1. Seed Germination

Definition and Process of Germination

• Germination is the process by which a plant grows from a seed into a seedling.
• Seeds remain dormant until conditions are favorable for germination.
• Germination begins with the absorption of water, followed by the resumption of various metabolic processes and structural changes in the seed’s organelles.
• The seed coat breaks open, and the root (radicle) emerges first, followed by the shoot (plumule) containing the leaves and stem.

Factors Affecting Germination

Water

• Water is essential for activating the processes in the seed that result in embryo growth.
• Germination begins with the absorption of water through the seed coat, causing the embryo’s cells to enlarge.
• Overwatering can cause poor germination due to a lack of oxygen.

Time

• The passage of time is necessary for germination, as seeds require a period of dormancy before they can germinate.

Temperature

• Optimal temperature is crucial for seed germination.
• Temperature affects the absorption of water, intake of oxygen, and chemical reactions in germinating seeds.
• There are three stages of temperature for seed germination: minimum, maximum, and optimum.
• The largest number of seeds of a particular species germinate at the optimum temperature, which is generally between 26.5 – 36°C.

Oxygen

• Oxygen is essential for seed germination, as it is required for cellular respiration.
• Germination percentage may be reduced if the oxygen percentage falls below 20%.
• Over-watered or poorly drained seedbeds can limit the oxygen supply, diminishing germination percentage.

Light

• Light can affect seed germination, as some seeds require light to germinate, while others need darkness.
• Sunlight supports the germination process by warming the soil.
• Some seeds, such as lettuce, require light exposure to germinate, while darkness inhibits germination in these plants.

2. Phases of Plant Growth and Plant Growth Rate

Arithmetic Growth

• Arithmetic growth occurs when one daughter cell continues to divide while the other matures.
• The elongation of roots is an example of arithmetic growth.
• Growth occurs in a linear manner, with a constant rate of increase.
• The formula for calculating arithmetic growth is: Lt = L0 + rt, where Lt is the length at time t, L0 is the initial length, r is the growth rate, and t is the time interval.

Geometric Growth

• Geometric growth is characterized by a slow growth in the initial stages and rapid growth during the later stages.
• Both daughter cells derived from mitosis retain the ability to divide, but the rate slows down due to nutrient deficiency.
• Geometric growth can be represented by a sigmoid growth curve, which has three phases: lag phase, log phase, and stationary phase.
• Lag phase: The initial phase of slow growth.
• Log phase: Also known as the exponential phase, this phase exhibits rapid growth.
• Stationary phase: The growth rate slows down and becomes stable due to limiting factors.

Sigmoid Growth Curve

• The sigmoid growth curve is an S-shaped curve that represents the geometric growth in plants.
• The curve is divided into three phases: lag phase, log phase, and stationary phase.
• The formula for calculating geometric growth is: W1 = W0ert, where W1 is the final size, W0 is the initial size, r is the growth rate, t is the time of growth, and e is the base of natural logarithms.

3. Conditions of Growth

Role of Light in Plant Growth

• Light is essential for photosynthesis, the process that converts light, oxygen, and water into carbohydrates (energy) for plant growth.
• Light intensity influences leaf color, stem length, and flowering.
• Plants require different light intensities based on their specific needs.
• Light duration and quality also play a role in plant growth.

Role of Temperature in Plant Growth

• Temperature is a critical factor affecting plant growth and development.
• Each plant species has a suitable temperature range for optimal growth.
• Higher temperatures within the suitable range generally promote shoot growth, including leaf expansion and stem elongation.
• Extreme temperatures can inhibit plant growth.

Role of Water in Plant Growth

• Water is essential for seed germination, root growth, and nutrient transportation within the plant.
• Water is necessary for photosynthesis, the process by which plants use energy from the sun to create their own food.
• Water also plays a role in transpiration, a process that helps plants regulate their temperature.
• The availability of water can be a limiting factor for plant growth in certain environments.

Role of Nutrients in Plant Growth

• Nutrients are essential elements that plants use for growth, development, and reproduction.
• There are 17 different essential nutrients for plants, each with a specific function.
• Nutrients are divided into two categories: micronutrients and macronutrients.
• Macronutrients are used in large amounts, while micronutrients are used in smaller amounts.
• Plant roots absorb nutrients from the soil, and various factors such as rainfall, pH, temperature, and organic matter can affect nutrient uptake.

4. Differentiation, Dedifferentiation, and Redifferentiation in Plants

Differentiation in Plants

• Differentiation is the process by which unspecialized cells (meristematic cells) develop into specialized cells with specific functions.
• During differentiation, cells undergo structural and functional changes to perform specific tasks.
• Differentiation occurs in the apical meristem, which gives rise to the primary tissues of the plant.
• Examples of differentiated cells include root cells, leaf cells, and vascular tissue cells.

