RBSE Solutions Class 12 Biology Chapter 13 Plant Growth

RBSE Class 12 Biology Chapter 13 Multiple Choice Questions

Question 1. Which plant hormone was first of all discovered?
(a) Auxin
(b) Gibberellin
(c) Ethylene
(d) Cytokinin
Answer: (a) Auxin
In simple words: Auxin was the very first plant hormone that scientists found. It plays a big role in how plants grow, especially in their stems.

🎯 Exam Tip: Remember Auxin as the initial discovery in plant hormones; it's a fundamental concept in plant growth regulation.

Question 2. Which hormone is sprayed on rosette plant of Cauliflower to convert it into a long shoot?
(a) IAA
(b) ABA
Answer: (Not provided in source)
In simple words: The correct hormone helps a short, leafy cauliflower plant grow a tall shoot.

🎯 Exam Tip: Specific hormones like gibberellins are known to cause bolting (stem elongation) in rosette plants, which can be important for seed production.

Question 3. A hormone found in a gaseous state is?
(a) Auxin
(b) Gibberellin
(c) Cytokinin
(d) Ethylene
Answer: (d) Ethylene
In simple words: Ethylene is special because it's a plant hormone that works as a gas, helping with things like fruit ripening.

🎯 Exam Tip: Identify ethylene as the only gaseous plant hormone; its gas form allows it to influence distant parts of the plant, especially in ripening.

Question 4. Which hormone is most active in plants at the time of leaf shedding (in autumn season)?
(a) IAA
(b) ABA
(c) GA
(d) All of the options
Answer: (b) ABA
In simple words: During autumn, when leaves fall off, ABA (abscisic acid) is very active in plants, helping them prepare for winter.

🎯 Exam Tip: Associate ABA with stress responses and dormancy in plants, including leaf shedding, as it prepares plants for unfavorable conditions.

Question 5. Apical dominance is found due to?
(a) Auxin
(b) Gibberellin
(c) Cytokinin
(d) Ethylene
Answer: (a) Auxin
In simple words: Auxin is the hormone that makes the main tip of a plant grow taller, often stopping side branches from growing much.

🎯 Exam Tip: Understand that auxin produced at the shoot tip inhibits the growth of lateral buds, leading to apical dominance.

Question 6. Which of the plant hormone is still not isolated?
(a) Auxin
(b) Florigen
(c) Cytokinin
Answer: (Not provided in source)
In simple words: Scientists have found many plant hormones, but there is one that they have not yet been able to fully separate and study on its own.

🎯 Exam Tip: While many hormones are well-characterized, some, like Florigen, are still hypothesized to exist based on physiological effects rather than direct isolation.

Question 7. Which hormone promotes the growth of apical part?
(a) Cytokinin
(b) Auxin
(c) Gibberellin
(d) All of the options
Answer: (b) Auxin
In simple words: Auxin is the hormone responsible for making the main tip of the plant grow upwards and get longer.

🎯 Exam Tip: Connect auxin directly to apical growth and elongation of the shoot, as it drives the primary upward growth of plants.

Question 8. Which hormone is used to control dicotyledonous weeds in the field?
(a) IAA
(b) GA
(c) IBA
(d) 2 - 4D
Answer: (d) 2 - 4D
In simple words: The hormone 2-4D is often used by farmers to get rid of weeds that have two seed leaves, without harming crops like corn.

🎯 Exam Tip: Remember that 2-4D is a synthetic auxin commonly used as a selective herbicide for broadleaf weeds.

Question 9. Which growth regulator is known as a stress hormone in plants?
(a) IAA
(b) ABA
(C) IBA
(d) NAA
Answer: (b) ABA
In simple words: ABA, or abscisic acid, helps plants deal with tough times like drought or very cold weather, making it their "stress hormone."

🎯 Exam Tip: The role of ABA in promoting dormancy and drought resistance makes it a key regulator for plants under stress.

Question 10. Who discovered phytochrome?
(a) Borthwick and Hendricks
(b) Boysen - Jensen
(c) Garner - Allard
(d) Darwin - Went
Answer: (Not provided in source)
In simple words: Scientists found phytochrome, a special plant protein, that helps plants know about light and darkness, guiding their growth and flowering.

