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11.1: Primary Growth - Biology

11.1: Primary Growth - Biology


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All plant roots begin in primary growth, which is a lengthening of plant organs. Primary growth involves the root apical meristem, primary meristems, and primary tissues.

Figure (PageIndex{1}): These wheatgrass seeds have recently sprouted, producing fresh root and shoot growth. This would be an excellent place to look for roots in primary growth. Note that, even though it is a tangled mess, the individual roots are not branching very much. Instead, they are growing like long, thin threads. At the tip of each of these threads, you would find a root apical meristem. Photo by Maria Morrow, CC BY-NC.

Video (PageIndex{1}): This video by Ben Montgomery provides a walkthrough of the internal structures and organization of plant roots, including lateral root formation. Sourced from YouTube.


Welcome to the Living World

All plant cells are descendants of the zygote (fertilized egg).

Growth is an irreversible permanent increase in size of an organ or its parts or an individual cell.

It involves metabolic processes that consume energy.

Plant Growth Generally is Indeterminate

Plant growth continues throughout the life due to the presence of meristems.

Meristematic cells have capacity to divide & self-perpetuate.

The growth where new cells are always added to the plant body by the meristem is called open form of growth.

  • It occurs due to root apical meristem & shoot apical meristem.
  • It causes the elongation of the plants along the axis.
  • It occurs due to lateral meristems, vascular cambium & cork-cambium.
  • It causes increase in the girth of organs.

At cellular level, growth occurs due to increase in the amount of protoplasm.

  • Cell number: E.g. A maize root apical meristem can produce more than 17,500 new cells per hour.
  • Cell size: E.g. Cells in a watermelon can increase in size by up to 3,50,000 times.
  • Length: E.g. Growth of a pollen tube.
  • Surface area: E.g. Growth in a dorsi-ventral leaf.
  • Meristematic phase: It occurs in the meristems at the root apex & the shoot apex. Here, cells have rich protoplasm and large nuclei. Cell walls are primary, thin & cellulosic with abundant plasmodesmata.
  • Elongation phase: It occurs in cells proximal (just next, away from the tip) to the meristematic zone. The cells have increased vacuolation, size and new cell wall deposition.
  • Maturation phase: It occurs in the cells further away from the apex, i.e., more proximal to the phase of elongation. The cells attain maximal size in terms of wall thickening and protoplasmic modifications.

It is the increased growth per unit time.

The growth rate may be arithmetic or geometrical.

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In this, following mitotic division, only one daughter cell continues to divide while the other differentiates & matures.

On plotting the length of the organ against time, a linear curve is obtained.

Mathematically, it is expressed as Lt = L0 + rt

Here, both daughter cells continue mitotic cell division.

In most systems, the initial growth is slow (lag phase), then it increases rapidly (log or exponential phase).

If nutrient supply is limited, the growth slows down leading to a stationary phase.

On plotting the parameter of growth against time, we get a typical sigmoid (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.


11.1: Primary Growth - Biology

Figure 1. There must be an area of growth, similar to how the bones in your fingers, arms, and legs grow longer. There is, and it is called the apical meristem, which is shown here.

Most plants continue to grow throughout their lives. Like other multicellular organisms, plants grow through a combination of cell growth and cell division. Cell growth increases cell size, while cell division (mitosis) increases the number of cells. As plant cells grow, they also become specialized into different cell types through cellular differentiation. Once cells differentiate, they can no longer divide. How do plants grow or replace damaged cells after that?

The key to continued growth and repair of plant cells is meristem. Meristem is a type of plant tissue consisting of undifferentiated cells that can continue to divide and differentiate.

Apical meristems are found at the apex, or tip, of roots and buds, allowing roots and stems to grow in length and leaves and flowers to differentiate. Roots and stems grow in length because the meristem adds tissue “behind” it, constantly propelling itself further into the ground (for roots) or air (for stems). Often, the apical meristem of a single branch will become dominant, suppressing the growth of meristems on other branches and leading to the development of a single trunk. In grasses, meristems at the base of the leaf blades allow for regrowth after grazing by herbivores—or mowing by lawnmowers.

Apical meristems differentiate into the three basic types of meristem tissue which correspond to the three types of tissue: protoderm produces new epidermis, ground meristem produces ground tissue, and procambium produces new xylem and phloem. These three types of meristem are considered primary meristem because they allow growth in length or height, which is known as primary growth.

Figure 2. Microphotograph of the root tip of a broad bean show rapidly dividing apical meristem tissue just behind the root cap. Numerous cells in various stages of mitosis can be observed.

Secondary meristems allow growth in diameter (secondary growth) in woody plants. Herbaceous plants do not have secondary growth. The two types of secondary meristem are both named cambium, meaning “exchange” or “change.” Vascular cambiumproduces secondary xylem (toward the center of the stem or root) and phloem (toward the outside of the stem or root), adding growth to the diameter of the plant. This process produces wood, and builds the sturdy trunks of trees. Cork cambiumlies between the epidermis and the phloem, and replaces the epidermis of roots and stems with bark, one layer of which is cork.

Figure 3. Primary and secondary growth

Woody plants grow in two ways. Primary growth adds length or height, mediated by apical meristem tissue at the tips of roots and shoots—which is difficult to show clearly in cross-sectional diagrams. Secondary growth adds to the diameter of a stem or root vascular cambium adds xylem (inward) and phloem (outward), and cork cambium replaces epidermis with bark.

Watch this time-lapse video of plant growth. Note that there isn’t any narration in the video.

In Summary: How Plants Grow

Most plants continue to grow as long as they live. They grow through a combination of cell growth and cell division (mitosis). The key to plant growth is meristem, a type of plant tissue consisting of undifferentiated cells that can continue to divide and differentiate. Meristem allows plant stems and roots to grow longer (primary growth) and wider (secondary growth).


