Plant Growth and Development

Growth

Growth in living organisms is defined as an irreversible increase in the size or volume or weight of an organism or its parts. In plants, growth is a very conspicuous phenomenon, involving cell division, cell enlargement, and cell differentiation.

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Plant Growth Generally Is Indeterminate

Plants exhibit a unique characteristic called indeterminate growth. This means that plants retain the capacity for unlimited growth throughout their life.

• This potential for growth is due to the presence of meristems (apical meristems at root and shoot tips, lateral meristems like cambium), which are regions of actively dividing cells.
• Even as differentiated, mature tissues are formed, these meristematic regions remain active, adding new cells and allowing the plant to continue growing in length and/or girth.

However, plant growth is also determinate in some aspects. For example, leaves, flowers, and fruits grow to a certain size and then stop growing (determinate growth of organs).

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Growth Is Measurable

Growth can be measured using various parameters, depending on the plant part and the type of growth:

• Increase in fresh weight.
• Increase in dry weight (more accurate measure of accumulated organic matter).
• Increase in length (e.g., of a stem or root).
• Increase in girth or diameter (e.g., of a woody stem).
• Increase in number of cells (especially relevant for microbial growth or initial stages of tissue culture).
• Increase in surface area (e.g., of a leaf).

Example: Growth of a single maize root apical meristem can produce over 17,500 new cells per hour. Increase in surface area in a dorsiventral leaf.

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Phases Of Growth

Growth at the cellular level (in meristematic regions) can be divided into three sequential phases:

1. Meristematic Phase:
   • Cells in this phase are actively dividing (mitotic division).
   • Located at the root and shoot apices.
   • Cells are rich in protoplasm, have large conspicuous nuclei, and thin primary cell walls with abundant plasmodesmatal connections.
2. Elongation Phase:
   • Cells proximal to the meristematic zone enter this phase.
   • Cells in this region undergo rapid enlargement (elongation and expansion).
   • Vacuolation increases, cell wall extension and new cell wall material deposition occur.
3. Maturation Phase:
   • Cells further away from the apex, proximal to the elongation phase.
   • Cells attain their final size and shape and undergo differentiation to mature into specific cell types (parenchyma, xylem, phloem, etc.).
   • Cell walls thicken (secondary wall formation may occur).

*(Image shows a longitudinal section of a root tip, highlighting the root cap, and the distinct zones of meristematic activity, elongation, and maturation, possibly showing cellular changes in each zone)*

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Growth Rates

The increased growth per unit time is called the growth rate. Growth rate can be expressed mathematically.

Types of Growth Rates:

• Arithmetic Growth: In this type of growth, only one daughter cell from a mitotic division continues to divide, while the other daughter cell differentiates and matures.
  • Example: Elongation of a root or stem.
  • If plotted over time, it shows a linear curve.
  • Equation: $ L_t = L_0 + rt $
    • $L_t$ = length at time t
    • $L_0$ = length at time zero
    • $r$ = growth rate (rate of elongation per unit time)
    • $t$ = time

  *(Image shows a linear graph with size/length on Y-axis and time on X-axis)*

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• Geometric Growth: In this type of growth, both daughter cells from a mitotic division retain the ability to divide and continue to do so.
  • This is common in the initial stages of growth in a culture (e.g., microbial growth, embryonic development) where resources are unlimited.
  • If plotted over time, it shows an exponential curve.
  • Equation: $ W_1 = W_0 e^{rt} $
    • $W_1$ = final size/weight
    • $W_0$ = initial size/weight
    • $e$ = base of natural logarithms
    • $r$ = relative growth rate (growth per unit initial size)
    • $t$ = time

  *(Image shows an exponential graph with size/weight on Y-axis and time on X-axis)*

Sigmoid Growth Curve:

When growth of an entire organism or a population is measured over time, it typically follows an S-shaped curve called the Sigmoid curve. This reflects geometric growth initially, followed by a plateau as resources become limited.

• Lag phase: Initial period of slow growth.
• Log phase (Exponential phase): Period of rapid, exponential growth (geometric growth).
• Stationary phase: Growth slows down and eventually stops as resources become limited or waste products accumulate.

