Plant Growth And Development Class 11 Biology Chapter 13 Notes

Plant Growth and Development Class 11 Biology Chapter 13 Notes

Growth

Growth is a fundamental characteristic of living beings.
It involves an irreversible, permanent increase in the size of an organ, its parts, or an individual cell.
Growth is typically associated with metabolic processes, including both anabolic (building) and catabolic (breakdown) processes.
It requires energy and can be illustrated by examples like the expansion of a leaf.

Plant Growth

Plant growth is generally indeterminate, meaning it can continue throughout a plant’s life.
This continuous growth is possible due to the presence of meristems in the plant’s body.
Meristems contain cells that can divide and self-perpetuate, allowing for the addition of new cells to the plant body. This is known as open form growth.
If meristems stop dividing, plant growth would cease. However, this rarely happens.
Root apical meristem and shoot apical meristem are responsible for primary growth and elongation of plants.
In dicotyledonous plants and gymnosperms, lateral meristems like vascular cambium and cork-cambium contribute to secondary growth, increasing the girth of plant organs.

Measuring Growth

Growth at the cellular level primarily involves an increase in the amount of protoplasm.
Protoplasm increase is challenging to measure directly, so various parameters are used to measure growth.
Parameters for measuring growth include: increase in fresh weight, dry weight, length, area, volume, and cell number.
Examples of growth measurement:
- A maize root apical meristem can generate over 17,500 new cells per hour, indicating growth in terms of cell number.
- Watermelon cells can increase in size by up to 350,000 times, indicating growth in terms of cell size.
- The length of a pollen tube is used to measure its growth.
- The increase in surface area is a measure of growth in dorsiventral leaves.

Phases of Growth

Meristematic Phase:
- Represents the constantly dividing cells at the root and shoot apex.
- Cells in this phase are rich in protoplasm, with large nuclei and thin, cellulosic primary cell walls.
- Abundant plasmodesmatal connections between cells.
Elongation Phase:
- Proximal to the meristematic zone.
- Characterized by increased vacuolation, cell enlargement, and the deposition of new cell walls.
Maturation Phase:
- Further away from the apex, closer to the phase of elongation.
- Cells in this phase attain their maximum size in terms of wall thickening and protoplasmic modifications.
- Most of the tissues and cell types studied in earlier classes represent this phase.

Growth Rates

Growth Rate Definition: Growth rate is the increased growth per unit time and can be expressed mathematically.
Arithmetic Growth:
- In arithmetic growth, following mitotic cell division, one daughter cell continues to divide, while the other differentiates and matures.
- It is exemplified by a root elongating at a constant rate, resulting in a linear curve when plotting length against time.
- Mathematically expressed as Lt = L0 + rt
  - Lt: length at time ‘t’
  - L0: length at time ‘zero’
  - r: growth rate or elongation per unit time.
Geometrical Growth:
- Geometrical growth usually involves an initial slow growth (lag phase) followed by rapid exponential growth (log or exponential phase) when both progeny cells from mitotic division can continue to divide.
- With limited nutrient supply, growth slows down, leading to a stationary phase.
- When plotted against time, it results in a sigmoid or S-curve.
- Sigmoid curves are characteristic of living organisms growing in natural environments.
Exponential Growth Formula: W1 = W0 * e^(rt)
- W1: final size (weight, height, number, etc.)
- W0: initial size at the beginning of the period
- r: growth rate
- t: time of growth
- e: base of natural logarithms
Relative Growth Rate (r):
- It measures the ability of a plant to produce new plant material, often referred to as the efficiency index.
- The final size, W1, depends on the initial size, W0.
Comparing Growth:
- Two ways to quantitatively compare growth in living systems:
  1. Absolute Growth Rate: Measurement and comparison of total growth per unit time.
  2. Relative Growth Rate: The growth of a given system per unit time expressed on a common basis, e.g., per unit initial parameter.
Comparison of Leaves A and B:
- Leaves A and B show different sizes but both show an absolute increase in area over time.
- However, one of them has a much higher relative growth rate, likely due to more efficient utilization of resources, which results in a faster rate of increase in area.