→ Growth is one of the most conspicuous events in any living organism. It is an irreversible increase expressed in parameters such as sizes, area, length, height, volume, cell number, etc. It conspicuously involves increased protoplasmic materials.

→ In plants, meristems are the sites of growth. Root and shoot apical meristems sometimes along with intercalary meristem, contribute to the elongation growth of the plant axis.

→ Growth is indeterminate in higher plants. Following cell division in root and shoot apical meristem cells, the growth could be arithmetic or geometrical.

→ Growth may not be and generally is not sustained at a high rate throughout the life of cell/tissue/organ/organism.

→ One can define three principal phases of growth- the lag, log, and senescent phase.

→ When a cell loses the capacity to divide it leads to differentiation. Differentiation results in the development of structures that are commensurate with the function the cells finally have to perform.

→ General principles for differentiation for cells, tissues, and organs are similar.

→ A differentiated cell may differentiate and then redifferentiate.

→ Since differentiation in plants is open, the development could also be flexible, i.e, the development is the sum of growth and differentiation. Plant exhibit plasticity in development.

→ Plant growth and development are under the control of both intrinsic and extrinsic factors.

→ Intercellular intrinsic factors are the chemical substances, called plant growth regulators (PGR).

→ There are diverse groups of PGRs in plants, principally belonging to five groups: auxins, gibberellins, cytokinins, abscisic acid, and ethylene. These PGR’s are synthesized in various parts of the plant they control different differentiation and developmental events.

→ Any PGR has diverse physiological effects on plants. Diverse PGRs also manifest similar effects. PGRs may act synergistically or antagonistically.

→ Plant growth and development are also affected by light, temperature, nutrition, oxygen status, gravity, and such external factors.

→ Flowering in some plants is induced only when exposed to a certain duration of photoperiod. Depending on the nature of photoperiod requirements, the plants are called short-day plants, long-day plants, and day-neutral plants.

→ Certain plants also need to be exposed to low temperatures so as to hasten to flower later in life. This treatment is known as vernalization

→ Vernalisation: Vernalisation is the low-temperature requirement of some plants for flowering. The cold treatment given to shoot tips or seeds is called vernalization.

→ Photoperiodism: Flowering in certain plants depends not only on a combination of light and dark exposures but also on their relative durations. This is termed photoperiodism.

→ Short-day plants/Long-day plants: The former group of plants is called short-day plants while the later ones are termed long-day plants.

→ Stress hormone: 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.

→ Apical dominance: In most higher plants, the growing apical bud inhibits the growth of the lateral (axillary) buds, a phenomenon called apical dominance.

→ Dedifferentiation: Plants show another interesting phenomenon. 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 dedifferentiation.

→ Differentiation: 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 differentiation.

→ Absolute growth rate: Measurement and the comparison of total growth per unit time is called the absolute growth rate.

→ Relative 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.

→ The open form of growth: The cell(s) 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. This form of growth wherein new cells are always being added to the plant body by the activity of the meristem is called the open form of growth.


Primary growth

Scruff believes that sex is not the primary concern of users.

He has picked pre-primary brawls with Christie, Perry, and Marco Rubio.

“You try to always scratch where the itch is,” Huckabee said about his campaigning and rhetoric in the 2008 primary.

Even then, most of us doubted he would show up and actually sign the papers allowing him to enter the 1992 New Hampshire primary.

It was Dec. 20, 1991, the deadline for the New Hampshire primary.

Besides this fundamental or primary vibration, the movement divides itself into segments, or sections, of the entire length.

What the ear hears is the fundamental pitch only the overtones harmonize with the primary or fundamental tone, and enrich it.

Water itself is of course essential to the growth of every plant, but the benefits of Irrigation reach far beyond this.

That—and no existing institution and no current issue—is the primary concern of the present age.

Potatoes also are extensively planted, and I never saw a more vigorous growth.


Plant Biology - Primary Growth and Secondary Growth

Do plants grow from the top or the bottom? If you carve your name in a tree trunk, will it be at the same place in 10 years or will it move up the trunk? To know the answers to these questions, you need to understand primary growth and secondary growth.

First, let’s look at primary growth. Primary growth extends the length of a plant both aboveground and belowground. Since humans generally live aboveground, we usually only see the aboveground parts of a plant: the shoot system. The entire shoot system, no matter how large or small, owes its beginnings to a small region of the plant called the shoot apical meristem.

An apical meristem is a region of high cell division (lots and lots of mitosis) that contributes to the extension of the plant. The shoot apical meristem is an apical meristem that is in the shoot system, as opposed to the root apical meristem that is, you guessed it, in the roots. It is only through the activity of the shoot apical meristem that the plant grows taller.

The shoot apical meristem is found at the tip of the plant stem, so growth extends upward from the top of the stem, not the bottom. Those bottom leaves aren’t going anywhere until they fall off the plant. That means if you carve your name into the trunk of a tree, it will still be there many years later (but don’t do that, it hurts the tree like a tattoo hurts human skin watch this clip from Fern Gully if you don’t believe us).

One more meristem is the intercalary meristem. This is a region of rapid cell division at the base of nodes. This type of meristem is only found in monocots, so don't go looking for it on eudicots. You’ll be looking a long time. These are particularly important to monocots because they allow stems to elongate quickly and also for leaves to regrow quickly if they have been damaged.

Just like a human body has all its different parts (arms, legs, torso, head), a plant body has parts that are the same on every plant, though they may look different in different species.

The parts of a shoot system are the:

  • Stem (nodes + internodes )
    • nodes are where leaves attach to the stem
    • internodes are the spaces on the stem in between the leaves

    A leaf is made up of a blade and a petiole. The blade is the flat green part that you usually think of as the leaf, and the petiole is just the little stem that attaches the blade to the main stem. In between the leaf primordia, where new leaves form, and the stem below, are the axillary buds. These will form branches, which will have their own apical meristems on the ends. Axillary buds are often protected by bud scales. A bud scale is a modified leaf that covers the delicate bud until it starts to grow into a shoot.