*(Image shows an S-shaped graph with size/weight on Y-axis and time on X-axis, highlighting the lag, log, and stationary phases)*

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Absolute and Relative Growth Rates:

• Absolute growth rate: The total growth per unit time.
• Relative growth rate: The growth per unit initial size (e.g., increase in dry weight per gram of initial dry weight per unit time). This is a better measure when comparing growth rates of organisms or parts of different initial sizes.

Answer:

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Conditions For Growth

Growth is a complex process requiring several environmental and internal factors:

• Water: Essential for cell enlargement (turgor pressure), enzymatic activities, and as a medium for metabolic reactions.
• Oxygen: Essential for aerobic respiration to release energy needed for growth processes.
• Nutrients: Macronutrients and micronutrients provide energy and materials for cell division, synthesis of protoplasm, and cell wall formation.
• Temperature: Growth occurs within an optimal temperature range. Extreme temperatures inhibit growth.
• Light: Essential for photosynthesis (producing food/energy). Also influences growth through photomorphogenesis (light-regulated development).
• Gravity: Influences the direction of root and shoot growth (geotropism).
• Plant Growth Regulators (PGRs): Internal factors (hormones) that play crucial roles in regulating and coordinating growth and development.

Differentiation, Dedifferentiation And Redifferentiation

Following the phase of elongation, cells from the meristematic region undergo differentiation. This is a key process in plant development.

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Differentiation

Differentiation is the process by which cells derived from meristems mature to perform specific functions. These cells lose their ability to divide (or divide slowly).

• During differentiation, cells undergo specific structural and physiological changes.
• Example: Cells from the root apical meristem differentiate into various cell types of the root (parenchyma, xylem, phloem, epidermal cells, etc.).
• Differentiation is an irreversible process in most cases, leading to the formation of permanent tissues.

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Dedifferentiation

Under certain conditions, differentiated, mature cells that have lost the ability to divide can regain the capacity to divide.

• This process is called dedifferentiation.
• This occurs to form meristems from mature cells.
• Example: Formation of interfascicular cambium (from medullary ray parenchyma cells) and cork cambium (from cortical cells) during secondary growth. These are secondary meristems.

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Redifferentiation

Cells produced by dedifferentiation (e.g., cells of vascular cambium or cork cambium) can again lose the ability to divide and differentiate into new cell types.

• This process is called redifferentiation.
• Example: Cells of vascular cambium (formed by dedifferentiation) redifferentiate to form secondary xylem and secondary phloem. Cells of cork cambium (formed by dedifferentiation) redifferentiate to form cork and secondary cortex.

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*(Image shows a simple flow chart: Meristematic cell $\rightarrow$ Differentiation $\rightarrow$ Differentiated cell (Permanent tissue) $\rightarrow$ Dedifferentiation $\rightarrow$ Dedifferentiated cell (Secondary meristem) $\rightarrow$ Redifferentiation $\rightarrow$ Redifferentiated cell (Secondary permanent tissue))*

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These processes of differentiation, dedifferentiation, and redifferentiation are crucial for the formation of complex tissues and organs and for secondary growth in plants.

Development

Development in plants refers to all the changes that an organism goes through during its life cycle, from seed germination to senescence (ageing) and death.

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Development includes:

• Growth: Irreversible increase in size.
• Differentiation: Maturation of cells to perform specific functions.

It is a sum of growth and differentiation. Development is a broader term that encompasses not just increase in size but also the qualitative changes in form and function.

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Plasticity

Plants have the ability to form different structures in response to their environment or different phases of life. This ability is called plasticity.

• Example 1: Heterophylly (different shapes of leaves) in cotton, coriander, and larkspur. In these plants, the leaves produced in the juvenile phase are different in shape from those produced in the adult phase.
• Example 2: Heterophylly due to environment. Leaves of buttercup ($Ranunculus \: aquatilis$) are different in shape depending on whether they are in water (finely dissected) or on land (lobed).

Plasticity is an example of development.

*(Image shows a Ranunculus plant with submerged, finely divided leaves and aerial, broader leaves)*

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Development in plants is controlled by both intrinsic (internal) factors and extrinsic (environmental) factors.

