Plant Growth and Development Chapter Notes | Biology Class 11 - NEET PDF Download

Plant Growth

Growth represents the remarkably robust, permanent increase in the dimensions of an organism. It is a fundamental trait observed across all living entities, driven by various metabolic processes. In the realm of plants, this phenomenon begins with the germination of seeds, progressing into seedlings, and eventually maturing into adult plants, showcasing an ongoing cycle of growth.

Vegetative Growth: This initial phase of plant development involves the production of leaves, stems, and branches without the emergence of flowers, termed as vegetative growth.

Reproductive Growth: Following the vegetative stage, plants transition into a phase where they produce flowers, which mark the reproductive aspect of their lifecycle. This phase is referred to as the reproductive stage.

Types of Growth

Primary and Secondary Growth: The elongation of the plant body occurs through the mitotic division of meristematic cells found at the root and shoot apex, referred to as primary growth. Secondary growth, on the other hand, involves the enlargement of the plant body's diameter through secondary meristem.

Unlimited Growth: Plants exhibit continuous growth in both the root and shoot systems from germination until death or throughout their lifespan, characterized as 'Unlimited' or 'indeterminate' growth.

Limited Growth: Once leaves, fruits, and flowers reach a certain size, their growth ceases, known as 'limited' or 'determinate' growth.

Key Factors Influencing Plant Development:

• Temperature: Growth is accelerated with rising temperatures.
• Light: Factors such as light intensity, duration, and quality influence various physiological processes in plants.
• Water: Adequate water is essential for plant growth, and they respond to water scarcity as well.
• Soil Nutrients: Plants require sufficient nutrients for proper growth, and the quality and quantity of nutrients impact their development.

Plant Growth Regulators

Various substances such as auxin, cytokinin, gibberellin, etc., are applied to plants to regulate their growth. Development encompasses all the changes occurring throughout the life cycle of a plant. Plants follow different pathways due to varying environmental conditions and structural formations. The leaves of a young plant exhibit different structures compared to those of a mature plant. Development represents the entirety of growth and differentiation, influenced by both external and internal factors.

Cell Differentiation:

The process of cell differentiation involves three stages: cell division, cell enlargement, and cell differentiation. The initial phases contribute to the enlargement of plant cells, while the third stage brings about specialization in the cells. Differentiation entails significant alterations in the cell wall and cellular components. Once a cell differentiates, it loses its ability to divide. However, under certain conditions, a dedifferentiated cell may regain the capacity for division, a phenomenon known as dedifferentiation. Dedifferentiation commonly occurs in plants during secondary growth and in wound healing processes. Dedifferentiated cells can undergo division and generate new cells, eventually redifferentiating to become part of permanent tissue. This process is termed "redifferentiation". Meristematic cells serve as prime examples of redifferentiated cells.

Development:

Development refers to the combined processes of growth and differentiation, regulated by both external and internal factors. Among the internal factors governing growth and development are "plant hormones".

Phases of Growth in Plants

Meristematic Phase:

• Consists of rapidly dividing isodiametric cells with abundant protoplasm and prominent nuclei.
• Primary cell walls are thin, made of cellulose, and linked by plasmodesmata.

• Cells near meristematic cells elongate due to vacuole growth, known as the zone of elongation.
• Cell expansion and new cell wall production occur during this phase.

• Cells reach maximal size, stop dividing, and undergo specialization for specific functions.
• Cell growth involves cell division, expansion, and differentiation processes.

Phases of Plant Growth

• Formative or Meristematic Phase: The meristematic zone at the plant's tip consists of rapidly dividing cells with thin primary cell walls.
• Elongation Phase: Cells in this phase elongate due to vacuole enlargement and new cell wall formation.
• Maturation Phase: Cells in the maturation phase cease dividing and reach their largest size.

Plant Differentiation

Plant differentiation involves cells maturing to carry out specific functions, with structural modifications occurring as cells specialize.

Differentiation Process in Plants

• Dedifferentiation Process: Dedifferentiation refers to cells losing and regaining the ability to divide, crucial for generating new cells.
• Redifferentiation Process: Redifferentiation involves cells losing their ability to divide but becoming specialized for specific functions.

Plant Growth Overview

• Plants exhibit various stages of growth due to their indeterminant nature enabled by meristems.
• Growth can be measured through parameters like increase in fresh weight, dry weight, length, area, volume, and cell number.

Phases of Growth

• Developmental/Meristematic Phase: Involves cell division through mitosis leading to growth and differentiation.
• Cell Enlargement/Elongation and Differentiation: Tissues and organs expand through various processes.
• Cell Maturation: Cells acquire specific sizes and structures based on their functions.

Growth Rates and Conditions

• Growth rate is the increased growth per unit of time.
• Conditions for growth include water, oxygen, appropriate temperature, light, and gravity.

Example of Plant Growth and Development

• Observation of stem growth.
• Plant development involves stretching of stems and roots, as well as thickening in woody plants.

Development Process

• Progression from seed germination to plant growth, flowering, and fruiting.
• Development entails growth and differentiation leading to the formation of complex plant structures.

Plasticity in Plant Growth

• Plants exhibit plasticity in leaf shapes between juvenile and mature phases known as heterophylly.
• Examples include Cotton, Coriander, and Larkspur.

Plant Growth Regulators

• Plant growth regulators are natural chemical compounds regulating growth processes like cell division, elongation, and responses to stimuli.