    Most of the parts named above are visible as they originate on the shoot apical meristem. The shoot apical meristem is comprised of leaf primordia, which turn into leaves, and the apical dome, where the stem elongates. Under a microscope, the tip of a plant shoot looks like this:

    Sometimes stems are modified, and specialized stems may look and function differently than "regular" stems. For example, a rhizome is a stem that grows horizontally underground. Just because a plant part is growing underground doesn’t mean it’s a root! A rhizome can have axillary buds that shoots grow out of. Irises have rhizomes, as do ginger and potato plants. Many people know that potatoes grow underground and are called tubers, but they actually are not roots. Potatoes are enlarged ends of rhizomes, storing sugars and acting as storage organs for the plant. A rhizome looks like this:

    In the picture above, see how the tubers are at the ends of the rhizomes, and the true roots are below?

    Primary Growth of Roots

    The root system also has an apical meristem, known as the root apical meristem. This acts in much the same way as the shoot apical meristem, causing extension growth. The main difference is this growth goes down into the ground, and roots, not leaves and branches, come from the root apical meristem.

    Roots have really important jobs, and they don’t get a lot of credit for their hard work because they are underground all the time. Roots are responsible for:

    1. Anchoring the plant into the ground
    2. Absorbing water and nutrients
    3. Storing nutrients
    4. Associating with soil microbes in symbiotic relationships

    As roots grow, they travel downward through the soil, dodging rocks and other obstacles that might be in their way. Just as you should wear a helmet when riding a motorcycle or playing hockey, roots have their own type of helmet: a root cap. The root cap protects the root apical meristem as the root pushes its way through the soil. It also secretes slimy ooze that lubricates the soil around the tip of the root, aiding the root on its journey through the harsh soil.

    Roots can take on many different forms, and root form depends on whether the plant is a eudicot or monocot. In eudicots, the first root to form is the primary root. It grows straight down and is the dominant root, also known as a taproot. The taproot can produce lateral roots that grow out to the sides. Common eudicots include tomato plants, roses, maple trees, oak trees, and raspberry bushes.

    In eudicots, branch roots soon join the taproot in its hunt for nutrients. These branch roots form from an area called the pericycle. Branch roots don’t grow as long as taproots, but they expand the plant’s ability to take up water and nutrients from the ground.

    In monocots, the primary root usually dies soon after the plant germinates and is replaced by roots that form on the stem, called adventitious roots. Adventitious roots are lateral roots that anchor the plant. Monocots don’t have taproots but instead have shallow, fibrous root systems that trap lots of soil. Some examples of monocots are corn, orchids, lilies, and magnolias.

    When seeds first start to germinate, the most important thing for the young plant is to get a good hold in the ground. The plant produces more roots than shoots when it is young, but as it gets older the amount of root structure is roughly the same as the amount of shoot structure. In fact, the underground root system often mirrors the aboveground shoot system.

    Secondary Growth of Shoots

    Now we know how a plant gets taller and its roots get longer. But what about wider? Even a big tree with an enormous trunk starts out as a puny seedling. Popeye eats a lot of spinach to grow big and strong, but what do spinach plants eat?

    The width of a plant, or its girth, is called secondary growth and it arises from the lateral meristems in stems and roots. As with apical meristems, lateral meristems are regions of high cell division activity. However, the cells they make grow outward rather than upward or downward. Eudicots use lateral meristems to add to their width monocots, however, do not experience secondary growth. We’ll come back to them later.

    The lateral meristems that produce secondary growth are called cambiums, which just means a tissue layer that adds to plant growth. The two important ones for secondary growth are the vascular cambium and the cork cambium. The vascular cambium produces more vascular tissue (xylem and phloem), which provide support for the shoot system in addition to transporting water and nutrients. Because the xylem and phloem that come from the vascular cambium replace the original (primary) xylem and phloem, and add to the width of the plant, they are called secondary xylem and secondary phloem. Here is what that looks like:

    The vascular cambium is only one cell thick and forms a ring around the stem of a plant. On its interior, it adds secondary xylem and on its exterior, it adds secondary phloem. In trees, the layers of secondary xylem form wood. The layers of the secondary phloem form bark. Over time, the tree sheds older layers of bark and replaces them with newer layers. If you look at a cross section of a stem, the width of the wood gets bigger over time but the bark always remains a narrow band.

    Over time, the older wood in the inner part of the trunk goes becomes transformed. It doesn’t turn into an alien and fight Decepticons, but it does increase its defenses. The inner wood goes goes through a genetic process that makes it harder and more resistant to decay. The wood’s cells are dead, and it is now called heartwood. Heartwood is sometimes, but not always, darker than the surrounding wood. You can think of it as the "heart" of the tree, keeping the tree strong and sturdy because it is in the middle of the tree. However, it does not actually contribute to keeping the tree alive—trees can live with their heartwood completely decayed!

    The wood that surrounds the heartwood is called sapwood. Sapwood is the living wood where transport of water occurs. Sapwood, unlike heartwood, is vital to the tree’s health because it is carrying the water and nutrients the tree needs to survive. Sapwood is softer than heartwood, so if you have to decide which to build your house out of, choose the heartwood.

    In this cross section of a stem, the stuff in the middle, labeled Pi, is called the pith. The pith is made up of primary cells (originating from an apical meristem). The area labeled with an X is the xylem, and the P is the phloem. The area labeled BF is a region of bast fibers, which are strong supporting fibers in the phloem. These are not present in all plants. The outer dark region labeled C is the cortex, which surrounds the vascular tissue. And last but not least is the epidermis, which is the outermost layer of cells.