🎯 Exam Tip: Phytochrome is crucial for photoperiodism and other light-regulated processes, so understand its role in plant development.

RBSE Class 12 Biology Chapter 13 Very Short Answer Questions

Question 1. What do you understand by the term grand period of growth (GPG)?
Answer: The grand period of growth (GPG) is the total amount of time during which a plant experiences its fastest and most complete growth, going through all its different growth stages.
In simple words: GPG is the entire time a plant takes to grow fully, from start to finish.

🎯 Exam Tip: Define GPG clearly by emphasizing "total time period" and "maximum growth" across "all phases."

Question 2. Which substance is used for artificial ripening of fruits?
Answer: Ethaphone, also known as 2-Chloro ethyl phosphoric acid, is the substance commonly used to ripen fruits faster artificially.
In simple words: Ethaphone is used to make fruits ripen quicker.

🎯 Exam Tip: Remember Ethaphone (or ethephon) as a commercial product that releases ethylene, the natural fruit-ripening hormone.

Question 3. What is the optimum temperature for chilling (Cold) treatment?
Answer: The best temperature for chilling treatment, which helps plants grow or flower, is typically between 0°C and 5°C.
In simple words: Chilling treatment works best for plants when the temperature is very cold, from 0 to 5 degrees Celsius.

🎯 Exam Tip: Note the specific temperature range (0°C - 5°C) for optimal chilling treatment, which is critical for processes like vernalization.

Question 4. What is Zeatin?
Answer: Zeatin is the first natural cytokinin hormone discovered. Letham and Miller found it in 1964 from the endosperm, which is the food-storage part, of maize.
In simple words: Zeatin is a natural plant hormone, first found in corn, that helps cells divide.

🎯 Exam Tip: Highlight Zeatin's significance as the first naturally occurring cytokinin and its origin from maize.

Question 5. What is Vernalin?
Answer: Vernalin is a hormone-like substance that plants make in their leaves when exposed to cold temperatures. It is thought to be a chemical that prepares the way for florigen, a hormone that causes flowering.
In simple words: Vernalin is a substance in plants that helps them flower after getting cold.

🎯 Exam Tip: Understand Vernalin as a precursor to florigen, linking cold treatment (vernalization) to the initiation of flowering.

Question 8. Name two artificially synthesized auxins.
Answer: Two examples of auxins that are made artificially are NAA (Naphthalene Acetic Acid) and IBA (Indole-3 Butyric Acid).
In simple words: NAA and IBA are two man-made auxins.

🎯 Exam Tip: Be able to name and write the full forms of common synthetic auxins like NAA and IBA, recognizing their use in horticulture.

Question 9. Which hormone is used to promote parthenocarpy?
Answer: Both Auxin and Gibberellin hormones are used to cause parthenocarpy, which means fruit development without fertilization.
In simple words: Auxin and Gibberellin help make fruits grow without seeds.

🎯 Exam Tip: Remember that both auxins and gibberellins can induce parthenocarpy, leading to seedless fruits, and are often applied externally.

Question 10. What is dormancy?
Answer: Dormancy is when a seed temporarily stops germinating, even if conditions are good for growth. It's like the seed is taking a break before it can sprout.
In simple words: Dormancy is when a seed rests and does not grow, even if everything is right.

🎯 Exam Tip: Clearly define dormancy as a temporary suspension of growth, highlighting that it occurs despite favorable environmental conditions.

Question 11. Which seeds are called photoelastic?
Answer: Seeds whose germination is affected by how much light they get are called photoelastic seeds. This means light plays a role in whether they will sprout.
In simple words: Photoelastic seeds are those that need specific light conditions to sprout.

🎯 Exam Tip: Note that "photoelastic" here refers to seeds sensitive to light duration (photoperiod) for germination, although "photoblastic" is the more common botanical term for light-sensitive seeds. Always use the term provided in the question.

