Plant Growth and Development class 11th NEET notes

What is Plant Growth and Development?

Plants, like all living organisms, undergo a series of changes throughout their life cycle. These changes are collectively referred to as growth and development. While growth refers to an irreversible increase in size, volume, or number of cells, development encompasses the progression from a single cell to a mature, functional plant with specialized structures.

Plant growth is indeterminate, meaning it can continue throughout the plant's life, especially in certain regions like the tips of roots and shoots (called meristems). Development, on the other hand, includes differentiation, maturation, and senescence—all tightly regulated by genetic factors and influenced by external environmental cues such as light, water, temperature, and gravity.

Phases of Growth

Growth in plants is a permanent and irreversible increase in size, volume, or number of cells. It is a fundamental characteristic of all living organisms, but in plants, growth continues throughout their lifetime due to the presence of meristems.

Plant growth occurs in a sequential manner, and this progression is divided into three distinct phases:

1. Meristematic Phase – Active cell division occurs in the apical meristems (root and shoot tips).
2. Elongation Phase – Cells increase in size by elongating and expanding, usually behind the meristematic zone.
3. Maturation Phase – Cells attain their final size, shape, and function; cell division stops.
Each phase is critical for the development and structure of the plant, and together they ensure continuous growth in different parts of the plant body.

Growth Curve

The growth curve represents the pattern of growth in living organisms over time. In plants, when growth is plotted against time, it typically results in a sigmoid (S-shaped) curve, especially during periodic measurement of parameters like height, weight, volume, or number of cells.

This sigmoid growth curve has four main phases

1. Lag Phase – Initial slow growth as cells adjust to new conditions.
2. Log (Exponential) Phase – Rapid growth due to active cell division.
3. Decelerating Phase – Growth slows down as resources become limited.
4. Stationary Phase – Growth stops; equilibrium is reached.
This curve helps in understanding how external and internal factors influence plant growth over time and is essential for studying plant development and productivity.

Conditions for Growth

Plant growth is a highly regulated process that depends on various internal and external factors. For growth to occur efficiently, certain essential conditions must be met. These conditions ensure proper cell division, elongation, and differentiation.
The major external conditions required for plant growth include:

1. Water – Necessary for cell enlargement and turgidity.
2. Oxygen – Required for cellular respiration to provide energy.
3. Nutrients – Essential elements like nitrogen, phosphorus, potassium, etc., support metabolic and structural functions.
4. Temperature – Growth occurs optimally within a specific temperature range.
5. Light – Influences processes like photosynthesis and photoperiodism.
Apart from these, plant hormones (phytohormones) also play a vital internal role in regulating and coordinating growth processes.

Understanding these conditions helps in improving crop productivity and plant health under natural or controlled environments.

Plant Growth Regulators

Plant Growth Regulators (PGRs), also known as plant hormones, are chemical substances that influence various physiological processes in plants. They regulate growth, development, flowering, fruiting, dormancy, and senescence.
These regulators are produced in minute quantities in specific parts of the plant and are transported to other regions to perform their functions.

Plant growth regulators are broadly classified into two types:

1. Growth Promoters – Promote growth activities
Examples: Auxins, Gibberellins, Cytokinins
2. Growth Inhibitors – Inhibit or slow down growth

Examples: Abscisic Acid (ABA), Ethylene

Auxins

Auxins are the first discovered class of plant growth regulators, playing a crucial role in growth and developmental processes. The most common natural auxin is IAA (Indole-3-acetic acid).
Auxins are produced mainly in the shoot apical meristems and young leaves, from where they move downward (basipetal transport) to exert their effects.

Characteristics of Auxins:

Promote cell elongation in stems and coleoptiles
Help in root initiation in stem cuttings
Involved in apical dominance (suppression of lateral buds)
Delay leaf and fruit abscission
Aid in vascular tissue differentiation

Gibberellins

Gibberellins are a group of plant growth regulators that play a significant role in promoting stem elongation, seed germination, and flowering. The most common and extensively studied gibberellin is GA₃ (Gibberellic Acid).