    In temperate areas with a distinct summer and winter, the vascular cambium takes a nice long rest during the winter, kicking its feet up and watching marathons of Friday Night Lights for a few months. When it starts up its cell division again in the spring, the new cells are much bigger than the last cells made during the fall because water and nutrients are more available in spring. The parts of the wood with wider cells made in the spring are called springwood. Wood made later in the season is called summerwood and is often composed of thinner cells. This cycle of growth in the spring and summer and Friday Night Lights in the winter, repeats every year and forms annual tree rings.

    The cork cambium makes cork, which is a tough, insulating layer of cells. These cells have wax in them, which helps them protect the stem from water loss. The cork layer also protects the plant from insects and pathogens such as fungi and bacteria, and can insulate the tree from fire. This cork is indeed the same cork found in wine bottles, which usually comes from the cork oak tree (Quercus suber). Harvesting cork from these trees maintains the ecosystem: in areas of Europe where cork harvesting has been abandoned, the cork oak habitats have become overgrown by flammable shrubs, causing an increase in wildfires. Cork is also part of the bark, and it falls off over time.

    Monocots don’t have secondary growth. Usually monocots do not get very wide. However, some monocots, such as palms, can get pretty thick in the middle. How? When palms shed their leaves, they don’t lose the entire leaf. The base of the leaf stays attached to the stem, and layers of old leaf bases accumulate over time. This makes the palm stem wider even without having secondary growth.

    Brain Snack

    Not all roots are underground. Mangroves are tropical plants that grow in flooded soil, and they have huge aboveground root systems to help them get air while the rest of their roots are underwater. Many orchids and other plants are epiphytes, which grow completely on top of other plants. Their roots dangle in the air or are plastered onto tree trunks, and are usually pretty small since the orchid has found someone else to lean on and don’t need the roots to keep them upright.


    Primary Phloem and Secondary Phloem | Plants

    2. It is found in the primary plant body of all vascular plants.

    3. Primary phloem occurs in all types of organs

    4. It occurs towards the periphery.

    5. Primary phloem is differentiated into proto- phloem and metaphloem.

    6. A radial system is absent.

    7. Phloem fibres are fewer. They are restricted to outer part.

    8. Primary phloem shows an irregular arrangement of various types of cells.

    9. Sieve tubes are comparatively fewer.

    10. Sieve tubes are longer and narrower.

    11. Phloem parenchyma is less abundant.

    12. Crystals and other depositions are rare.

    13. Sciereids are usually absent.

    Difference # Secondary Phloem:

    1. Secondary phloem develops from a lateral meristem called vascular cambium.

    2. It is found only during secondary growth of dicots and gymnosperms with the exception of annuals.

    3. Secondary phloem is restricted to stems and roots of perennial dicots and gymnosperms.

    4. It is formed inner to the primary phloem.

    5. There is no such distinction.

    6. It is traversed by radial system of phloem rays.

    7. Phloem fibres are more abundant. They commonly occur in patches or bands.

    8. Secondary phloem has a more regular arrangement.

    9. Sieve tubes are comparatively more numerous.

    10. Sieve tubes are shorter but wider.

    12. The cells often contain crystals and depositions of various substances.

    13. Sciereids are formed in the secondary phloem of several plants.

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    Primary growth is the process that increases the length of the plant while secondary growth is the process that increases the girth of the plant. Thus, this is the key difference between primary and secondary growth. A further difference between primary and secondary growth is that the primary growth is a result of cell division in the primary meristems while secondary growth is a result of the cell division in the secondary meristems.

    Below infographic shows more details on the difference between primary and secondary growth.


    Primary Growth

    Most primary growth occurs at the apices, or tips, of stems and roots. Primary growth is a result of rapidly dividing cells in the apical meristems at the shoot tip and root tip. Subsequent cell elongation also contributes to primary growth. The growth of shoots and roots during primary growth enables plants to continuously seek water (roots) or sunlight (shoots).

    The influence of the apical bud on overall plant growth is known as apical dominance, which diminishes the growth of axillary buds that form along the sides of branches and stems. Most coniferous trees exhibit strong apical dominance, thus producing the typical conical Christmas tree shape. If the apical bud is removed, then the axillary buds will start forming lateral branches. Gardeners make use of this fact when they prune plants by cutting off the tops of branches, thus encouraging the axillary buds to grow out, giving the plant a bushy shape.


    All plant organs are made up of various kinds of tissues which occupy specific locations within an organ and perform specific designated functions. Thus, the development of a plant follows a very precise pattern, during this period complex body organisation is formed, i.e., produces, roots, leaves, branches, flowers, fruits, seed which finally dies.

    Topic 1 Growth, Differentiation and Development

    The life of a plant initiates from a single cell called zygote. All the structures of plants such as roots, stems, leaves, flowers, fruits and seeds arise from a single cell in a very orderly sequence.

    Growth
    It is regarded as an essential, fundamental and one of the most conspicuous characteristics of a living being. Growth can be defined as a dynamic, irreversible permanent increase in size of an organ, its parts or even an individual cell.
    The growth is generally accompanied by the metabolic processes, i.e„ anabolic and catabolic reactions occurring in an organism (mainly protein synthesis). Thus, growth in living organism is an intrinsic phenomenon (unlike, non-living organisms in which growth is extrinsic).

    Plant Growth Generally is Indeterminate
    Growth of plant is unique as they retain the capacity for unlimited growth throughout their life. In plants the growth is generally confined only to the meristematic tissues present at certain locations in the body. Meristems in the plant have certain cells that have the capacity of dividing and self-perpetuation.

    The new cells produced by the action of division of meristematic cells, soon loose, the ability of dividing and make up the plant frody.

    The form of growth in which newly producing cells are always being added to the body erf the plant by the activity of meristems is called open form of growth.

    If the meristem ever ceases to divide, the growth of I the plant will not occur and they may undergo a j period of dormancy depending upon the seasonal j changes in the climate.

    Regions of Growth
    Apical, lateral and intercalary are the special regions, where growth is localised in plants.
    At the apex of every root and shoot apical meristems are present which are responsible for the elongation of plant along their axis. This is known as primary growth of the plant.

    Note:
    A meristematic tissue consists of a group of cells, which remain in active and continuous state of division, and they retain their power of division. It consists of immature, living,
    thin-walled cells, which are rich in cytoplasm.

    In the mature plant, meristem is also found in intercalary and lateral regions. .
    The lateral meristems, vascular cambium and cork cambium appear later in life in dicotyledonous plants and gymnosperms and are responsible for the increase in the girth of the stem. This increase in girth is known as secondary growth of the plant.

    Growth is Measurable
    As stated that at cellular level, growth is the consequence of increase in the amount of protoplasm. It is difficult to measure the increase in the protoplasm directly, so it can generally be measured by measuring some quantity of it that is more or less proportional to it.

    Hence, growth can be easily measured by a variety of parameters such as
    (i) Dry weight (ii) Fresh weight
    (iii) Length (iv) Area
    (v) Volume (vi) Cell number
    Growth can be expressed in terms of increase in the cell number, e.g., Single root apical meristem in maize which give rise to more than 17,500 new cells per hour.

    It can also be expressed as an increase in the size of the cell, e.g., Cells in watermelon increases about 3,50,000 times per hour.

    Growth can be measured in terms of its length, e.g., Pollen tube and can also be measured in terms of surface area, e.g., In a dorsiventral leaf.

    Phases of Growth
    Under favourable conditions, growth of plant shows a characteristic course. The period of growth is generally divided into three phases.
    All three phases can be easily understood by taking the example of root tip.

    i. Meristematic Phase
    The cells that are constandy dividing, i.e., both at the apex of the root and shoot represents the meristematic phase of growth.

    Features shown by this phase are

    • rich in protoplasm.
    • has large conspicuous nuclei.
    • cell walls are primary in nature.
    • a thin, cellulosic, has plasmodesmatal connections.

    This phase is also known as division phase.

    ii. Elongation Phase
    This phase lies just behind the growing parts, i.e., behind the meristematic zones away from the tip.

    Features shown by this phase are

    Enlargement of cell during this phase occurs in all direction. Maximum elongation is seen in conducting tissues and fibres.

    iii. Maturation Phase
    Just behind the phase of elongation, occurs a phase of maturation. It occurs further away from the apex, i.e., more proximal to the elongation phase.

    Features shown by this phase are

    Growth Rate

    The growth rate is defined as the increased growth per unit time. Rate of growth can be expressed mathematically. It shows increase that may be arithmetic or geometrical in nature.

    i. Arithmetic Growth
    Somatic cells increases in number due to mitosis. In this type of growth, following mitotic cell division, only one daughter cell continues to divide, while others follow differentiation and attains maturity.

    Expression of arithmetic growth can be exemplified by a root elongating at a constant rate. A linear curve is obtained on plotting the length of root against time.

    ii. Geometrical Growth
    In living organisms, during geometric type of growth rate, pattern follows three important phases
    (a) Lag Phase (initial or the beginning phase) It is mainly characterised by very slow growth.

    (b) Log Phase (exponential phase) It is the middle phase of the system and is characterised by very fast and rapid growth of the plant body. After initiation of growth, it increases rapidly at an exponential rate.

    During this phase, both progeny cells undergoing mitotic cell division retain the ability to divide and continue dividing till the next phase appears till the time nutrient supply is appropriate.

    (c) Stationary Phase (steady phase) This phase occurs when either the plant reaches maturity or the supply of nutrients become limited. Due to these mentioned factors, the growth of the plant slows down to come to a halt.

    Under favorable conditions, the characteristic course of growth is observed. Thus, if we plot the parameter of growth rate against time, the typical shaped, a sigmoid curve is seen.

    It shows a characteristic feature of all living organism growing in a natural environment. This curve is typical for all cells, tissues and organs of a plant.

    and, r — relative growth rate that measures the ability of the plant to produce new plant material, known as efficiency index.
    The final size (W1) depends on the initial size (W0).

    Quantitative Comparisons of Growth Rate
    The quantitative comparisons between the growth of living systems is done in following two ways
    i. Absolute Growth Rate
    It is known to be the measurement and comparison of total growth per unit time.

    ii. Relative Growth Rate
    It is the growth of the given system per unit time expressed on a common basis, e.g., Per unit initial parameter.

    Leaves A and B shown in the figure have grown 5 cm2 in one day. Although their sizes are different, i.e., 5 cm2 and 50 cm2 respectively but both of them shows absolute increase in area in the given time to give leaves A and B, i.e., 5 cm2 in both cases. Out of these two the relative growth rate is higher or faster in leaf A.

    Conditions or Factors for Growth
    The growth of a plant is influenced by a variety of external and internal factors. Growth of plant involves synthesis of protoplasm, cell division, cell enlargement and cell differentiation.

    Some of the factors due to which growth of plants is influenced are mentioned below
    i. Water
    It is the first and the foremost requirement of the plants for the enlargement of cell, maintaining turgidity of growing cells, for extension of growth. It also acts as a medium for many enzymatic activities. In water stress conditions growth of the plants seems to get retarded.

    ii. Oxygen
    It helps in releasing metabolic energy essential for growth activities.

    iii. Nutrients
    These acts as (macro and micro essential nutrients) major raw materials for protoplasmic synthesis and also acts as a source of energy. However, under nutrient deficient conditions the growth of the plant is affected.
    Details of each and every essential nutrient has already been studied in chapter 12.

    iv. Light
    The requirement of the light to the plants for its growth is called photo-periodism. It helps in synthesis of food. It also determines the root and shoot growth. Along with light, gravity also serves as art environmental signal that affects certain phases/stages of growth.

    v. Temperature
    For normal and appropriate growth of plant optimum temperature range is necessary, i.e., 25-30°C (this happens because enzymatic reactions are very fast at optimum temperature range).

    Differentiation, Dedifferentiation and Redifferentiation
    Differentiation
    During growth, meristematic cell divides by mitotic division to form daughter cells. The cells from root and shoot apical meristem, cambium or other meristems tends to differentiate and mature to perform specific functions. This act leading to maturation is known as differentiation.
    e.g., Cell tends to loose their protoplasm, in order to form tracheary element. These cells also develop a very strong, elastic, lignocellulosic secondary cell wall in order to carryout water to long distance even under extreme conditions.

    Dedifferentiation
    The living differentiated cells also show another interesting phenomenon during which they regain the capacity to divide mitotically under certain conditions. The dedifferentiated cell can act as a meristem, e.g., Formation of meristems-interfasicular cambium and cork cambium from fully differentiated parenchyma cells.

    Redifferentiation
    The products of dedifferentiated.cells or tissue when lose the capacity to divide but mature taperform specific functions is known as redifferentiation, e.g, Secondary cortex and cork.
    Parenchyma cells that are made to divide to form callus under controlled laboratory conditions are examples of dedifferentiated tissue. From the above discussion, it is very much clear that growth in plants is open in spite of differentiation shown by them.

    It is so because cells/tissue that arise out of the single or same meristem shows different structures after attaining maturity. Thus, the final structure at maturity of a cell/tissue arising from the same tissue is also determined by the location of the cells, e.g, Cells positioned away from the root apical meristems differentiate as root cap cells, while those which are pushed to the periphery develops and matures as epidermis.

    Development
    It is the process that includes a series of changes that an organism goes through during its life cycle, i.e., from germination till senescence.

    In broad terms development is the sum total of both growth and differentiation in plants.
    The developmental process, of growth and differentiation is controlled by several intrinsic and extrinsic factors
    (i) Intrinsic factors includes, both intracellular (genetic) or intercellular factors (such as plant growth regulators).
    (ii) Extrinsic factors includes, light, temperature, water, oxygen, nutrition, etc.

    Plasticity
    Plasticity refers to a phenomenon in which plants follows different pathways in response to environment or phases of life forming different kinds of structures, e.g., Heterophylly, the phenomenon in plants by which more than two types of leaves occurs on the same plant.

    Topic 2 Plant Growth Regulators

    It has been suggested from sufficient evidences that the plants have certain chemical substances, which help to the control the mechanism of growth in the plant.

    Plant growth regulators are variously described as plant growth substances, plant hormones or phytohormones. These are the small, simple organic molecules of diverse chemical composition produced naturally in higher plants that controls the growth and other physiological functions. These are required in a very small amount by the plant.

    Classification of Plant Growth Regulators
    The plant growth regulators falls under the following categories
    (i) Indole compounds, e.g., Indole Acetic Acid (IAA)
    (ii) Adenine derivatives, e.g., forfuryl amino purine, kinetin
    (iii) Carotenoid derivatives, e.g., Abscisic acid (ABA)
    (iv) Terpenes, e.g., Gibberellic acid (mainly )
    (v) Gases, e.g., Ethylene .

    On the basis of junctions they perform in a living plant body in broad terms, PGRs are divided into two groups
    1. Plant Growth Promoters
    PGRs that shows growth promoting activities such as cell division, cell enlargement, tropic growth, pattern formation, flowering, fruiting, seed formation, etc., are called plant growth promoters, e.g., auxins, gibberellins and cytokinins.

    2. Plant Growth Inhibitors
    These perform function in response to wounds and stresses i.e., of biotic and abiotic origin. These are also involved in various growth inhibiting activities like dormancy and abscission, e.g., Abscisic acid.

    The gaseous form of PGR, i.e., ethylene, can fit in either category and may function both as promoter and inhibitor. But largely it functions as an inhibitor of growth activities.

    Discovery of Plant Growth Regulators
    It is interesting to know that the discovery of all five major groups of plant growth regulators have been done accidentally. All these help in understanding the phenomenon of development and abnormalbehaviour in plants.

    1. Discovery of Auxin
    This was the first growth hormone to be discovered. It come into existence through the observation of Charles Darwin and his son Francis Darwin.

    They observed the coleoptiles of canary grass that responded to unilateral illumination by growing towards the source of light (phenomenon known as photo-periodism).

    After performing series of experiments they came to the conclusion that coleoptile tip was the site that has the property of transmittable influence due to which bending of complete coleoptile was caused. The first PGR i.e., auxin was isolated by FW Went in 1928, from coleoptile tip of oat seedlings.

    2. Discovery of Gibberellins
    In early part of 20 century. The bakane (foolish seedlings), was reported to be caused by a fungal pathogen Gibberella fujikuroi, symptoms shown by the plant were elongated stems, little or no production of grains and plant became weak thus, it was later identified that the active substances was gibberellic acid.

    The Japanist plant pathologist E Kurosawa, reported the appearance of symptoms of the disease in uninfected rice seedlings when they were treated with sterile filtrate of fungus.

    3. Discovery of Cytokinins

    F Skoog and his coworkers, while studying the nutritional requirements of tissue culture derived from the internodal segments of tobacco stems, observed that from that internodal segments, a callus (i.e., a mass of undifferentiated cells) proliferated, only when the nutrient medium containing auxin was supplemented with the extract of vascular tissues or yeast or coconut milk (water of endosperm of coconut) or DNA.

    It was later found that the active substances were a modified form of adenine which was crystallised and identified as Kinetin. Further the compounds that exhibited kinetin like properties were termed as cytokinins.

    4. Discovery of Abscisic Acid
    With the progression in the research on plant growth regulators three independent researchers reported the purification and chemical characterisation of three different kinds of inhibitors (during mid I960), i.e., inhibitor B, abscission II and dormin. Later, three were proved to be chemically identical in nature and were named Abscisic Acid (ABA).