RBSE Class 12 Biology Chapter 13 Short Answer Questions

Question 1. What is sigmoid growth curve?
Answer: The sigmoid growth curve is an S-shaped graph that shows how growth occurs over time. It represents the typical pattern of growth in living organisms, with phases of slow start, rapid increase, and then slowing down.
The whole growth curve can be divided into four parts:
1. Lag Phase: This is the first period of growth. During this time, the cell undergoes internal changes and uses stored food, causing a slight loss in dry weight. New cells are created through cell division, and the size grows slowly.
2. Log Phase (Exponential Phase): In this phase, the cells divide and grow very quickly. The growth rate is at its maximum, meaning the organism is increasing in size and number very fast.
3. Decline Phase: After the log phase, the growth rate starts to slow down. This happens as cells mature, and the metabolic activities also become less intense.
4. Stationary or Steady Phase: In this final stage, the cells become fully mature, and the overall growth rate becomes almost constant. According to Sachs (1882), the time taken for this maximum growth is called the grand period of growth.
In simple words: The sigmoid growth curve looks like an "S" and shows how plants grow slowly at first, then very fast, and then slow down again until they stop growing bigger.

🎯 Exam Tip: Draw and label a sigmoid curve, clearly marking the lag, log (exponential), and stationary phases, and explain the key characteristics of each phase.

Question 2. Differentiate between free and bound auxin.
Answer: Free auxins are those plant hormones that can be easily taken out or separated from plant cells. On the other hand, bound auxins are those that are harder to remove and need special organic liquids to be extracted from the cells.
In simple words: Free auxins are easy to take out from cells, while bound auxins are hard to take out.

🎯 Exam Tip: The key difference lies in the ease of extraction: "free" means easily isolated, while "bound" requires harsher methods like organic solvents, indicating they are chemically linked or sequestered.

Question 3. What is "Baknae" disease?
Answer: Baknae disease, also known as foolish seedling disease, affects paddy crops. In Japan, some rice plants grew unusually tall but failed to produce flowers. These plants were infected by a fungus called Gibberella fuji-Kuro. Hori (1898) studied this disease. The plants looked unusual in the field, leading to the name "foolish seedling," as "Baknae" means foolish in Japanese.
In simple words: Baknae disease is a sickness in rice plants caused by a fungus, making them grow too tall and thin without flowering, which is why it's called "foolish seedling" disease.

🎯 Exam Tip: Remember Baknae disease is caused by Gibberella fuji-Kuro in rice, resulting in exaggerated stem elongation and inhibited flowering, providing historical context for gibberellin discovery.

Question 4. What do you understand by the term apical dominance?
Answer: Apical dominance is a plant phenomenon where the growth of side (lateral and axillary) buds is held back, either partly or fully, by the main top bud (apical bud). If this apical bud is cut off, the side buds then become active and start growing, making the plant look bushier. The strong influence of the apical bud causes the plant to grow taller rather than wider.
In simple words: Apical dominance means the top part of a plant grows the most, stopping side branches from growing much, but if you cut the top, the side branches will grow.

🎯 Exam Tip: Explain apical dominance by describing the suppression of lateral buds by the apical bud and how removing the apical bud promotes lateral branching.

Question 5. What is meant by bolting effect?
Answer: (Not provided in source)
In simple words: The bolting effect is when a plant, especially a leafy one, suddenly grows a tall stalk for flowering, often triggered by temperature changes.

🎯 Exam Tip: Define bolting as the rapid elongation of the floral stalk, especially in rosette plants, often induced by gibberellins or environmental cues like cold.

Question 7. Why the gardeners often cut the top or apical part of Hina (“Mehandi”) bush?
Answer: Gardeners often trim the top or apical part of a Hina (Mehandi) bush to encourage it to grow thicker and bushier. When the apical bud is removed, the side buds (lateral and axillary buds) become active and start growing. This stops the main stem from growing taller and instead promotes the growth of side branches, making the hedge dense and full.
In simple words: Gardeners cut the top of Mehandi bushes so that the side branches grow more, making the bush thick instead of just tall.

🎯 Exam Tip: Relate trimming to the removal of apical dominance, explaining that this promotes the development of lateral buds for a denser plant.

Question 8. What is phytochrome and what is its significance?
Answer: Phytochrome is a special bright blue or bluish-green protein found in plants that acts like a light sensor. It helps plants detect light. Phytochrome exists in two forms: PFR (phytochrome far-red, P700) and PR (phytochrome red, P660), which can change into each other. These two forms are believed to control the making of hormones needed for flowering in plants, showing how important light is for plant growth and development.
In simple words: Phytochrome is a plant protein that senses light. It has two forms that can switch, and this helps plants know when to flower based on light.

