Growth and its Phases

Just like any other organism, plants too grow in height and size over

time. So how does plant growth occur? Or how flowers and fruits in a

tree appear and fall periodically? All events in plants, right from the

zygote stage to the full-grown plant take place in an orderly manner.

This is called development. Development is the sum of growth and

differentiation. Let us understand plant growth and its phases in more

detail.

Plant Growth And Its Phases

Growth is the most fundamental characteristic of any living organism.

It is defined as “an irreversible, permanent increase in the size of an

organ or its parts, or even of an individual cell”. Generally, growth is

accompanied by metabolic processes (catabolic and anabolic) that

spend energy.

Browse more Topics under Plant Growth And Development

● Plant Growth Regulators

● Photoperiodism

● Vernalisation

Characteristics Of Plant Growth

Plant Growth Is Indeterminate

Plants have the unique ability to grow indefinitely throughout their life

due to the presence of ‘meristems’ in their body. Meristems have cells

that can divide and self-propagate. This is called ‘open form of

growth’ because new cells are constantly added to the plant body by

the cells in the meristem.

Meristems in the roots and shoots of plants are responsible for

‘primary growth of the plant’. These increase the height of the plant.

On the other hand, lateral meristems increase the width of the plant.

This is known as the ‘secondary growth of the plant’.

Plant Growth Is Measurable

At a cellular level, growth simply means an increase in the amount of

protoplasm. Since this is hard to measure, growth is measured in a

quantity proportional to this increase. Therefore, growth is measured

in terms of increase in cell number, area, volume, length etc.

Did you know that the cells in a watermelon can increase in size up to

about 3,50,000 times! On the other hand, a single apical meristem in

maize roots can produce about 17,500 new cells per hour! In the first

case, growth is in terms of the size of the cell whereas in the second

case, it is in terms of cell number.

Phases Of Plant Growth

Meristematic Phase

The cells in the root and shoot apex of a plant are constantly dividing.

They represent the meristematic phase of growth. The cells in these

regions have large nuclei and are rich in protoplasm and their cell

walls are thin and contain cellulose.

Elongation Phase

The cells in the zone just after the meristematic region represent the

phase of elongation. The characteristics of cells in this zone are cell

enlargement, increased vacuole formation and new cell wall

deposition.

Maturation Phase

Just close to the phase of elongation, but away from the apex lies the

phase of maturation. The cells in this region reach their maximum size

with respect to their protoplasm and cell wall thickening.

Rates Of Growth

The growth rate is defined as increased growth per unit time. An

organism or its parts can give rise to more cells in a number of ways.

The increase in growth may be arithmetic or geometrical and thus, the

rate of growth can be expressed mathematically.

Arithmetic Growth

Following mitotic cell division, only one cell continues to divide while

the other begins to differentiate. A simple example of this is the

elongation of a root at a constant rate. When you plot this increase in

length against time, you get a linear curve as shown below. This is

expressed mathematically as –

Lt = L0 + rt (where, Lt is the length at time ‘t’, L0 is the length at time

‘zero’, r is the growth rate)

Geometric Growth

Most biological systems show an initial growth phase which is slow.

This is the ‘lag phase’. This phase is followed by a period of

exponential growth and is called ‘log phase’ or ‘exponential phase’.

Here, following mitotic cell division both the daughter cells continue

to divide.

However, due to limited nutrient supply, the growth slows down

giving rise to the ‘stationary phase’. When we plot this growth, we get

a sigmoidal curve or S-curve as shown below. This growth curve is

typical for all cells, tissues, organs and is characteristic of an organism

living in a natural environment. Mathematically, it is expressed as –

W1 = W0 ert (where W1 is final size, W0 is initial size, e is base of

natural logarithm, r is the growth rate and t is the time of growth).

Note: Here, r is the growth rate and is also the measure representing

the ability of the plant to produce new plant material. This is the

‘efficiency index’.

Growth rate can also be described as absolute or relative. The absolute

growth rate is the measurement of total growth per unit time. The

relative growth rate is also the growth of a given system per unit time

but relative to another parameter like initial size, weight etc.