• Intrinsic factors:
  • Genetic factors (genes control developmental processes).
  • Plant Growth Regulators (PGRs) or plant hormones.
• Extrinsic factors:
  • Light
  • Temperature
  • Water
  • Oxygen
  • Nutrients
  • Gravity

These factors interact with each other to determine the overall developmental path of the plant.

Plant Growth Regulators

Plant Growth Regulators (PGRs) are small, simple molecules of diverse chemical composition that regulate plant growth and development. They are often called plant hormones or phytohormones.

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PGRs are signalling molecules produced in one part of the plant and transported to other parts where they exert their effects at very low concentrations.

Based on their functions, PGRs can be broadly classified into two groups:

1. Plant Growth Promoters: Involved in growth promoting activities like cell division, cell enlargement, pattern formation, tropic growth, flowering, fruiting, and seed formation. Examples: Auxins, Gibberellins, Cytokinins.
2. Plant Growth Inhibitors: Involved in growth inhibiting activities like dormancy and abscission. They also play roles in stress response. Example: Abscisic Acid (ABA).
3. Ethylene: Can act as both a promoter and an inhibitor, but is largely involved in inhibition of elongation and promoting senescence and abscission.

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Characteristics

• Small molecules, diverse chemical nature (indoles, terpenes, adenines, carotenoids, gases).
• Produced in specific parts of the plant, often in meristematic or actively growing tissues.
• Translocated to target tissues, sometimes over long distances.
• Active at very low concentrations ($10^{-6}$ to $10^{-2}$ M).
• Have multiple and often overlapping effects on plant growth and development.

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The Discovery Of Plant Growth Regulators

The discovery of PGRs was based on observations of plant growth and development.

• Auxins: First plant hormone discovered. Charles Darwin and his son Francis Darwin observed that the coleoptile of canary grass bends towards light (phototropism). They concluded that something was transmitted from the tip of the coleoptile to the bending region. Later, F.W. Went isolated Auxin from the tips of oat coleoptiles (Avena curvature test).
• Gibberellins: Discovered when Japanese scientists studied the 'bakane' (foolish seedling) disease in rice seedlings, caused by the fungus Gibberella fujikuroi. Infected seedlings grew abnormally tall. The active substance was isolated as gibberellic acid.
• Cytokinins: Discovered as kinetin (a modified form of adenine, a nitrogen base) from autoclaved herring sperm DNA. Kinetin was found to promote cell division in tobacco pith culture. Later, naturally occurring cytokinins were isolated from coconut milk, corn kernels (e.g., zeatin).
• Ethylene: Discovered when it was observed that ripened oranges released a volatile substance that hastened the ripening of unripe bananas stored nearby. This volatile substance was later identified as ethylene, a gaseous hormone.
• Abscisic Acid (ABA): Discovered as three different inhibitors ('inhibitor-B', 'abscisin II', 'dormin') independently. Later, it was shown that all three were chemically the same molecule, named Abscisic Acid. It was initially thought to be involved in abscission (falling of leaves/fruits) and bud dormancy.

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Physiological Effects Of Plant Growth Regulators

PGRs influence various aspects of plant life:

• Cell division and elongation.
• Seed germination.
• Flowering.
• Fruiting.
• Rooting.
• Senescence (ageing) and abscission (falling of leaves, fruits).
• Dormancy (seeds and buds).
• Apical dominance.
• Tropic movements (response to light, gravity).

The effect of a PGR depends on:

• The type of PGR.
• Its concentration.
• The specific plant species.
• The specific tissue or organ.
• The developmental stage of the plant/organ.
• The presence and concentration of other PGRs (PGRs often interact with each other, either synergistically or antagonistically).

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Auxins

Auxins are the first plant hormones to be discovered. The term 'auxin' is applied to Indole-3-acetic acid (IAA) and other natural and synthetic compounds having certain growth regulating properties.

Sites of Synthesis:

Primarily produced in the growing apices of stems and roots.