🎯 Exam Tip: Describe phytochrome as a photoreceptor existing in two interconvertible forms (PR and PFR) and explain its role in controlling photoperiodic responses like flowering.

Question 9. Why abscisic acid is called stress hormone?
Answer: Abscisic acid (ABA) is known as a stress hormone because it helps plants survive in tough environmental conditions. For instance, when leaves lack water, the amount of ABA increases. This increase makes the stomata (small pores on leaves) close, which helps the plant save water. By controlling these stress responses, ABA allows the plant to cope with challenges like drought.
In simple words: ABA is called a stress hormone because it helps plants handle difficult times like not enough water by closing the leaf pores.

🎯 Exam Tip: Explain that ABA mediates plant responses to various stresses, notably water deficiency, by inducing stomatal closure and promoting dormancy, hence its designation as a "stress hormone."

Question 10. Write short notes on the following.
1. Dormancy.
2. After the ripening period.
Answer:
1. Dormancy: Dormancy is a state where a seed temporarily stops germinating, even if environmental conditions are suitable for growth. This period of suspended growth helps the seed survive until more favorable times arrive. It's like the seed taking a rest before it can sprout.
2. After the ripening period: Some plant seeds do not sprout right away after they are formed. Instead, they go into a resting phase. During this resting period, internal changes happen within the seeds that make them capable of germinating. This necessary waiting time is called the after-ripening period, and seeds only sprout after this period is completed.
In simple words: Dormancy is when seeds rest before growing, even if conditions are good. The after-ripening period is a specific resting time seeds need to complete internal changes before they can sprout.

🎯 Exam Tip: Distinguish between dormancy (a general state of suspended growth) and after-ripening (a specific internal maturation period required for germination).

RBSE Class 12 Biology Chapter 13 Essay Type Questions

Question 1. Write notes on the following:
1. Phases of Growth.
2. Growth Kinetics.
Answer:
1. Phases of Growth: Plant growth is a complex process completed in three main phases:
    • The phase of Cell Division: This is the first step where meristematic cells continuously divide through mitosis, creating new cells. These cells are metabolically very active, have dense cytoplasm, a clear nucleus, small vacuoles, and thin, elastic cell walls made of cellulose. This phase is also known as the phase of formation.
    • The phase of cell elongation: In this second phase, the newly formed cells stop dividing and begin to stretch and grow longer in a specific direction. A large central vacuole develops, pushing the cytoplasm into a thin layer against the cell wall. The nucleus moves closer to the cell wall, and new cell wall material is made to allow for enlargement. This phase involves an irreversible increase in cell size and dry weight, also called the phase of enlargement.
    • The phase of Cell differentiation: In this final phase, the cells mature and develop into specialized permanent tissues. They undergo specific changes in shape and size according to their intended function. For example, mechanical tissue cells develop highly thickened walls with deposits of cellulose, suberin, or lignin. This phase is also referred to as the phase of maturation, where cells acquire their final physiological and biochemical properties.
2. Growth Kinetics: Growth kinetics describes how an organism or its parts grow over time, often expressed as a growth rate. This rate can be measured by an increase in cell number or dry weight per unit of time. Plant growth can be expressed in two main ways: arithmetically or geometrically.
    • Arithmetic Growth: In this type of growth, from the two cells formed after each division, one cell continues to divide, while the other develops into permanent tissue. This leads to a continuous, steady addition of new cells to the body.
    • Geometric Growth: Here, both daughter cells formed from a single cell division continue to divide, causing the cell number to double after each division (e.g., 1-2-4-8-16). This rapid form of growth is often seen in the initial divisions of a zygote. This type of growth is characterized by an exponential increase in biomass over time.

🎯 Exam Tip: When describing growth phases, clearly define the key events in each (division, elongation, differentiation). For growth kinetics, differentiate arithmetic and geometric growth with examples and their characteristic rates.