Conditions For Growth

● Water – Water is essential for cell enlargement, extension and

for keeping plant cells upright. It also provides the medium for

enzymatic activities which is needed for growth. Therefore,

plant growth and its phases are highly dependent on water.

● Oxygen – Metabolic energy is needed for plant growth

activities. Oxygen helps to release this metabolic energy.

● Nutrients – Macro and micronutrients are sources of energy for

plants. They are also needed to make protoplasm.

● Light – Light controls growth and its phases in plants.

● Temperature – Every plant has an optimum temperature range

suitable for its growth. Changes in this range are harmful to

plant growth.

Differentiation, Dedifferentiation And Redifferentiation

Differentiation is when the cells have stopped dividing and are

beginning to mature and perform special functions. For example, to

form tracheids (elongated cells that carry water in the xylem), the cells

lose their protoplasm. They also develop strong, elastic cell walls to

carry water across long distances.

Dedifferentiation is the phenomenon where differentiated cells that

have lost their ability to divide, regain the capacity to divide under

specific conditions. Example – fully differentiated parenchyma cells

can go back to their earlier meristem form and divide.

Redifferentiation is the phenomenon where dedifferentiated cells

divide and once again produce cells that can no longer divide but

mature to perform specific functions. Example – the meristems

obtained after dedifferentiation (described above) can divide and again

produce cells that stop dividing but go on to mature.

Just like growth, differentiation in plants is also open. This is because

cells that arise from the same meristem have different structures once

they have matured. Also, the final structure of the cells at maturity is

dependent on the location of the cell. For example, cells that are away

from the root apical meristem become root caps, whereas cells pushed

to the periphery become epidermis.

Plant Development

It is defined as all the changes that an organism goes through during

its life cycle, right from seed germination to senescence. Development

of plants (i.e. growth and differentiation) is influenced by extrinsic

factors (light, temperature, water) and intrinsic factors (genes and

plant growth regulators).

Plants respond in different ways to environment and phases of life and

give rise to different forms of structures. This ability of plants is called

‘plasticity’. Example – Leaves of a young cotton plant are different in

shape from a mature cotton plant. Also, leaves of the buttercup plant

that grow in the air have a different shape than those that grow in

water. This phenomenon of producing different forms is called

‘heterophylly’.

Solved Example For You

Question: What is the initial, slow phase of geometric growth called?

a. Elongation phase

b. Lag phase

c. Log phase

d. Exponential phase

Solution: The answer is ‘b’. In geometric growth, the initial phase of

slow growth is called ‘lag phase’.

Plant Growth Regulators

We all know that plants need light, water, oxygen and nutrition to

grow and develop. All these qualify as extrinsic factors. While

extrinsic factors are important, did you know that plant growth

depends on intrinsic factors too? They can be intracellular genes or

intercellular chemicals. These chemicals are called Plant Growth

Regulators. Let’s learn about them in more detail below.

Plant Growth Regulators

Plant Growth Regulators are defined as small, simple chemicals

produced naturally by plants to regulate their growth and

development.

Characteristics

Plant Growth Regulators can be of a diverse chemical composition

such as gases (ethylene), terpenes (gibberellic acid) or carotenoid

derivates (abscisic acid). They are also referred to as plant growth

substances, phytohormones or plant hormones. Based on their action,

they are broadly classified as follows:

● Plant Growth Promoters – They promote cell division, cell

enlargement, flowering, fruiting and seed formation. Examples

are auxins, gibberellins and cytokinins.

● Plant Growth Inhibitors – These chemicals inhibit growth and

promote dormancy and abscission in plants. An example is an

abscisic acid.

Note: Ethylene can be a promoter or an inhibitor, but is largely a Plant

Growth Inhibitor.

Browse more about Plant Growth and Development

Plant Growth and Development

● Growth and its Phases

● Vernalisation

● Photoperiodism

All plant growth regulators were discovered accidentally. Let’s take a

detailed look at each regulator and learn about it more closely:

Auxins

Discovery

Auxins were the first growth hormone to be discovered. They were

discovered due to the observations of Charles Darwin and his son,

Francis Darwin. The Darwins observed that the coleoptile (protective

sheath) in canary grass grows and bends towards the source of light.