Dedifferentiation in Plants

• Dedifferentiation is the process by which specialized cells revert to a less specialized state, regaining the ability to divide and differentiate again.
• Dedifferentiation occurs in response to injury or stress, allowing the plant to regenerate damaged tissues.
• Dedifferentiated cells can form new meristematic regions, such as callus tissue in wounded plants.
• Dedifferentiation is essential for plant tissue culture techniques, where cells are induced to revert to a less specialized state and then stimulated to form new plantlets.

Redifferentiation in Plants

• Redifferentiation is the process by which dedifferentiated cells regain their specialized functions and structures.
• Redifferentiation occurs after dedifferentiation and is essential for tissue repair and regeneration.
• During redifferentiation, cells undergo changes in gene expression, allowing them to regain their specialized functions.
• Examples of redifferentiation include the formation of new vascular tissue in a wounded plant or the development of new organs in a regenerated plant.

5. Sequence of Developmental Processes in a Plant Cell

Overview of Plant Cell Development

• Plant cell development involves a series of changes that occur in the cell from cell division to maturation.
• The source of all plant cells, such as xylem, phloem, and pith, is the apical and shoot meristem.
• Plant cell development can be divided into several stages: cell division, elongation, differentiation, and maturation.

Cell Division

• Cell division occurs at the meristem, where cells continuously divide to produce new cells.
• The process of cell division involves the replication of DNA and the separation of the replicated chromosomes into two daughter cells.

Elongation

• After cell division, the daughter cells undergo elongation, which is the process of increasing in size.
• Elongation is essential for the overall growth of the plant, as it contributes to the expansion of plant tissues.

Differentiation

• Differentiation is the process by which unspecialized cells (meristematic cells) develop into specialized cells with specific functions.
• During differentiation, cells undergo structural and functional changes to perform specific tasks, such as the formation of root cells, leaf cells, and vascular tissue cells.

Maturation

• Maturation is the final stage of plant cell development, where cells reach their fully differentiated state and perform their specialized functions.
• At this stage, cells have completed their growth and differentiation processes and are now functioning as part of the plant’s overall structure and physiology.

6. Growth Regulators in Plants

Auxin: Role in Apical Dominance, Rooting, and Leaf Abscission

• Auxin is a plant hormone that plays a crucial role in various aspects of plant growth and development.
• It is involved in apical dominance, where the growth of lateral buds is inhibited by the presence of the apical meristem.
• Auxin promotes adventitious root formation, which is essential for the development of new roots from non-root tissues.
• It also plays a role in leaf abscission, the process by which leaves are shed from the plant.

Gibberellin (GA): Stimulates Cell Division and Elongation, Breaks Dormancy, Speeds Germination

• Gibberellins are plant hormones that affect cell division and elongation, contributing to overall plant growth.
• They play a role in breaking seed dormancy, allowing seeds to germinate when conditions are favorable.
• Gibberellins also speed up the germination process, ensuring rapid growth and development of the plant.

Cytokinin: Role in Cell Division, Shoot Initiation, and Delay of Senescence

• Cytokinins are essential plant hormones that regulate various aspects of plant growth, including cell division, shoot meristem size, leaf primordia number, and leaf and shoot growth.
• They promote cell division and increase cell expansion during the proliferation and growth of plant tissues.
• Cytokinins also play a role in shoot initiation, maintaining the growth potential of shoot apical meristems.
• Additionally, they are involved in delaying leaf senescence, allowing leaves to remain functional for a longer period.

Ethylene: Ripening Agent, Stimulates Leaf and Fruit Abscission, Root Hair Growth Inhibition, Adventitious Root Growth During Flooding, and Epinasty

• Ethylene is a gaseous plant hormone that plays a key role in various aspects of plant growth, development, and senescence.
• It acts as a ripening agent, promoting the maturation of fruits.
• Ethylene stimulates leaf and fruit abscission, allowing the plant to shed leaves and fruits when necessary.
• It inhibits root hair growth and promotes adventitious root growth during flooding, helping plants adapt to waterlogged conditions.
• Ethylene also causes epinasty, a downward bending of leaves, stems, and other plant organs.

Abscisic Acid (ABA): Role in Seed Dormancy, Overcoming the Effect of Hormones

• Abscisic acid (ABA) is a plant hormone involved in various aspects of plant growth and development, including seed dormancy.
• ABA reversibly arrests embryo development at the brink of radicle growth initiation, inhibiting water uptake and preventing germination.
• It plays a role in overcoming the effect of other hormones, such as auxins and gibberellins, allowing seeds to remain dormant until conditions are favorable for germination.