Question 2. How growth is measured? Describe different factors affecting growth.
Answer: Growth in plants can be measured in several ways:
Measurement of Growth:
    • By counting the increase in the number of cells produced.
    • By measuring the increase in the size of cells, tissues, or plant parts like leaves, flowers, and fruits.
    • By observing an increase in the dry weight of the plant.
    • By measuring the increase in the length and thickness (girth) of the stem and root.
    • By simple linear measurement using a scale.
    • By Auxanometer: Plant growth, especially linear growth, can be precisely measured using an instrument called an auxanometer. A simple arc auxanometer works by attaching a silk thread to the plant's stem tip. This thread runs over a pulley with a pointer that moves along a graduated arc. As the plant grows, the pulley turns, and the pointer moves, showing the increase in length. The change in the pointer's position directly indicates the plant's growth.

Factors Affecting Growth: Plant growth is influenced by both external (environmental) and internal (physiological) factors.
1. Light: Light impacts growth in several ways:
    • Light Intensity: Very strong light can slow down plant growth and cause dwarfism, as it reduces the size of internodes and leaves.
    • Quality of Light: Plants absorb specific wavelengths of light. Red light is most effective for growth, while ultraviolet light can hinder it.
    • Duration of Light: The length of time a plant is exposed to light (photoperiod) greatly affects its vegetative and reproductive structures. A specific period of light is often needed for flowering, and without it, flowering may not occur.
2. Water: The amount of available water directly affects the growth rate. Water is essential for all biological processes in plants, including absorption, osmosis, transpiration, photosynthesis, respiration, and germination.
3. Temperature: Plant growth depends on cellular metabolism. Since temperature changes directly influence metabolic activity, the growth rate is also affected, either directly or indirectly. There is an optimal temperature range for plant growth.
In simple words: We can measure how much a plant grows by checking its cell count, size, weight, or length using tools like an auxanometer. Many things make plants grow, like how much light they get, how much water they have, and the temperature around them.

🎯 Exam Tip: List various methods of growth measurement (e.g., cell count, dry weight) and then systematically describe how external factors like light (intensity, quality, duration), water, and temperature influence these growth parameters.

Question 3. What is auxin? Critically analyse the physiological effects of auxin on growth.
Answer: The term auxin comes from a Greek word meaning "to grow" or "increase in growth." Auxins are a group of plant hormones that encourage the growth of stems and coleoptiles (protective sheaths for new shoots). Indole Acetic Acid (IAA) is a naturally occurring auxin. Any other natural or man-made product that acts like IAA is also called an auxin. These hormones are vital for many plant growth processes.

Here are the main ways auxins affect plant growth:
1. Apical dominance: This is when the main tip of a plant grows strongly, stopping the side branches from growing much. Auxins made at the tip cause this. If the tip is removed, the side branches start to grow more. This is why gardeners trim hedges to make them bushy.
2. Cell elongation: Auxins help plant cells get longer. This is their main job. A high amount of auxin at the top of the plant makes cells stretch, helping the shoot grow upwards. This also makes shoots grow towards light and away from gravity.
3. Root initiation: Auxins help new roots grow from stems, especially from nodes or the base. Plant growers often use auxins like BA and NAA to make roots form quickly on stem cuttings, helping new plants grow. Auxins work best for root growth at very low concentrations. If a stem cutting is dipped in auxin and then planted, roots form fast.
4. Parthenocarpy: This is when fruits grow without the flower being fertilized. Fruits made this way are seedless and are called parthenocarpic fruits. To make such fruits, gardeners remove the male parts of flowers and spray auxin on the stigma. Many seedless fruits like tomatoes, apples, cucumbers, and lemons are grown this way using external auxin.
5. Prevention of lodging: Lodging happens when strong winds cause tall, thin crop plants like wheat to bend and fall over. Spraying young plants with auxin can make the lower part of the stem stronger, reducing the chances of them falling over in strong winds.
6. Storage: Auxins can stop seeds from sprouting and buds from growing. This helps in storing crops like potatoes for a longer time. For example, potato tubers sprayed with NAA can be stored for about three years.
7. Thinning of flowers: In some mango varieties, too many flowers can grow, leading to many small, low-quality fruits. Spraying NAA can reduce excessive flowering, helping to produce fewer but larger fruits.
8. Effect on abscission: Abscission is the premature falling of leaves, flowers, and fruits. This happens because a special layer, called the abscission layer, forms due to a lack of auxin. Auxins prevent this layer from forming. Spraying auxins like NAA and 2,4-D can help stop or delay plants from losing leaves, flowers, and fruits too early.
9. Eradication of weeds: Unwanted plants in a crop field are called weeds. They compete with the main crop for water, minerals, light, and space, preventing the crop from growing well. Auxins can be used to get rid of these weeds. Broad-leaved weeds can be killed by spraying 2,4-D (2,4-Dichlorophenoxyacetic acid). Grass weeds can be destroyed using a synthetic hormone called "Delapen" (2,2-Dichloropropionic acid).
10. Shortening of internodes: In plants like apple and pear, fruits grow on short branches. Spraying NAA can shorten the internodes (the spaces between leaf attachments) of long branches. This increases the number of short branches and, in turn, the number of fruits.
In simple words: Auxin is a plant hormone that helps plants grow in many ways, like making stems longer, helping roots start, making fruits without seeds, and even controlling when leaves fall off or fighting weeds. It is found naturally and can also be made artificially to help with farming.