This phenomenon is ‘phototropism’. In addition, their experiments

showed that the coleoptile tip was the site responsible for the bending.

Finally, this led to the isolation of the first auxin by F. W. Went from

the coleoptile tip of oat seedlings.

(Source: Wikimedia Commons)

Types

First isolated from human urine, auxin is a term applied to natural and

synthetic compounds that have growth regulating properties. Plants

produce natural auxins such as Indole-3-acetic acid (IAA) and Indole

butyric acid (IBA). Natural auxins are found in growing stems and

roots from where they migrate to their site of action. Naphthalene

acetic acid (NAA) and 2, 4-dichlorophenoxyacetic (2, 4-D) are

examples of synthetic auxins.

Effects

● Promote flowering in plants like pineapple.

● Help to initiate rooting in stem cuttings.

● Prevent dropping of fruits and leaves too early.

● Promote natural detachment (abscission) of older leaves and

fruits.

● Control xylem differentiation and help in cell division.

Applications

● Used for plant propagation.

● To induce parthenocarpy i.e. the production of fruit without

prior fertilization.

● 2, 4-D is widely used as a herbicide to kill dicotyledonous

weeds.

● Used by gardeners to keep lawns weed-free.

Note: The growing apical bud in higher plants inhibits the growth of

the lateral buds. This phenomenon is ‘Apical Dominance‘. Removal of

the apical bud allows the lateral buds to grow. This technique is

commonly used in tea plantations and hedge-making.

(Source: Wikimedia Commons)

Why do we get certain fruits in one particular season and not the

others? Learn about Photoperiodism here to know the answer.

Gibberellins

Discovery

It is the component responsible for the ‘bakane’ disease of rice

seedlings. The disease is caused by the fungal pathogen Gibberella

fujikuroi. E. Kurosawa treated uninfected rice seedlings with sterile

filtrates of the fungus and reported the appearance of disease

symptoms. Finally, the active substance causing the disease was

identified as gibberellic acid.

Types

There exist more than 100 gibberellins obtained from a variety of

organisms from fungi to higher plants. They are all acidic and are

denoted as follows – GA1, GA2, GA3 etc. GA3 (Gibberellic acid) is

the most noteworthy since it was the first to be discovered and is the

most studied.

Effects

● Increase the axis length in plants such as grape stalks.

● Delay senescence (i.e. ageing) in fruits. As a result, their

market period is extended.

● Help fruits like apples to elongate and improve their shape.

Applications

● The brewing industry uses GA3 to speed the malting process.

● Spraying gibberellins increase sugarcane yield by lengthening

the stem.

● Used to hasten the maturity period in young conifers and

promote early seed production.

● Help to promote bolting (i.e. sudden growth of a plant just

before flowering) in cabbages and beet.

Cytokinins

Discovery

F. Skoog and his co-workers observed a mass of cells called ‘callus’ in

tobacco plants. These cells proliferated only when the nutrient

medium contained auxins along with yeast extract or extracts of

vascular tissue. Skoog and Miller later identified the active substance

responsible for proliferation and called it kinetin.

Types

Cytokinins were discovered as kinetin. Kinetin does not occur

naturally but scientists later discovered several natural (example –

zeatin) and synthetic cytokinins. Natural cytokinins exist in root

apices and developing shoot buds – areas where rapid cell division

takes place.

Effects

● Help in the formation of new leaves and chloroplast.

● Promote lateral shoot growth and adventitious shoot formation.

● Help overcome apical dominance.

● Promote nutrient mobilisation which in turn helps delay leaf

senescence.

Abscisic Acid

Discovery

Three independent researchers reported the purification and

characterization of three different inhibitors – Inhibitor B, Abscission

II and Dormin. Later, it was found that all three inhibitors were

chemically identical and were, therefore, together were given the

name abscisic acid. Abscisic acid mostly acts as an antagonist to

Gibberellic acid.