Physiological Effects:

• Cell elongation: Promotes elongation of cells in stems and coleoptiles (major effect).
• Apical dominance: The apical bud inhibits the growth of lateral buds. Auxin produced at the shoot apex is transported downwards and inhibits lateral bud growth. Removal of the shoot apex (decapitation) promotes the growth of lateral buds (a practice in tea plantations and hedge-making).
• Initiation of rooting: Promotes root initiation in stem cuttings. Used in plant propagation.
• Prevent premature fruit and leaf drop: At low concentrations.
• Promote abscission: At higher concentrations (often with ethylene).
• Induce flowering: In some plants (e.g., pineapple).
• Promote parthenocarpy: Development of fruit without fertilisation (seedless fruits, e.g., tomatoes).
• Herbicides: Synthetic auxins like 2,4-D (2,4-dichlorophenoxyacetic acid) are widely used as selective herbicides to kill dicotyledonous weeds in monocotyledonous crops (like wheat and maize).
• Auxin also controls xylem differentiation and helps in cell division.

Auxins are transported polarly (unidirectionally) downwards from the apex.

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Gibberellins

There are over 100 known gibberellins ($GA_1, GA_2, GA_3$, etc.). Gibberellic acid ($GA_3$) is one of the first and most intensively studied forms.

Sites of Synthesis:

Produced in various parts of the plant, including apical buds, root tips, young leaves, embryos, and developing seeds.

Physiological Effects:

• Increase in length of stem: Promotes elongation of internodes (especially in plants with rosette habit).
• Bolting: Induces sudden elongation of stem just before flowering in rosette plants (like cabbage, beet).
• Delay senescence: Can delay ageing of leaves and fruits.
• Speed up malting process: In brewing industry (breaking down starch to sugars in barley seeds).
• Increase fruit size: Used to increase the size of fruits like grapes and apples.
• Juvenile to adult transition: Can influence the transition from juvenile to adult phase in some conifers, leading to early seed production.
• Seed germination: Breaks seed dormancy and promotes germination (counteracts ABA).

Gibberellins are transported non-polarly.

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Cytokinins

Cytokinins are adenine derivatives, involved in promoting cell division.

Sites of Synthesis:

Synthesised in regions of rapid cell division, such as root apices, developing shoot buds, young fruits, and seeds. They are transported upwards in the xylem.

Physiological Effects:

• Cell division (Cytokinesis): Promote cell division (in conjunction with auxin). The ratio of auxin to cytokinin in tissue culture media determines whether callus develops into roots (high auxin:cytokinin) or shoots (low auxin:cytokinin).
• Promote lateral shoot growth and adventitious shoot formation.
• Break apical dominance: In conjunction with auxin, cytokinins promote the growth of lateral buds (counteracts auxin effect).
• Delay leaf senescence: By mobilising nutrients.
• Chloroplast development: Promotes the growth of chloroplasts in leaves.
• Development of new leaves.

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Ethylene

Ethylene is a gaseous plant hormone.

Sites of Synthesis:

Produced in large amounts by tissues undergoing senescence (ageing) and ripening fruits. Also produced by actively growing tissues like root tips, shoot apices, and developing flowers/seeds.

Physiological Effects:

• Fruit ripening: Promotes ripening of fruits by increasing the rate of respiration (respiratory climactic). Used commercially for ripening fruits like bananas, mangoes, apples.
• Promote senescence and abscission: Promotes ageing and shedding of leaves, flowers, and fruits.
• Promote horizontal growth of seedlings and swelling of the axis in dicot seedlings.
• Inhibit elongation: Inhibits elongation of stem and root in certain cases.
• Promote flowering: In some plants (e.g., pineapple, mango).
• Break dormancy: Breaks bud and seed dormancy in some cases.

Ethylene is transported by diffusion as a gas.

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Abscisic Acid (ABA)

Abscisic Acid is a growth inhibitor and plays a role in stress responses.

Sites of Synthesis:

Produced in various parts of the plant, particularly in chloroplasts of mature leaves, roots, and developing seeds.

Physiological Effects:

• Induce dormancy: Induces dormancy in buds and seeds (counteracts gibberellins). Helps in storage of seeds under unfavourable conditions.
• Promote abscission: Promotes shedding of leaves, flowers, and fruits (often with ethylene).
• Close stomata: Causes closure of stomata during water stress, thus conserving water. ABA is often called the 'stress hormone'.
• Inhibit seed germination: In some cases (counteracts gibberellins).
• Inhibit growth: Can inhibit shoot growth.