[Intext Question]

Physiological Effects Of Plant Growth Regulators

• Growth is the constant expansion of an organism's size, accompanied by various metabolic processes.
• Plants undergo continuous development throughout their life cycle, responding to their environment.
• Development in plants is the result of differentiation and growth controlled by internal and external factors.

Plant Growth Regulators

Classification of PGRs

• Plant Growth Regulators (PGRs) are categorized based on their roles in plant growth processes.
• Two main classes are growth-promoting PGRs and growth-inhibiting PGRs.

Plant Promoters

• Promoter hormones like Auxin, Gibberellin, and Cytokinin stimulate plant growth.
• For example, Auxins aid in root formation and apical dominance control in plants.
• Gibberellins promote stem elongation, fruit enlargement, and delay senescence.
• Cytokinins encourage cell division and lateral shoot growth.

Plant Inhibitors

• Inhibitor hormones like Abscisic Acid and Ethylene hinder plant growth.
• For instance, Ethylene accelerates fruit ripening and leaf senescence.
• Abscisic Acid inhibits seed germination and promotes stomatal closure under stress.

Physiological Effects of Specific PGRs

Auxins

• Auxins like IAA and synthetic auxins promote rooting and flowering in plants.
• They also regulate apical dominance and leaf abscission.

Gibberellins

• Gibberellins stimulate stem elongation, fruit growth, and seed germination.
• They are used in agriculture to increase crop yield and hasten maturation.

Cytokinins

• Cytokinins promote cell division, lateral shoot growth, and delay leaf senescence.
• They are crucial for reversing apical dominance in plants.

Ethylene

• Ethylene accelerates ripening, germination, and root hair development in plants.
• It plays a vital role in plant growth responses to environmental stimuli.

Abscisic Acid

• Abscisic Acid regulates seed dormancy, stomatal closure, and stress responses in plants.
• It interacts with other PGRs to modulate growth and development processes.

Vernalization

Growth is essential for all living organisms. In plants, growth involves an irreversible increase in size, affecting their height, weight, and volume. Plant reproduction occurs through flowers, which develop from the vegetative apex transforming into a reproductive structure.

Factors Influencing Plant Reproduction

• Photoperiod (duration of light)
• Temperature

Role of Temperature in Flowering

• Many plants require cold treatment to transition into the flowering stage after receiving the correct photoperiod. Temperature significantly influences flowering, with light playing a secondary role. Winter treatment is essential for crops like cereals to initiate flowering.
• Biennial plants, such as wheat and barley, require cold conditions before flowering. They may not flower until they have undergone the necessary winter treatment.

Vernalization Process

• Some plants need vernalization, a cold treatment, to flower. By exposing seeds to cold temperatures, biennial plants can be induced to flower in the same year they are planted.
• The USSR developed vernalization to expedite crop production by reducing the time between sowing and harvesting.

Devernalization

• When the vernalization process is reversed by subjecting vernalized seeds to high temperatures, it's called devernalization. Devernalized plants can undergo vernalization again by exposing them to cold temperatures.

Comparison: Photoperiodism vs. Vernalization

• Definition: Photoperiodism is determined by the length of day and night, while vernalization requires cold temperatures for flowering.
• Hormones: Florigen hormone regulates photoperiodism, while vernalin hormone controls vernalization.
• Pigments: Phytochrome pigment is crucial for photoperiodism, whereas vernalization does not involve any specific pigment.
• Stimulus: Leaves detect the stimulus in photoperiodism, whereas meristematic cells of the shoot tip and embryonal cells respond to vernalization.
• Flowering: Photoperiodism triggers flowering after a specific photoperiod, whereas vernalization induces flowering after cold treatment.

Seed Dormancy

• Growth is defined as an irreversible increase in dry weight, mass, or volume of a cell, organ, or organism.
• Plant growth occurs in three phases: formative, enlargement, and differentiation.
• Seed dormancy refers to seeds being unable to germinate even under favorable conditions due to a need for a rest period.

Types of Seed Dormancy

• Innate Dormancy: Seeds are incapable of germination even under suitable conditions due to immature embryos in certain species.
• Enforced Dormancy: Seeds can't germinate due to environmental factors like moisture, oxygen, light, and temperature.
• Induced Dormancy: Seeds fail to germinate even under favorable conditions after being subjected to adverse conditions.

Causes of Seed Dormancy

• Factors like temperature, light, seed coat impermeability, and embryo immaturity contribute to seed dormancy.

Methods of Breaking Seed Dormancy

• Natural Breaking: Dormancy ends when seeds encounter suitable environments or natural actions like temperature changes.
• Artificial Reduction: Methods like applying pressure and treatments with heat or cold can break dormancy.

Artificial Reduction of Seed Dormancy

• Methods include applying hydraulic pressure to weaken tough seed coats and using hot water to remove inhibitors.

Treatment to Break Dormancy in Seeds

• Embryo Treatments: Techniques like stratification and high-temperature treatment help overcome dormancy.
• Seed Coat Treatment: Scarification processes make hard seed coats permeable to water or gases.
• Chemical Treatments: Plant growth regulators or chemicals aid in inducing germination.

Importance of Seed Dormancy

• Seed dormancy allows seeds to remain dormant and unharmed during adverse conditions, aiding in preservation and dispersion.
• Dormancy helps in seed storage for future use and is advantageous for desert plants.

Role of Hormones in Seed Dormancy

• Abscisic acid (ABA) maintains seed dormancy, while other hormones like auxin, gibberellin, ethylene, and cytokinin play roles in germination and dormancy regulation.