🎯 Exam Tip: When asked about physiological effects, list the distinct actions clearly and provide a brief explanation for each, ideally with a relevant example if available.

Question 4. Describe main experiments conducted by a different scientist in context with the discovery of auxins.
Answer: Several scientists performed important experiments that led to the discovery of auxins. These include:
1. Charles Darwin (1881)
2. Boysen-Jensen (1910-1913)
3. Paal (1919)
4. F.W. Went (1928)

**Definition of Auxins:** Auxins are organic compounds that can make shoots grow longer and stop roots from growing longer when present in low amounts (below \( 10^{-3} \)M or \( 0.001 \)M).

**Discovery of Auxins and Key Experiments:**
* **Charles Darwin (1881):** Known for his theory of evolution, Darwin was the first to realize that a substance promoting growth was present at the tip of plants. He and his son, Francis Darwin, studied canary grass (Phalaris canariensis) and described their findings in their book "The power of movements in plants." They observed that a plant's coleoptile (the protective sheath covering the first shoot) bends towards light when lit from one side. They concluded that when light shines on one side, a substance forms at the tip, moves downwards, and causes the unlit side to grow more, making the plant bend towards the light.
* **Boysen-Jensen (1910-1913):** Boysen-Jensen continued Darwin's work. He found that if a horizontal slit was made in the dark side of a coleoptile and a mica sheet was inserted, the coleoptile would not bend towards the light. However, if the mica sheet was inserted into a slit on the lit side, the coleoptile would still bend towards the light. These results showed that a substance responsible for bending is made at the tip and moves down the dark side.
* **Paal (1919):** Paal also performed similar experiments and confirmed that specific chemicals at the plant's apex (tip) promote growth. These chemicals are made at the tip and can dissolve in water.
* **F.W. Went (1926-1928):** Went is credited with truly discovering auxins. He managed to isolate this growth-regulating substance. He proved that the tip of the plant produces this substance. Went's experiment involved placing several decapitated (tip-removed) Avena (oat) coleoptiles on an agar-agar block. Agar-agar is a jelly-like substance that absorbs chemicals. After some time, he removed the coleoptile tips. He then cut the agar block into small pieces and placed each piece off-center on top of a decapitated coleoptile stump. He observed that the coleoptile bent even in darkness. He also found that an untreated agar block caused no bending. This clearly showed that a substance diffused from the coleoptile tip into the agar block, then moved down the coleoptile, causing one side to grow longer and the plant to bend. Went also observed that when a coleoptile is lit from one side, the amount of auxin on that side decreases to about 35%, while it migrates to the unlit side, which then contains about 65% of the auxin.
In simple words: Scientists like Darwin, Boysen-Jensen, Paal, and Went did experiments with plant shoots (coleoptiles) to figure out how they grow and bend towards light. They found out that the tip of the shoot makes a special chemical, now called auxin, that moves down and makes cells on one side grow more, causing the plant to bend. Went was the first to successfully collect and identify this chemical.

🎯 Exam Tip: When describing experiments, mention the scientist, the plant used (if specific), the experimental setup, the observation, and the conclusion drawn. Focus on the progression of discoveries.