Effects

● Regulate abscission and dormancy.

● Inhibit plant growth, metabolism and seed germination.

● Stimulates closure of stomata in the epidermis.

● It increases the tolerance of plants to different kinds of stress

and is, therefore, called ‘stress hormone’.

● Important for seed development and maturation.

● It induces dormancy in seeds and helps them withstand

desiccation and other unfavourable growth factors.

Ethylene

Discovery

A group of cousins showed that a gaseous substance released from

ripe oranges hastens the ripening of unripe oranges. Consequently,

they found that the substance was ethylene – a simple gaseous Plant

Growth Regulator. Ripening fruits and tissues undergoing senescence

produce ethylene in large amounts.

Effects

● Affects horizontal growth of seedlings and swelling of the axis

in dicot seedlings.

● Promotes abscission and senescence, especially of leaves and

flowers.

● Enhances respiration rate during ripening of fruits. This

phenomenon is ‘respiratory climactic’.

● Increases root growth and root hair formation, therefore

helping plants to increase their absorption surface area.

Application

Ethylene regulates many physiological processes and is, therefore,

widely used in agriculture. The most commonly used source of

ethylene is Ethephon. Plants can easily absorb and transport an

aqueous solution of ethephon and release ethylene slowly.

● Used to break seed and bud dormancy and initiate germination

in peanut seeds.

● To promote sprouting of potato tubers.

● Used to boost rapid petiole elongation in deep water rice plants.

● To initiate flowering and synchronising fruit-set in pineapples.

● To induce flowering in mango.

● Ethephon hastens fruit ripening in apples and tomatoes and

increases yield by promoting female flowering in cucumbers. It

also accelerates abscission in cherry, walnut and cotton.

In summary, one or the other plant growth regulator influences every

phase of growth or development in plants. These roles could be

individualistic or synergistic; promoting or inhibiting. Additionally,

more than one regulator can act on any given life event in a plant.

Along with genes and extrinsic factors, plant growth regulators play

critical roles in plant growth and development. Factors like

temperature and light affect plant growth events (vernalisation) via

plant growth regulators.

Read more about Vernalisation here in detail.

Solved Example for You

Question: Match the plant growth regulator and the scientist

associated with it.

Plant Growth Regulator Scientist

1. Gibberellins a. Independent researchers

2. Auxins b. E. Kurosawa

3. Abscisic acid c. F. Skoog

4. Cytokinins d. F. W. Went

Solution: 1-b, 2-d, 3-a, 4-c.

Photoperiodism

Have you ever wondered why certain flowers bloom only in certain

seasons? Why do we get certain fruits in one particular season and not

the others? This is because seasonal changes involve changes in the

length of day and night and some plants need a certain amount of light

to flower. This is photoperiodism. Photoperiodism applies not only to

plants but animals too! Let’s try and understand this concept in more

detail.

Photoperiodism

‘Photo’ means ‘light’ and ‘period’ means ‘length of time’. Therefore,

by definition, photoperiodism is the reaction of plants and animals to

the length of day and night.

Photoperiodism in Plants:

Most flowering plants have the ability to sense changes in season (i.e.

the length of day and night) and flower at the right time. To do this,

they make use of photoreceptor (light-sensitive) proteins called

‘phytochrome’.

Plants need exposure to light for a ‘critical duration’. This duration is

different for different plants. Based on this critical duration, plants can

fall into the following three categories:

Long Day Plants (LDP)

● These plants flower when the days are longer.

● They require more than the critical duration of light to flower

(usually 14-16 hours).

● The light period is very critical in LDP plants. Prolongation of

the light period or a brief exposure to light during the dark

period boosts flowering in these plants.

● One usually does not find LDP plants in places where the

length of a day is too short.

● They are also called ‘Short Night Plants’.

● Examples – spinach, radish, hibiscus etc.

Short Day Plants (SDP)

● These plants flower when the days are shorter.

● They need less than the critical duration of light (about 8-10

hours) and a continuous dark period (about 14-16 hours) to

flower.