The concepts of arithmetic and geometrical growth rates help us understand the dynamics of growth in living organisms and their response to environmental conditions.

Conditions for Growth

Water: Water is essential for plant growth. It’s required for cell enlargement and turgidity, which aids in extension growth. Water also serves as a medium for enzymatic activities necessary for growth.
Oxygen: Oxygen is crucial for releasing metabolic energy, which is essential for various growth activities in plants.
Nutrients: Plants require a range of nutrients, including macro and micro essential elements, for the synthesis of protoplasm and as a source of energy.
Temperature: Each plant organism has an optimal temperature range best suited for its growth. Deviations from this range can be detrimental to a plant’s survival.
Light: Light is a significant environmental signal that affects various phases and stages of plant growth, particularly in the process of photosynthesis.
Gravity: Gravity also plays a role in certain aspects of plant growth, such as root and stem orientation.

These conditions are fundamental for supporting the growth and development of plants. The availability and balance of these factors influence the overall health and productivity of plant organisms.

Differentiation, Dedifferentiation, and Redifferentiation

Differentiation: Differentiation refers to the process in which cells derived from meristems and cambium undergo structural changes in their cell walls and protoplasm to mature and perform specific functions. For example, cells might develop strong secondary cell walls to transport water as tracheary elements.
Dedifferentiation: Dedifferentiation is a phenomenon in plants where living, fully differentiated cells that have lost the capacity to divide can regain this division capacity under specific conditions. For instance, dedifferentiation can lead to the formation of meristems such as interfascicular cambium and cork cambium from fully differentiated parenchyma cells.
Redifferentiation: After dedifferentiation and division, cells redifferentiate to perform specific functions. For example, cells originating from dedifferentiation may lose their division capacity again but mature into specific functional cells.
Tissues Resulting from Redifferentiation in Woody Dicotyledonous Plants: In woody dicotyledonous plants, some tissues that are products of redifferentiation include the cambium (interfascicular cambium and cork cambium) and certain parenchyma cells that transform into specialized cell types within the wood and bark.
Tumor and Parenchyma Cells in Plant Tissue Culture: A tumor is an abnormal mass of cells resulting from uncontrolled cell division. In the context of plant tissue culture, parenchyma cells that are induced to divide under controlled laboratory conditions are often called callus. Callus can be further manipulated to redifferentiate into specific tissues.
Open Differentiation: Differentiation in plants is open, meaning it can be indeterminate or determinate, and it’s influenced by various factors.
Examples of Open Differentiation: The final structure of a cell or tissue at maturity is determined by its location within the plant. Examples of open differentiation include:
- Cells positioned away from root apical meristems differentiating into root-cap cells.
- Cells pushed to the periphery of an organ maturing as epidermal cells.
- In leaves, cells near the upper surface differentiating into upper epidermis, while cells on the lower surface become lower epidermis.

Development in Plants

Development encompasses all changes an organism goes through during its life cycle, from seed germination to senescence.
Diagrammatic Representation: The process of development in a higher plant can be represented diagrammatically, applicable to tissues and organs.
Plasticity: Plants exhibit the ability to follow different pathways in response to their environment or life phases, resulting in the formation of various structures. This adaptability is known as plasticity. Examples include heterophylly in cotton, coriander, and larkspur, where leaves of juvenile plants differ in shape from those in mature plants.
Environment-Induced Changes: Environmental conditions can influence the development of plant structures. An example is the difference in leaf shapes produced in the air and underwater in buttercup, representing heterophyllous development due to the environment.
Relationship Between Growth, Differentiation, and Development: Growth, differentiation, and development are closely related in a plant’s life. Development is often considered as the sum of growth and differentiation processes.
Factors Influencing Plant Development: Plant development, including growth and differentiation, is influenced by both intrinsic and extrinsic factors.
- Intrinsic factors include genetic and chemical factors such as plant growth regulators.
- Extrinsic factors encompass environmental elements like light, temperature, water, oxygen, and nutrition. These external factors play a significant role in shaping a plant’s development.