Question 5. What is seed dormancy? Describe various reasons for dormancy and various methods of breaking seed dormancy.
Answer: Normally, a fully grown seed should sprout if the conditions are good. However, in some plants, even with favorable conditions, the seeds do not sprout right away. This temporary halt in seed germination is called **dormancy**. The length of this resting period can vary greatly among different plants.

**Reasons for Dormancy:**
1. **Hard seed coat:** Some seeds have a very hard outer layer that prevents water and oxygen from getting in, which are needed for germination. Sometimes, this hard coat also physically stops the embryo from growing. Example: Amaranthus, Gram, Pea.
2. **Immature embryo:** In some plants, seeds are released before the embryo inside is fully developed. These seeds will not sprout until the embryo matures completely. Example: Ginkgo biloba.
3. **After-ripening period:** Some seeds do not sprout right after they are formed. They need a special resting phase during which internal changes occur, allowing them to eventually germinate. This period is called the "after-ripening period," and it can last from weeks to months.
4. **Specific temperature and light requirements:** Certain seeds need a particular temperature range or exposure to specific light conditions to sprout.
5. **Presence of germination inhibitors:** Fleshy fruits often contain substances like abscisic acid (ABA), coumarin, and phenolic acid that stop germination. These inhibitors gradually wash away when the seeds are in the soil, allowing germination to occur. Applying gibberellin can also neutralize their effect.

**Methods of Breaking Seed Dormancy:** Depending on the cause of dormancy, different methods are used to encourage germination:
1. **Scarification:** This involves weakening or scratching the hard seed coat to allow water absorption. Seeds can also be softened by soaking them in dilute sulfuric acid, hot water, or fat solvents.
2. **Chilling treatment:** Seeds that need cold to break dormancy are exposed to artificial chilling. Example: Cherry, Oak.
3. **Exposure to alternate temperature:** Some seeds break dormancy when subjected to alternating high and low temperatures.
4. **Light:** For light-sensitive seeds (photoelastic seeds), exposure to red light can increase germination efficiency.
5. **Pressure:** Keeping some seeds at high atmospheric pressure (e.g., 2000 atmospheric pressure at 18-20°C) can increase germination by softening the seed coat and increasing its permeability.

**Dormancy Periods of Various Plants:**

🎯 Exam Tip: Clearly define dormancy, then list and explain both its causes and the methods to overcome it. A table, if applicable, can greatly enhance clarity for specific examples.

Question 6. Write Essay on Following:
1. Gibberellin and Cytokinin.
2. Growth inhibitor substances.
3. Photoperiodism.
4. Senescence and Abscission.
5. Vernalization.
Answer:
**1. Gibberellins:**
* Gibberellins were first found in Japan while studying a disease in rice plants.
* Around 1890, a disease appeared in Japanese rice fields where some rice plants grew unusually tall and thin but did not produce flowers.
* This disease was called "Bakanae disease," which means "foolish disease" in Japanese. Hori (1898) studied it and discovered that these plants were infected by a fungus named *Gibberella fuji-Kuro*.
* The infected seedlings, which were extra tall, thin, and did not bear flowers, fruits, or seeds, were called "foolish seedlings" because of their unusual appearance.
* Kurosawa (1926) later proved that the fungus's secretions could cause the same disease in healthy plants.
* Yobuta and Hayashi (1939) extracted a pure crystalline substance from the fungus culture and named it Gibberellin.
* Brian et al. (1954) isolated a pure form of a single gibberellin and named it Gibberellic acid.
* Over 100 different gibberellins (GA1, GA2, GA3, etc., up to GA100) have been found in fungi and higher plants. GA3 was the first one discovered and is the most common.

**2. Cytokinins:**
* Cytokinins are a type of plant hormone that helps with cell division and delays aging in plants.
* Skoog and his team first discovered that coconut milk and yeast extract contained substances that could promote cell division in tobacco pith tissue.
* Miller et al. (1955) isolated the active substance from herring sperm DNA and called it Kinetin (6-furfuryl aminopurine).
* Letham (1963) officially named Kinetin as Cytokinin.
* Letham and Miller (1964) later isolated a similar substance from maize endosperm and called it Zeatin.
* Zeatin is known as the first naturally occurring cytokinin.

**3. Growth Inhibitor Substances:**
* These are hormones or substances that slow down plant growth. They are important for controlling and balancing a plant's overall growth and development.
* Abscisic acid (ABA) is the most significant growth-inhibiting hormone.