7. Seed Dormancy

Definition, Causes, and Types of Seed Dormancy

• Seed dormancy is a state in which seeds do not germinate despite the presence of necessary conditions, such as temperature, humidity, oxygen, and light.
• It is caused by factors such as hard seed coat impermeability or a lack of supply and activity of the enzymes required for germination.
• Seed dormancy can be classified into two categories: exogenous and endogenous.
• Exogenous dormancy is caused by conditions outside the seed’s control, such as environmental factors.
• Endogenous dormancy is caused by internal factors, such as hormonal structures or seed coat impermeability.
• Some common factors affecting seed dormancy include light, temperature, hard seed coat, period after ripening, germination inhibitors, immaturity of the seed embryo, impermeability of seed coat to water or oxygen, mechanically resistant seed coat, and presence of high concentrate solutes.

Methods to Overcome Seed Dormancy

Scarification

• Scarification is a process that weakens the seed coat, allowing the seed to absorb water and germinate.
• Mechanical scarification involves using sandpaper, a hammer, a knife, or a tumbler to weaken the seed coat.
• Hot water scarification involves dropping seeds into hot water (77 to 100°C), removing them from heat, and allowing them to cool and soak for 24 hours.
• Acid scarification involves exposing seeds to a concentrated acid, such as sulfuric acid, for a set period of time before rinsing and neutralizing the acid with baking soda.

Temperature Treatments

• Some seeds require exposure to low temperatures (0-5°C) for a period of time before they can germinate.
• This process, called stratification, can be achieved by placing seeds in moist planting medium in a cold environment for a period of time.

Light Treatments

• Some seeds require exposure to light in order to germinate, such as lettuce (Lactuca sativa).
• Providing continuous or periodic exposure to light can help overcome seed dormancy in these species.

8. Vernalisation

Definition and Process of Vernalisation

• Vernalisation is the process by which exposure to a prolonged period of cold temperatures promotes the flowering of certain plants.
• It is a form of dormancy-breaking mechanism that ensures plants flower at the appropriate time, usually in spring or summer.
• During vernalisation, plants undergo physiological changes that make them competent to flower when the temperature rises.
• Vernalisation is a temperature-dependent process, with most plants requiring temperatures between 0-10°C for a specific duration.

Examples of Vernalisation in Plants: Wheat, Beet, Cabbage, Turnips, and Onions

• Wheat: Winter wheat varieties require vernalisation to induce flowering and produce a crop.
• Beet: Some sugar beet varieties require vernalisation to transition from vegetative to reproductive growth.
• Cabbage: Biennial cabbage varieties require vernalisation to initiate the formation of a flower head.
• Turnips: Turnips are biennial plants that require vernalisation to produce flowers and seeds.
• Onions: Some onion varieties require vernalisation to initiate bulb formation and flowering.

Difference Between Vernalisation and Photoperiodism (Table)

9. Photoperiodism

Definition and Process of Photoperiodism

• Photoperiodism is the physiological reaction of plants to the length of the day.
• It is a process that allows plants to measure and sense day and night time lengths, using light-sensitive receptors such as phytochrome.
• Photoperiodism influences various aspects of plant growth and development, including flowering, seed germination, and dormancy.
• Plants use photoperiodism to determine when to flower, ensuring that they develop flowers at appropriate times of the year.

Classification of Plants Based on Photoperiodism: Long-Day Plants, Short-Day Plants, and Day-Neutral Plants

• Plants can be classified into three groups based on their photoperiodic requirements for flowering: long-day plants, short-day plants, and day-neutral plants.

Long-Day Plants

• Long-day plants require a certain duration of light exposure longer than a critical day length to induce flowering.
• Examples of long-day plants include lettuce, spinach, and wheat.

Short-Day Plants

• Short-day plants require a certain duration of light exposure shorter than a critical day length to induce flowering.
• Examples of short-day plants include rice, soybean, and chrysanthemum.

Day-Neutral Plants

• Day-neutral plants are not affected by the photoperiod and can flower under various day lengths.
• Examples of day-neutral plants include tomato, cucumber, and sunflower.

Role of Photoperiodism in Plant Growth and Development

• Photoperiodism plays a crucial role in regulating plant growth and development, including the timing of flowering, seed germination, and dormancy.
• It helps plants adapt to seasonal changes, ensuring that they flower and reproduce at the appropriate times.
• Photoperiodism also affects other aspects of plant growth, such as stem elongation, leaf expansion, and the formation of storage organs.