Characteristics of Plant Growth Regulators (PGRs)

Chemical Diversity: PGRs are small, simple molecules with diverse chemical compositions. They can include indole compounds (e.g., indole-3-acetic acid or IAA), adenine derivatives (e.g., kinetin), carotenoid derivatives (e.g., abscisic acid or ABA), terpenes (e.g., gibberellic acid or GA3), and even gases (e.g., ethylene or C2H4).
Various Names: PGRs are referred to by different names in literature, including plant growth substances, plant hormones, or phytohormones.
Two Functional Groups: PGRs can be broadly categorized into two groups based on their functions in a plant’s body:
- Growth Promoters: These PGRs are involved in promoting various growth-related activities, such as cell division, cell enlargement, pattern formation, tropic growth, flowering, fruiting, and seed formation. Examples include auxins, gibberellins, and cytokinins.
- Stress and Inhibitory Regulators: PGRs in this group play a crucial role in plant responses to wounds and stresses, whether biotic or abiotic in origin. They are also involved in growth-inhibiting activities like dormancy and abscission. Abscisic acid (ABA) belongs to this group. Ethylene, while somewhat versatile, largely acts as an inhibitor of growth activities.

These PGRs have significant roles in regulating various aspects of plant growth, development, and responses to environmental conditions and stressors.

The Discovery of Plant Growth Regulators

Auxin (Indole-3-Acetic Acid, IAA):
- Charles Darwin and his son Francis Darwin observed that canary grass coleoptiles responded to light by growing towards the light source (phototropism).
- They concluded that the tip of the coleoptile was responsible for this response.
- F.W. Went isolated auxin from oat seedling coleoptile tips, recognizing it as the growth-promoting substance responsible for phototropism.
Gibberellins (GA):
- The ‘bakanae’ disease in rice seedlings was caused by the fungal pathogen Gibberella fujikuroi.
- E. Kurosawa found that symptoms of the disease appeared in rice seedlings when treated with sterile filtrates of the fungus.
- These filtrates were identified as containing gibberellic acid (GA), which was found to promote stem elongation and other growth processes in plants.
Cytokinins (Kinetin):
- F. Skoog and his colleagues observed that callus (undifferentiated cell mass) proliferation occurred in tobacco stem segments when supplemented with certain nutrients in addition to auxins.
- Kinetin was later identified as the cytokinesis-promoting substance that worked in combination with auxins.
Abscisic Acid (ABA):
- In the mid-1960s, three independent research groups purified and characterized three different inhibitors: inhibitor-B, abscission II, and dormin.
- These inhibitors were later confirmed to be chemically identical and named abscisic acid (ABA).
- ABA is involved in various growth-inhibiting activities, such as promoting dormancy and abscission.
Ethylene (C2H4):
- H.H. Cousins confirmed the release of a volatile substance from ripened oranges that hastened the ripening of stored unripened bananas.
- This volatile substance was later identified as ethylene, a gaseous PGR that can either promote or inhibit various growth activities in plants.

The discovery of these major groups of plant growth regulators was often the result of accidental observations and experimentation, leading to a better understanding of how plants respond to their environment and regulate their growth and development.

Physiological Effects of Auxins

Root Initiation: Auxins like indole-3-acetic acid (IAA) and indole butyric acid (IBA) help initiate root development in stem cuttings. This is widely used in plant propagation techniques, such as cloning plants.
Flowering Promotion: Auxins can promote flowering in some plants, such as pineapples. They stimulate the formation of flowers, which is important for fruit production.
Regulation of Leaf and Fruit Drop: Auxins play a dual role in the abscission (shedding) of plant parts. They help prevent the premature drop of young leaves and fruit but promote the abscission of older, mature leaves and fruits. This is important for the plant’s resource allocation and reproductive success.
Apical Dominance: In most higher plants, the growing apical bud inhibits the growth of lateral (axillary) buds, a phenomenon known as apical dominance. When the apical bud is removed (decapitation), it allows the lateral buds to grow. This principle is applied in practices like pruning tea plants and hedge-making.
- Explanation: The apical bud produces auxins, which move downward and inhibit the growth of lateral buds. When the apical bud is removed, the inhibitory effect of auxins is eliminated, allowing lateral buds to grow.
Parthenocarpy: Auxins induce parthenocarpy, a process in which fruits develop without fertilization. This is seen in some plants, like tomatoes, where auxins promote fruit formation even without pollination.
Herbicide Use: Synthetic auxins like 2, 4-D are widely used as herbicides to control and kill dicotyledonous weeds. These herbicides do not significantly affect mature monocotyledonous plants. Gardeners use them to prepare weed-free lawns.
Xylem Differentiation and Cell Division: Auxins play a role in controlling the differentiation of xylem (vascular tissue responsible for water transport) and promoting cell division in plants.

Auxins have diverse effects on plant growth and development, and their application in agriculture and horticulture has significant implications for crop production and weed control.

Physiological Effects of Gibberellins

Stimulating Stem Elongation: Gibberellins (GAs) are known for their ability to promote the elongation of the stem and other plant parts. They are particularly effective in increasing the length of the axis in various plant species.
Enhancing Fruit Growth: Gibberellins can stimulate the elongation and improvement of the shape of fruits. For example, they are used to elongate grape stalks and improve the shape of apples. This can extend the market period for fruit sales.
Delaying Senescence: Gibberellins have the ability to delay senescence, the natural aging and deterioration of plant parts. This property is valuable for extending the time fruits can remain on the tree, allowing for better fruit quality and marketability.
Malting Process in Brewing: Gibberellic acid (GA3) is used in the brewing industry to accelerate the malting process, which is essential for beer production.
Increasing Sugarcane Yield: Spraying sugarcane crops with gibberellins can increase the length of sugarcane stems. This increase in stem length leads to higher sugar yields, potentially raising the yield by as much as 20 tonnes per acre.
Early Seed Production: Gibberellins can be used to hasten the maturity period in juvenile conifers, leading to early seed production. This can be beneficial for forestry and conservation efforts.
Promoting Bolting: Gibberellins promote bolting, which is the internode elongation that occurs just before flowering in plants like beets, cabbages, and those with a rosette growth habit. This is significant for the reproduction of these plants.

Gibberellins have a wide range of physiological effects on plant growth and development, making them valuable tools in agriculture, horticulture, and various industries.

Physiological Effects of Cytokinins:

Cytokinesis Stimulation: Cytokinins are named for their specific effects on cytokinesis, the process of cell division. They promote and facilitate cytokinesis, resulting in increased cell division in plant tissues.
Discovery and Sources: Kinetin, one of the first cytokinins discovered, is a modified form of adenine, a purine, and was initially isolated from autoclaved herring sperm DNA. While kinetin does not naturally occur in plants, the search for natural substances with cytokinin-like activities led to the discovery of zeatin from sources like corn kernels and coconut milk. Natural cytokinins are synthesized in regions of the plant where rapid cell division is occurring, such as root apices, developing shoot buds, and young fruits.
Promoting Growth and Development: Cytokinins have several growth-promoting effects, including:
- Production of new leaves.
- Stimulation of chloroplast development in leaves, enhancing photosynthesis.
- Promotion of lateral shoot growth, leading to the development of branches.
- Induction of adventitious shoot formation, which can be important for plant propagation.
Overcoming Apical Dominance: Cytokinins help overcome apical dominance, a phenomenon where the growth of lateral buds is inhibited by the apical bud. By promoting lateral shoot growth, cytokinins contribute to a more balanced and bushy plant structure.
Nutrient Mobilization: Cytokinins play a role in nutrient mobilization, helping to transport nutrients within the plant. This is essential for the overall health and growth of the plant.
Delaying Leaf Senescence: Cytokinins are involved in the regulation of leaf senescence, delaying the natural aging and deterioration of leaves. This allows the plant to maintain healthier leaves for longer periods.

Cytokinins are crucial plant growth regulators that influence various aspects of plant development and growth, particularly in tissues undergoing rapid cell division and differentiation.

Physiological Effects of Ethylene

Horizontal Growth and Morphological Changes: Ethylene influences the growth and development of plants. It can cause horizontal growth of seedlings and is involved in the swelling of the axis and apical hook formation in dicot seedlings.
Senescence and Abscission: Ethylene promotes senescence, the natural aging and deterioration of plant organs, especially leaves and flowers. It also induces abscission, which is the shedding or dropping of plant parts.
Fruit Ripening: Ethylene is highly effective in fruit ripening. It enhances the respiration rate during the ripening of fruits, a phenomenon referred to as “respiratory climacteric.” This rise in respiration rate is a key indicator of fruit ripening.
Breaking Seed and Bud Dormancy: Ethylene plays a role in breaking seed and bud dormancy, initiating germination in peanut seeds and sprouting in potato tubers.
Stimulating Internode and Petiole Elongation: In deep water rice plants, ethylene promotes rapid internode and petiole elongation, helping leaves and upper parts of the plant to remain above water.
Root Growth and Root Hair Formation: Ethylene stimulates root growth and root hair formation, increasing the surface area available for nutrient and water absorption in plants.
Flowering and Fruit Set: Ethylene is used to initiate flowering and synchronize fruit-set in pineapples. It can also induce flowering in mango plants.
Widespread Use in Agriculture: Due to its ability to regulate various physiological processes, ethylene is one of the most widely used plant growth regulators in agriculture. Ethephon is a commonly used compound that serves as a source of ethylene. It is readily absorbed and transported within the plant, releasing ethylene slowly.
Fruit Ripening and Abscission Acceleration: Ethephon is used to hasten fruit ripening in tomatoes and apples and to accelerate abscission in flowers and fruits, which is important for thinning cotton, cherries, walnuts, and other crops.
Increasing Yield in Cucumbers: Ethylene can promote the formation of female flowers in cucumber plants, increasing the yield of cucumbers.

Ethylene has diverse effects on plant growth and development, and its applications in agriculture and horticulture are extensive, making it an important plant growth regulator.

Physiological Effects of Abscisic Acid (ABA)

Inhibiting Seed Germination: ABA is known to inhibit seed germination. It plays a crucial role in regulating when and under what conditions seeds will sprout. ABA induces seed dormancy, helping seeds withstand desiccation and unfavorable conditions for growth.
Stomatal Closure: ABA stimulates the closure of stomata (small openings in the plant’s surface) in response to various environmental stresses. This closure helps plants conserve water and cope with conditions such as drought.
Stress Hormone: ABA is often referred to as the “stress hormone” because it helps plants tolerate a range of stressors, such as drought, salt, and extreme temperatures. Its action in closing stomata reduces water loss and helps the plant withstand adverse conditions.
Antagonistic Action with Gibberellins (GAs): In most situations, ABA acts as an antagonist to gibberellins (GAs). While GAs promote growth, ABA tends to inhibit it, contributing to the regulation of plant development.
Role in Seed Development and Maturation: ABA is involved in seed development and maturation. It induces dormancy in seeds, which is essential for their long-term viability and the ability to withstand environmental challenges.

Abscisic acid, like other plant growth regulators, has a multifaceted role in plant growth and development. It functions as a general plant growth inhibitor, plays a vital role in seed dormancy and maturation, and helps plants cope with various environmental stresses. The interactions and balance of different plant growth regulators are critical in regulating different aspects of plant growth and development.