ABA is transported in the phloem and xylem.

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Interactions of PGRs:

The physiological response of a plant to a PGR is often the result of the interaction of multiple PGRs. These interactions can be:

• Synergistic: Two PGRs enhance each other's effect (e.g., auxin and cytokinin in cell division).
• Antagonistic: Two PGRs oppose each other's effect (e.g., auxin vs. cytokinin in apical dominance, gibberellins vs. ABA in seed germination/dormancy, ethylene vs. auxin in some elongation effects).

The overall development of a plant is controlled by a complex interplay of these internal hormonal signals and external environmental factors.

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PGR	Function (Examples)	Nature
Auxins	Cell elongation, Apical dominance, Root initiation, Parthenocarpy, Herbicide (2,4-D)	Indole derivative (IAA)
Gibberellins	Stem elongation, Bolting, Seed germination, Fruit size (grapes)	Terpenes
Cytokinins	Cell division, Lateral shoot growth, Delay senescence	Adenine derivatives
Ethylene	Fruit ripening, Senescence, Abscission, Triple response (dicot seedlings)	Gaseous (modified amino acid - methionine)
Abscisic Acid (ABA)	Dormancy (buds/seeds), Abscission, Stomatal closure (stress hormone)	Carotenoid derivative

Photoperiodism

Photoperiodism is the response of plants to the relative lengths of day (light) and night (dark periods). This phenomenon influences flowering in many plants, as well as other processes like bud dormancy and vegetative growth.

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The stimulus for flowering is perceived by the leaves.

Based on their photoperiodic response, plants are classified into three groups:

• Short-day plants (SDP): These plants require a photoperiod shorter than a critical day length for flowering. They actually require a continuous dark period longer than a critical dark period. If the long night period is interrupted by a brief exposure to light, flowering is inhibited.

  Example: Tobacco, Soybean, Chrysanthemum, Rice.

• Long-day plants (LDP): These plants require a photoperiod longer than a critical day length for flowering. They require a continuous dark period shorter than a critical dark period. If the short night period is interrupted by a brief exposure to light, it usually does not inhibit flowering.

  Example: Wheat, Barley, Radish, Spinach.

• Day-neutral plants (DNP): These plants flower irrespective of the photoperiod. They flower when they reach a certain stage of maturity, regardless of the day length.

  Example: Tomato, Maize, Cucumber, Cotton.

*(Image shows diagrams for SDP, LDP, DNP under varying day/night lengths, indicating flowering or no flowering in each scenario, highlighting the critical dark period for SDP and critical day length for LDP)*

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Significance of Photoperiodism:

• Ensures flowering occurs at an appropriate time of the year for successful pollination and seed set.
• Important for geographical distribution of plant species.
• Used in agriculture and horticulture to manipulate flowering time.

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Phytochrome: The Photoperiod Receptor

The pigment that perceives the light/dark stimulus for photoperiodism is phytochrome. Phytochrome exists in two interconvertible forms:

• Pr (Phytochrome red): Absorbs red light (around 660 nm). In the presence of red light, it is converted to Pfr.
• Pfr (Phytochrome far-red): Absorbs far-red light (around 730 nm). In the presence of far-red light or darkness, it is converted back to Pr.

It is the Pfr form that is generally considered the physiologically active form. The relative amounts of Pr and Pfr depend on the light conditions, particularly the ratio of red to far-red light and the duration of darkness.

The critical factor in the flowering of both SDP and LDP is not the length of the light period, but the length of the critical dark period.

• In SDP, flowering requires a dark period longer than a critical length. Pfr promotes flowering in LDP and inhibits it in SDP. In darkness, Pfr is converted to Pr. If the night is long enough, enough Pfr is converted to Pr to allow flowering in SDP.
• In LDP, flowering requires a dark period shorter than a critical length (or a light period longer than a critical length). A short night ensures that sufficient Pfr remains at the end of the night to promote flowering.

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Florigen: The Flowering Hormone

It is hypothesised that a floral hormone called Florigen is produced in the leaves in response to the appropriate photoperiodic stimulus. This hormone is thought to be translocated to the shoot apices, where it induces flowering. Florigen is believed to be a complex molecule, possibly involving gibberellins and other signals.