They were first discovered from a fungus Gibberella fujikuroi, which caused excessive elongation in rice plants — a condition known as “foolish seedling disease.”

Characteristics of Gibberellins:

Stimulate cell division and elongation in stems and leaves
Promote bolting (sudden elongation) in rosette plants like cabbage
Break seed and bud dormancy
Enhance fruit size and improve grape bunch compactness
Delay senescence (aging) in some fruits like citrus
Used in malting of barley during brewing

Cytokinins

Cytokinins are a class of plant growth regulators that primarily promote cell division (cytokinesis) and influence various aspects of plant growth and development. The first discovered cytokinin was Kinetin (from herring sperm DNA), and Zeatin is a naturally occurring cytokinin found in corn.

Cytokinins are synthesized mainly in root apical meristems and transported upward to other plant parts.

Characteristics of Cytokinins:

Promote cell division and differentiation, especially in combination with auxins
Encourage shoot formation in tissue culture
Delay senescence (aging) of leaves by promoting nutrient mobilization
Overcome apical dominance, promoting lateral bud growth
Involved in chloroplast development and leaf expansion

Abscisic Acid (ABA)

Abscisic Acid (ABA) is a plant growth inhibitor that primarily regulates stress responses, seed dormancy, and growth suppression. It is often referred to as the “stress hormone” of plants.

Unlike growth promoters like auxins and gibberellins, ABA slows down growth and helps the plant adapt to adverse conditions, such as drought or cold.

Characteristics of ABA:

Induces and maintains seed dormancy, preventing premature germination
Promotes stomatal closure during water stress to reduce transpiration
Inhibits growth and cell elongation
Plays a key role in abscission (shedding of leaves and fruits)
Helps plants survive abiotic stress (drought, salinity, cold)

Ethylene

Ethylene is a unique gaseous plant hormone that plays a major role in fruit ripening, senescence, and stress responses. It is the only gaseous plant growth regulator and is active at very low concentrations.

Produced in almost all parts of the plant, especially during ripening, aging, and stress, ethylene acts both as a growth promoter and inhibitor, depending on the context.

Characteristics of Ethylene:

Promotes fruit ripening (e.g., in bananas, mangoes, and tomatoes)
Stimulates leaf and fruit abscission
Accelerates senescence (aging of plant parts)
Induces flowering in some plants (e.g., pineapple)
Causes epinasty (downward bending of leaves)
Involved in seed germination and response to mechanical stress

Each PGR has a unique role, and often, the balance and interaction among them determine the final outcome in plant development. These regulators are crucial for agricultural practices such as rooting, fruit ripening, and tissue culture.

Seed Dormancy and Germination

Seeds are the reproductive units of flowering plants, capable of developing into a new plant. However, a seed does not always germinate immediately after dispersal. It undergoes two key physiological stages:

Seed Dormancy

Seed dormancy is a temporary suspension of growth, even under favorable environmental conditions. It is a survival mechanism that prevents germination during unsuitable seasons and ensures that seeds sprout only when conditions are ideal.

Dormancy can be caused by:

Hard seed coat
Immature embryo
Chemical inhibitors

Seed Germination

Germination is the process by which a dormant seed resumes growth, forming a seedling. It begins with water absorption (imbibition) and involves metabolic activation, leading to the emergence of the radicle (root) and plumule (shoot).
Conditions necessary for germination:

Water
Oxygen
Suitable temperature
In some cases, light or darkness

Vernalisation and Photoperiodism

Plants respond to environmental signals to regulate their flowering and growth. Two such important environmental factors are temperature and light duration, which influence when and how a plant flowers.

Vernalisation

Vernalisation is the process of inducing flowering by exposure to prolonged cold or low temperatures. It is essential for certain plants (especially winter varieties) to experience a cold period before they can initiate flowering.

Common in: Wheat, Barley, Cabbage

Function: Prevents premature flowering and ensures reproductive success in spring

Photoperiodism

Photoperiodism is the response of plants to the duration of light and dark periods (day length). It plays a key role in regulating flowering time.
Based on their photoperiodic response, plants are classified into: