Understand the relationship between plant growth, differentiation, and development.
Recognize the factors that influence plant growth, including intrinsic and extrinsic factors.
Learn about the role of meristems in indeterminate plant growth.
Distinguish between different types of plant growth regulators and their effects.
Understand processes like photoperiodism, vernalisation, and seed dormancy.
Growth
Growth is a fundamental feature of all living organisms, characterized by an irreversible and permanent increase in the size of a cell, organ, or organism. It is a process driven by metabolic reactions, both anabolic and catabolic, which require energy. In plants, growth begins with the germination of seeds, contingent upon favorable environmental conditions. If conditions are unfavorable, seeds enter dormancy, suspending metabolic activity until favorable conditions return.
Growth Characteristics:
It is an irreversible increase in the size of cells, tissues, or organs.
Growth requires energy, primarily from metabolic processes.
Seed germination initiates plant growth under favorable environmental conditions.
Indeterminate Growth:
Plants exhibit indeterminate growth due to the presence of meristems, which are groups of cells that retain the ability to divide throughout the plant’s life.
Primary growth occurs at root apical meristems and shoot apical meristems, contributing to the elongation of the plant.
In dicots and gymnosperms, lateral meristems (vascular cambium and cork cambium) are responsible for secondary growth, increasing the girth of plant organs.
Growth Measurement:
Growth can be measured in various ways: by fresh weight, dry weight, length, area, volume, and cell number.
Example: A maize root apical meristem can produce over 17,500 new cells per hour, while watermelon cells can expand up to 350,000 times their original size.
Phases of Growth: Growth occurs in three distinct phases:
Meristematic Phase: Cells actively divide in the meristematic regions at the root and shoot apices. These cells are characterized by abundant protoplasm, large nuclei, and primary, thin cell walls.
Elongation Phase: Cells near the meristematic zone elongate due to vacuolation, cell wall expansion, and new cell wall deposition.
Maturation Phase: Cells further from the meristem reach their maximum size, complete wall thickening, and undergo protoplasmic modifications, leading to fully functional tissues.
Growth Rates:
Arithmetic Growth: In this form, after mitotic division, one daughter cell continues to divide while the other differentiates. The growth rate remains constant, producing a linear growth curve.
Geometric Growth: Both daughter cells continue to divide after mitosis, resulting in exponential growth. This pattern is represented by a sigmoid (S-shaped) curve, consisting of lag, log, and stationary phases.
Differentiation, Dedifferentiation, and Redifferentiation
Differentiation refers to the process where cells derived from meristems become specialized for specific functions. During differentiation, cells undergo structural changes in their protoplasm and cell walls.
Differentiation:
Cells undergo structural and functional changes to perform specific roles.
For example, tracheary elements lose their protoplasm and develop lignified secondary cell walls for water conduction under tension.
Dedifferentiation:
Some differentiated cells regain their ability to divide under certain conditions. This process is termed dedifferentiation.
Example: Formation of interfascicular cambium and cork cambium from fully differentiated parenchyma cells.
Redifferentiation:
After dedifferentiated cells divide, they can again lose their ability to divide and differentiate into specific tissues. This process is called redifferentiation.
Example: Tissues like xylem and phloem in a woody dicot plant are products of redifferentiation.
Important Note: Differentiation in plants is considered open because the final structure of the differentiated cell is influenced by its position in the organ.
Development
Development in plants encompasses the sum of growth and differentiation, occurring throughout the life cycle from germination to senescence.
Plasticity:
Plants exhibit plasticity, meaning they can follow different developmental pathways in response to environmental or life phase changes. This results in structures like heterophylly, where leaves of juvenile and mature plants, or leaves in water and air, differ in shape.
Intrinsic and Extrinsic Factors:
Development is controlled by intrinsic factors, such as genetic information and plant growth regulators (PGRs), and extrinsic factors like light, temperature, water, and nutrients.
Plant Growth Regulators
Plant Growth Regulators (PGRs) are chemicals that influence various developmental processes in plants. These regulators are divided into two groups: growth promoters and growth inhibitors.
PGR Type
Examples
Functions
Auxins
IAA, IBA, NAA, 2,4-D
Cell elongation, root initiation, apical dominance, fruit development, parthenocarpy, herbicide
Gibberellins
GA1, GA3, GA4
Stem elongation, bolting, seed germination, fruit setting, delaying senescence
Auxins like Indole-3-Acetic Acid (IAA) promote cell elongation, root formation, and apical dominance. They prevent premature fruit drop and help initiate parthenocarpy.
Synthetic auxins like Naphthalene Acetic Acid (NAA) and 2,4-Dichlorophenoxyacetic Acid (2,4-D) are used to regulate growth and as herbicides.
Gibberellins:
Gibberellins stimulate stem elongation, break dormancy, and are used to improve fruit quality, enhance grape stalk length, and speed up the malting process in brewing.
Cytokinins:
Cytokinins stimulate cell division, promote leaf growth, and delay senescence. They are essential for the growth of lateral shoots and play a crucial role in overcoming apical dominance.
Ethylene:
Ethylene regulates fruit ripening, abscission, and enhances the respiratory rate during fruit ripening (a process called respiratory climactic). It is also used to control flowering in pineapples and ripening in tomatoes.
Abscisic Acid (ABA):
ABA acts as a growth inhibitor and is known as the stress hormone. It induces seed dormancy, facilitates stomatal closure during water stress, and regulates responses to environmental stresses.
Photoperiodism
Photoperiodism refers to a plant’s ability to measure the duration of light exposure to trigger flowering.
Short Day Plants (SDP):
Flowering occurs when the plant is exposed to light for a period shorter than a critical duration. Examples: Chrysanthemums, tobacco.
Long Day Plants (LDP):
These plants flower when exposed to light periods longer than the critical duration. Examples: Wheat, radish.
Day-neutral Plants:
These plants do not rely on specific light durations for flowering. Examples: Tomato, cotton.
Light Perception:
The leaves are the primary sites for perceiving light and dark durations. Once the necessary inductive photoperiod is met, a signal is sent to the shoot apices, inducing flowering.
Vernalisation
Vernalisation is the promotion of flowering by exposing plants to low temperatures.
Winter and Spring Varieties:
Winter varieties of crops like wheat and rye need exposure to cold temperatures before flowering, while spring varieties flower without cold treatment.
Biennials:
Biennial plants (e.g., carrot, cabbage) require cold treatment to initiate flowering in their second year of growth.
Important Note: Vernalisation ensures plants have adequate time to reach maturity and prevents premature flowering.
Seed Dormancy
Seed dormancy is a state where seeds fail to germinate even under favorable environmental conditions.
**Causes of Dormancy**:
Hard, impermeable seed coats, chemical inhibitors like abscisic acid, and immature embryos can induce dormancy.
Breaking Dormancy:
Dormancy can be overcome by methods like scarification (mechanically weakening the seed coat), chilling treatments, or the application of chemicals like gibberellic acid.
MCQ: What is the primary role of Abscisic Acid (ABA) in plants? Answer: Induces seed dormancy and regulates stress responses.