly.
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.

5. Abscisic acid (ABA)

Acts as a general plant growth inhibitor and an inhibitor of plant metabolism.
ABA inhibits seed germination.
ABA stimulates the closure of stomata and increases the tolerance of plants to various kinds of stresses. Therefore, it is also called the stress hormone. (NEET 2014)
ABA plays an important role in seed development, maturation and dormancy.
ABA helps seeds to withstand desiccation and other factors unfavourable for growth.
ABA acts as an antagonist to GAs.
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.

PHOTOPERIODISM

Plants require a periodic exposure to light to induce flowering. It is also seen that such
Plants are able to measure the duration of exposure to light.
Long day Plants - some plants require the exposure to light for a period exceeding a well defined critical duration.
Short day Plants - must be exposed to light for a period less than this critical duration before the flowering is initiated in them.
Day-neutral plants - where there is no such correlation between exposure to light duration and induction of flowering response.
This response of plants to periods of day/night is termed photoperiodism.
Interesting to note that while shoot apices modify themselves into flowering apices prior to flowering, they (i.e., shoot apices of plants) by themselves cannot percieve photoperiods.
site of perception of light/dark duration are the leaves.(NEET 2019)
Hormonal substance migrates from leaves to shoot apices for inducing flowering only when the plants are exposed to the necessary inductive photoperiod.

VERNALISATION

Plants for which flowering is either quantitatively or qualitatively dependent on exposure to low temperature. This phenomenon is termed vernalisation.
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, rye have two kinds of varieties: winter and spring varieties.
‘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.
They germinate, and over winter come out as small seedlings, resume growth in the spring, and are harvested usually around mid-summer.

Biennials are monocarpic plants that normally flower and die in the second season. Sugarbeet, cabbages, carrots are some of the common biennials.

Seed Dormancy

There are certain seeds which fail to germinate even when external conditions are favourable. Such seeds are understood to be undergoing a period of dormancy which is controlled not by external environment but are under endogenous control or conditions within the seed itself.
Impermeable and hard seed coat; presence of chemical inhibitors such as abscissic acids, phenolic acids, para-ascorbic acid; and immature embryos are some of the reasons which causes seed dormancy.
Overcome through natural means and various other man-made measures.

For example, the seed coat barrier in some seeds can be broken by mechanical abrasions using knives, sandpaper, etc. or vigorous shaking.

In nature, these abrasions are caused by microbial action, and passage through digestive tract of animals.
Effect of inhibitory substances can be removed by subjecting the seeds to chilling conditions or by application of certain chemicals like gibberellic acid and nitrates.