Vernalisation

Vernalisation is the phenomenon in which flowering is induced by a period of cold treatment.

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Some plants, particularly those that grow in temperate regions, require exposure to low temperatures for a certain duration before they can flower. This prevents flowering in the autumn and ensures that flowering occurs in the spring when conditions are more favourable.

Examples of plants requiring vernalisation:

• Biennial plants: Plants that complete their life cycle in two years. They grow vegetatively in the first year and require a cold period (winter) to flower and produce seeds in the second year (e.g., Sugarbeet, Cabbage, Carrots).
• Some annual plants: Some varieties of wheat, barley, and rye have two kinds: spring varieties (planted in spring, flower in summer) and winter varieties (planted in autumn, overwinter as seedlings, resume growth in spring, and flower in summer). Winter varieties require vernalisation.

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Mechanism of Vernalisation:

• The stimulus for vernalisation is perceived by the apical meristems of the shoot.
• The duration of cold treatment required varies depending on the species, usually from a few days to several weeks, at temperatures typically between $0^\circ C$ and $5^\circ C$.
• The vernalisation stimulus is thought to be perceived at the apex and then transmitted to other parts, likely involving hormonal signals (gibberellins can sometimes substitute for the cold treatment in some species).

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Significance of Vernalisation:

• Prevents premature flowering.
• Enables plants to withstand frost injury during winter.
• Shortens the vegetative period, allowing plants to flower earlier after the cold treatment.

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Vernalisation and photoperiodism are two important environmental factors that regulate flowering in plants, ensuring that reproduction occurs at the most favourable time.

Seed Dormancy

Seed dormancy is a state of suspended growth in seeds where they are unable to germinate even when external conditions are favourable (adequate moisture, oxygen, and temperature).

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Dormancy is an adaptation that prevents premature germination of seeds under unfavourable conditions, increasing the chances of survival of the species. It allows seeds to germinate at the appropriate time, ensuring the seedlings encounter suitable conditions for establishment and growth.

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Causes of Seed Dormancy:

Dormancy can be caused by various factors within the seed itself:

• Impermeable seed coat: A hard and tough seed coat may prevent the entry of water or oxygen, both essential for germination. (e.g., in legumes, some fruits).
• Mechanically resistant seed coat: A strong seed coat may prevent the expansion of the embryo during germination.
• Presence of chemical inhibitors: Some seeds contain chemical substances that inhibit germination (e.g., abscisic acid (ABA), phenolic acids, coumarins). These inhibitors must be removed or inactivated for germination to occur.
• Immature embryo: In some plants, the embryo is not fully developed at the time of seed dispersal and needs a period of further development before it can germinate.
• Requirement for after-ripening: Even with a mature embryo, some seeds require a period of dry storage under specific conditions before they can germinate. This process is called after-ripening.
• Requirement for chilling (Stratification): Some seeds require exposure to a period of low temperature before they can germinate. This requirement is linked to vernalisation in the parent plant's flowering process in some cases, but in the seed, it specifically breaks dormancy. This simulates overwintering.
• Requirement for light (Photoblasty): Some seeds require exposure to light (positive photoblastic) or darkness (negative photoblastic) for germination.

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Methods to Break Seed Dormancy:

Various methods can be used to break seed dormancy and induce germination:

• Mechanical abrasion: Breaking or scratching the hard seed coat (scarification) to allow water and oxygen entry. This can occur naturally in soil (e.g., microbial action, passage through digestive tracts of animals) or be done manually/mechanically.
• Stratification: Providing a period of cold treatment (chilling) under moist conditions to seeds.
• Chemical treatments: Using chemicals like gibberellins (GA$_3$) or nitrates, which can overcome the effect of inhibitors.
• Removal of inhibitors: Washing seeds to leach out inhibitory chemicals.
• Light exposure: Providing specific light treatments (red light is often effective for positive photoblastic seeds).
• Providing optimal environmental conditions: Ensuring adequate water, oxygen, and temperature, which may be sufficient to overcome weaker dormancy mechanisms.

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Abscisic Acid (ABA) is a key plant growth regulator involved in inducing and maintaining seed dormancy. Gibberellins (GAs) often act antagonistically to ABA, promoting germination.