**Abscisic Acid (ABA):**
* ABA is the most important natural growth inhibitor found in plants.
* It helps plants survive challenging environmental conditions, so it is also known as a "stress hormone."
* Wareing (1963) isolated a growth-inhibiting substance from Acer plant leaves and named it Dormin.
* Addicott et al. (1963) also isolated a substance from cotton flower buds and named it Abscisin.
* Later, it was confirmed that Dormin and Abscisin were the same substance, and it was officially named Abscisic Acid (ABA).
* The chemical formula of ABA is \( C_{15}H_{20}O_4 \).
* It is made of five carbon-containing isoprene units and has one carboxylic group (COOH).
* ABA is produced when carotenoids in chloroplasts break down, usually due to a lack of water.

**4. Photoperiodism:**
* Photoperiodism is how plants respond to the length of day and night, which affects their development, especially when they switch from growing leaves to making flowers.
* In 1920, W.W. Garner and H.A. Allard showed that the amount of light a plant gets controls flowering.
* They observed that a type of tobacco called "Maryland mammoth" did not flower in spring or summer, even with rich growth. But when its seeds were grown in a greenhouse in winter and then moved to the field, it flowered and produced fruits in early summer.
* These experiments proved that a specific period of light, called a critical photoperiod, is needed for flowering in this tobacco variety (it was a short-day plant).
* Hillman (1969) defined photoperiodism as the plant's growth response to the relative lengths of light and dark periods.
* Plants are categorized based on their photoperiodic response:
1. **Short-Day Plants:** Flower when the day length is shorter than a critical period.
2. **Long-Day Plants:** Flower when exposed to uninterrupted light for more than a critical period.
3. **Day-Neutral Plants:** Flower regardless of day length.

**5. Senescence:**
* Senescence is the aging process in an organism, leading to its death after its lifespan is complete.
* During senescence, the organism weakens, and its ability to function decreases.
* Metabolic substances are stored, and the dry weight of the plant reduces.
* A.C. Leopold (1961) described four types of senescence:
1. **Whole Plant Senescence:** The entire plant dies after flowering and fruiting (e.g., annuals like wheat, rice).
2. **Shoot Senescence:** The above-ground parts die, but underground stems and roots remain alive, developing new shoots in the next growing season (e.g., potato, ginger).
3. **Leaf Senescence:** In most perennial trees, leaves die and fall off in autumn, while the stem and roots remain alive, growing new leaves in spring.
4. **Gradual Senescence:** In most annual plants, older leaves die first, followed gradually by other leaves, stems, and roots. This is also called sequential senescence.

**6. Abscission:**
* Abscission is the natural process where plants shed leaves, flowers, and fruits.
* This is a biological process that starts due to changes in cells at the base of the part that will fall off.
* The middle layer and cell walls in this basal area are broken down by enzymes like pectinase and cellulase.
* As these breakdown, cells in this specific region start to separate.
* The tissues in this area become weak, and a special layer, called the abscission layer, forms.
* Cells just below the abscission layer become active and form cork cells, creating a protective layer or abscission zone.
* Strong winds or rain can cause the plant part to separate and fall off from this region.
* Abscission is often caused by a change in hormone balance, with abscisic acid playing a big role in this process. For instance, in biennial plants, vegetative growth occurs in the first year, and flowering completes in the second. Before flowering, these plants need a cold period (winter). Without cold treatment, they stay in the vegetative stage and do not reproduce.
* **Vernalization:** This is the process of using chilling (cold) treatment to encourage flowering. Choird (1960) defined vernalization as "Inducing ability of flowering by chilling treatment artificially."
In simple words: This essay covered several key plant processes: Gibberellins and Cytokinins are growth hormones; growth inhibitors like Abscisic Acid slow things down. Photoperiodism is how plants use light and dark periods to flower. Senescence is the aging of plant parts or the whole plant, while abscission is the shedding of leaves, flowers, or fruits. Vernalization is a method to make plants flower by giving them a cold period.

🎯 Exam Tip: For essay questions with multiple parts, organize your answer with clear headings for each topic. Provide key definitions, discoveries, and effects in a structured manner to ensure full coverage and easy understanding.