● The dark period is very critical for SDP plants and has to be

continuous. These plants will not flower if the dark period is

briefly interrupted by light.

● SDP plants are usually not found in places where the length of

a day is too long.

● They are also called ‘Long Night Plants’.

● Examples – soybean, tobacco, chrysanthemum etc.

Day Neutral Plants (DNP) ● These plants do not follow this restriction of critical duration.

● In other words, they are ‘neutral’ to the length of day or night.

● Examples – tomatoes, pea plants, rose etc.

Did you know that scientists use the concept of photoperiodism to

classify plants and to identify their location? As mentioned earlier,

photoperiodism exists in animals too.

Photoperiodism in Animals

Depending on the length of the day, animals also show behavioural

and biological changes. Day length affects their fur colour, migration,

hibernation and also sexual behaviour. For example, the singing

frequency of the canary bird depends on the length of the day.

Solved Example for you

Question: What are the plants that flower during seasons with long

days called?

a. Long Night Plants

b. Short Day Plants

c. Long Day Plants

d. Short Night Plants

Solution: Answer is ‘”c” and “d’. Long days also means shorter

nights. Therefore, these plants are called Long Day Plants or Short

Night Plants.

Vernalisation

Are plants only dependent on light to flower and germinate? Is there

any other factor that influences flowering? Yes! In addition to light,

another factor that influences flowering in plants is temperature. This

dependency on temperature is Vernalisation. Let’s learn this concept

and how it affects flowering in plants in more detail.

Vernalisation

Vernalisation is defined as the qualitative or quantitative dependence

of plants on exposure to a low temperature to flower. Temperature

affects flowering, metabolic activities, and germination of seeds in

plants.

Plants that grow in mild weather germinate at low temperatures

whereas those that grow in hot regions germinate at high temperatures.

Some plants need exposure to a low temperature to germinate.

Furthermore, a plant can be induced to flower in a growing season by

exposing it to low temperature. Therefore, it shortens the vegetative

phase and hastens flowering in plants.

Examples of Vernalisation

Food plants such as wheat and barley have a ‘spring variety’ and a

‘winter variety’. The ‘spring variety’ is usually planted in the spring

season. As a result, it flowers and produces grains by the end of the

growing season. The ‘winter variety’, however, is planted in autumn.

It germinates over winter, grows in the spring and is harvested in

summer. In contrast to the spring variety, the winter variety will not

flower or produce grains within the flowering season if planted in

spring.

Biennial plants are plants that take two years to flower. They grow

leaves, stem, and roots in the first year and then enter a period of

dormancy in the cold months. They need this period of cold or

vernalisation to flower in the subsequent months. Eventually, biennial

plants flower, produce fruit and die in the next spring/summer.

Examples are carrots, sugarbeet, and cabbages.

Factors Needed For Vernalisation

● Low Temperature – 50-day treatment between 2°C and 12°C.

● Water – Plants need proper hydration to receive the stimulus

from cold temperatures.

● Actively dividing cells – For vernalisation to work the cells

need to be actively dividing. It does not work on dry seeds and

therefore, the seeds need to be moist before exposure to low

temperature.

● Nutrients

● Aerobic respiration

Site Of Vernalisation

The site that perceives the cold stimulus can be different in different

plants. It could be the apical meristem in the shoots, the germinating

seed or the vegetative parts such as leaves.

Advantages

● Prevents plants from maturing too early in the growing season.

Therefore, they get enough time to mature.

● Induces early flowering and reduces the vegetative phase of

plants.

● It increases yield in plants.

● Provides resistance to cold and diseases.

● It enables biennial plants to behave like annual plants.

● Vernalisation allows plants to grow in regions they normally do

not grow.

● Also, it helps to remove the wrinkles on kernels of Triticale

(wheat and rye hybrid).

Solved Example For You

Question: Vernalisation reduces which phase in plants?

a. Growth phase

b. Vegetative phase

c. Differentiation phase

d. Flowering phase

Solution: The answer is ‘b’. Vernalisation reduces the vegetative

phase (phase where leaves are produced) and induces early flowering

in plants.