    5. Discovery of Ethylene
    Cousins (1910), confirmed the release of a volatile substance from ripened oranges that enhance the ripening of stored unripened bananas. This volatile substance was later identified to be a gaseous plant growth regulators, i.e., ethylene.

    Physiological Effects of Plant Growth Regulators

    All five categories of plant growtbregulators discussed above are described have under with their physiological effects on the growth of the plant
    1. Auxins
    Auxin (Gk. auxein to grow) was initially isolated from the urine of human, but later on, their presence was also found in plants and was proved to be the first PGR ever known. The real plant auxin is chemically known as Indole -3-Acetic Acid (IAA).

    The term is also applied to other natural and synthetic compounds having various growth regulating properties. Production of auxin generally takes place in the region of growing apices of the stems and roots from where they migrates to the site of their action.
    Auxins can move only through cell to cell by diffusion, i.e., they cannot move through vascular tissues.

    Types of Auxins
    There are generally two basic categories in which auxins are divided
    a. Natural Auxins
    It occur naturally in plants and fungi e.g., Indole Actic Acid (IAA) and Indole Butyric Acid (IBA).

    b. Synthetic Auxins
    These are prepared from synthetic compounds that causes several responses to IAA. They can easily move in all directions inside the plants, e.g., Naphthalene Acetic Acid (NAA), 2-4- dichlorophenoxyacetic acid (2, 4-D).
    All these types of auxins are extensively been used in agricultural and horticultural practices.

    • The compounds, which can be converted into auxins, are called auxin precursors, e.g., IAA is synthesised from tryptophan hormone.
    • The compounds, which inhibit the actions of auxins, are termed anti- auxins.
    • lndole-3 acetic acid is a derivative of an amino acid tryptophan.

    Functions of Auxins
    Auxins performs severaljunctions, these are as follows
    (a) Cell Elongation Auxin stimulate the elongation of cells of shoots.

    (b) Initiation of Roots In contrast to stem, higher concentration of auxin inhibits the elongation of shoots, but it initiates more lateral branches of roots.

    (c) Inhibition of Abscission Natural auxins delay abscission of young fruits and leaves and also used to control pre-harvest fruit drop.

    (d) Apical Dominance Presence of auxin in higher concentration (in higher plants) in shoot apex, promotes apical dominance. It is been seen commonly in many vascular plants, that presence of apical buds does not allow the lateral buds to grow. They only start developing into branches when the apical bud is removed.

    (e) Promotes Flowering Presence of auxin helps in promoting flowering in pineapple litchi, etc.

    (f) Parthenocarpy Auxins are used to unpollinated pistil and make them develop into parthenocarps, which carry a better market value.

    (g) Metabolism Application of auxin can enhance metabolism due to mobilisation of plant resources.

    Applications of Auxins
    As stated, use of synthetic auxins is widely accepted now-a-days in various agricultural and horticultural practices.

    Following are the applications of auxins
    (a) Eradication of Weeds Auxins are used as weedicides and herbicides. Application of 2, 4-dichlorophenoxyacetic acid (2, 4-D) is widely done in order to kill dicotyledonous weeds. It inflict does not affect mature, monocotyledonous plants.
    The growth of lateral buds into branches after decapitation.
    (b) Helps in Cell Division Besides cell elongation auxin may also be active in cell division.
    (c) Controls Xylem and Phloem Differentia¬tion Auxin controls differentiation of xylem and phloem is stems and roots. There are evidences that low concentration of auxin induces phloem differentiation while higher concentration of auxin is responsible for differentiation of both xylem and phloem tissues.

    2. Gibberellins
    These are another kind of plant growth regulators, which are known to be weakly acidic growth hormones. There are more than 100 different gibberellins reported from widely different organisms like fungi and higher plants. All of them are known to be acidic in nature, . thus, they are termed as Gibberellic Adds (i.e„ GA , GA1 , GA2 and so on). However, GA3 is the most important gibberellic acid which was first to be discovered. It was most extensively studied.

    Functions of Gibberellins
    Gibberellins show various important physiological effects
    (a) Elongation of Internodes It elongate the internodes so, as to increase the height of the plant. They cause an increase in length of axis and is also used in increasing length of grapes stalks.

    (b) Elongation of Genetically Dwarf Plants It has been seen that if gibberellins are administered to a dwarf plant (pea, maize, etc), it may help in overcoming dwarfism. It also causes fruits to elongate and improve their shape, e.g., in apples etc.

    (c) Bolting and Flowering The gibberellins also helps in promoting bolting (internode elongation) just prior to their reproductive phase or flowering. This is seen in rosette plants like beet, cabbage as these plants shows retarded internodal growth and profuse leaf development. Rosette plants require either long days or cold night for bolting process and for the initiation of flowering.

    (d) Breaking Dormancy It also helps in overcoming natural dormancy of buds, tubers, seeds, etc, and allows them to grow.

    Seed Dormancy

    The state of the seed is said to be the dormant state when it remains dry and non-germinating. Thus, by ‘breaking seed dormancy’, we simply mean, to make the seed to germinate.
    (e) Flowering This can also be induced in long day plants by the action of gibberellins.

    Applications of Gibberellins
    Gibberellins, apart from showing varied .physiological effects, also have numerous application.
    These are as follows
    (a) Delays Senescence Gibberellins can delay the ripening of fruits such as Citrus fruits, apples, etc. This can be also used for safe and prolonged storage of the fruits.

    (b) Malting Process The process of malting in brewing industry can be speedup by the use of GA3 .

    (c) Sugar Yield As carbohydrate is stored in the form of sugar in the stems of sugarcane. Thus, if crop of sugarcane is sprayed with gibberellins. It results in increased length of the stem. This, enhance, increases the yield of sugarcane as much as 20 tonnes per acre.

    (d) Early Seed Production like and when sprayed on juvenille conifers, hastens the maturity period of them leading to early seed production.

    Cytokinins
    These are growth promoters that are basic in nature. They have specific effects on cytokinesis (division of cytoplasm) and were discovered as kinetin (a modified form of adenine, a purine).
    Lethometal (1964) while searching for a substance with cytokinin like activity isolated Zeatin from corn kernels and coconut milk. Now presendy, several naturally occurring cytokinins and some synthetic compounds having cell division promoting activities have been identified after the discovery of Zeatin.

    Region of Synthesis of Cytokinins
    Natural cytokinins are known to be synthesised in the regions where rapid cell division takes place, e.g., root apex, developing shoot buds, young fruits, etc., out of these roots are the major source of synthesis of cytokinins, from where, they move upwards through xylem.

    Naturally Occurring Cytokinins
    Coconut milk factor The liquid endosperm of coconut is known as coconut milk. This contains some factors, that shows kinetin like activity and enhance, stimulate the growth in many plant tissues (in vitro). All these factors are collectively called as ‘coconut milk factor’. These represents an example of naturally occurring cytokinins?
    Zeatin It is also a naturally occurring cytokinin, isolated from maize grains. It is remarkably known to be more active than any other cytokinin.

    Functions of Cytokinins
    Cytokinins have following remarkable physiological effects
    (a) Promotes cell division This is one of the most common and important biological effect of kinetin on plants, i.e., to induce cell division in the presence of sufficient amount of auxin (IAA).
    (b) Reduces apical dominance They promote the growth of lateral buds by breaking apical dominance.
    (c) Morphogenesis Differentiation or morphogenesis of plants tissues/organs is seen to be in control, if ratio of cytokinins and auxins is proportionate.
    (d) Resistance They also increase resistance of plants to high or low temperature and diseases.
    (e) Delays senescence These also helps in delaying senescence (ageing) of leaves and other organs by controlling synthesis of protein and mobilisation of resources or nutrients.

    Applications of Cytokinins
    (a) Tissue culture Cytokinins are essential for tissue culture apart from cell division they are also involved in morphogenesis.

    (b) Shelf life Administration of cytokinins to harvest fruits and vegetables keeps them fresh for several days and increase their shelf life.
    Shelf life of flowers and cut shoots can also be increased by using cytokines.

    4. Ethylene
    It is a simple gaseous plant growth regulator, which is synthesized from the amino acid methionine. In plants synthesis of ethylene takes place in almost every part of the plant, i.e., roots, leaves, flowers, seeds, fruits, etc. Most important effect of ethylene is promotion of senescent changes in the plant. Thus, it is synthesized by tissue in large amounts that undergo senescence and also by ripening fruits due to this property it is also known as fruit ripening hormone.
    As ethylene is a volatile substance, its production in one plant may influence the growth of other plants near to it.

    Functions of Ethylene
    Ethylene shows various important physiological effects
    (a) In divot seedlings, ethylene influences the horizontal growth of seedling, swelling of the axis and formation of apical hook.
    (b) It is highly effective in fruit ripening. It also increases the rate of respiration. This rise in the respiration rate is called respiratory climacteric.
    (c) Helps in breaking seed and bud dormancy.
    (d) Initiation of germination in peanut seeds and sprouting of potato tubers is also due to the
    production of ethylene in plants.
    (e) In deep water rice plants, ethylene promotes rapid internode petiole elongation.
    (f) It proves to be helpful in increasing absorption surface of plants by promoting growth of root and formation of root hairs.
    (g) It also stimulates flowering in fruits like pineapple, mango and other related plants.
    Ethylene apart from so many positive responses also has negative feedback. Release of ethylene commonly inhibits the synthesis of auxins.

    Applications of Ethylene
    As ethylene helps in regulating these many physiological process in plants. It is known to be the most widely used PGR in agricultural field.

    Ethephon It is the most widely used compound as a source of ethylene. This tends to absorbs readily in an aqueous solution and transported within the plant. This slowly releases ethylene.

    (a) Ethephon is known to control fruit ripening (in tomatoes and apples).
    (b) It also helps in accelerating abscission in flowers and fruits (causes thining of fruits like cotton, cherry, walnut, etc).
    (c) Helps in promoting female flowers inhance, the yield of the fruits, e.g., Cucumber.

    5. Abscisic Acid
    It is slightly acidic growth hormone that functions as a growth inhibitor by interacting with other mentioned growth hormones, i.e., auxins, gibberellins and cytokinins.
    Thus, like other PGR, abscisic acid also has a wide range of effects on growth and development of plants.
    As its production is stimulated under stress (unfavourable conditions such as drought, water lodging, excessive temperature, etc). Thus, it is known as stress hormone. It acts antagonistically to gibberellic acid.
    This hormone is transported to all parts of the plants through the process of diffusion by conductive channels.

    Functions of Abscisic Acid
    Abscisic acid shows various important physiological effects
    (i) It has a primary role in regulating abscission and dormancy of buds and seeds. By inducing dormancy it helps the seeds to withstand the desiccation and other factors related to unfavorable growth.
    (ii) It acts as a general plant growth inhibitor and also inhibits metabolism of plants.
    (iii) It has its role in inhibition of seed germination.
    (iv) Also plays an important role in seed development and maturation.
    (v) Abscisic acid stimulates the closure of stomata.
    Abscisic acid is also known as dormin as promotes several kinds of dormancy in plants.

    The Mechanism of Stomatai Ciosmg by ABA
    ABA binds to receptors of the plasma membrane at the surface of the guard cells.

    The receptors in turn activate several interconnecting pathways, which causes a rise in pH in the cytosol promoting the transfer of Ca +2 from the vacuole to the cytosol.

    All this causes stomata to close, and opening of stomata occurs when conditions are just reverse to it.
    Interaction between Growth Regulators


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