Samacheer Class 11 Bio Botany - Plant Growth and Development

The correct answer is 1.5 inches. To calculate the actual growth in length of the plant, we use the principle of the arc length subtended by the pointer's movement on the pulley. First, we determine the radius of the pulley. Given the diameter of the pulley is 6 inches, the radius of the pulley is half of that, which is \(6/2 = 3\) inches. The distance traveled by the pointer is 5 inches, and the length of the pointer is 10 inches. The actual growth in length of the plant is proportional to the distance traveled by the pointer, scaled by the ratio of the pulley's radius to the pointer's length. Therefore, the actual growth in length = (Distance traveled by pointer \(\times\) Radius of the pulley) / Length of the pointer. Plugging in the values: \((5 \times 3) / 10 = 15 / 10 = 1.5\) inches. Thus, the actual growth in length of the plant is 1.5 inches.

The growth of plants is a complex process that can be quantitatively measured using several parameters, reflecting different aspects of their development. These parameters allow scientists to assess the rate and extent of plant growth accurately. One common method is to measure the increase in length or girth, particularly in organs like roots and stems, which exhibit apical and lateral growth respectively. Another significant parameter is the increase in fresh or dry weight, which provides an indication of the biomass accumulation over time. Fresh weight includes water content, while dry weight reflects the actual organic matter produced. Furthermore, growth can be quantified by measuring the increase in area or volume, which is particularly relevant for organs such as leaves and fruits. Finally, an increase in the number of cells produced, often determined through microscopic analysis, is a fundamental measure of growth, especially during early developmental stages and in meristematic tissues. These diverse parameters collectively offer a comprehensive understanding of plant growth dynamics.

Plasticity refers to the ability of plants to modify their form and structure in response to environmental conditions, demonstrating developmental flexibility without permanent genetic changes. A classic example of plasticity is seen in the Buttercup plant (Ranunculus), an aquatic species that exhibits environmental heterophylly. In this plant, leaves that develop in air have a normal shape and structure adapted for photosynthesis in the terrestrial environment, whereas leaves that develop submerged underwater are highly thin, delicate, and hairy, showing morphological adaptations for efficient carbon assimilation in the aquatic environment. These structural differences are reversible and depend entirely on the environmental conditions during leaf development. In contrast, developmental heterophylly, such as seen in juvenile leaves of cotton and coriander plants where young leaves have a distinctly different shape from mature leaves, is not considered plasticity because it represents a genetically programmed developmental sequence rather than a response to environmental variation. Plasticity demonstrates the remarkable ability of plants to adjust their growth patterns to optimize survival and function in different environmental conditions.

• Cytokinin promotes cell division in the presence of auxin (IAA).
• Induces cell enlargement associated with IAA and gibberellins
• Cytokinin can break the dormancy of certain light-sensitive seeds like tobacco and induces seed germination.
• Cytokinin promotes the growth of lateral bud in the presence of apical bud.
• Application of cytokinin delays the process of aging by nutrient mobilization. It is known as the Richmond Lang effect.
• Cytokinin:
• increases rate protein synthesis
• induces the formation of inter-fascicular cambium
• overcomes apical dominance
• induces the formation of new leaves, chloroplast and lateral shoots.
• Plants accumulate solutes very actively with the help of cytokinins.

Mechanism of photoperiodic induction of flowering.
* The physiological change on flowering due to the relative length of light and darkness (photoperiod) is called Photoperiodism.
* The photoperiod required to induce flowering is called critical day length. Eg. 12 hours in Maryland Mammoth’s Tobacco Xanthium 15.05 hours.
Photoperiodic induction:
* An appropriate photoperiod in 24 hours cycle constitutes one inductive cycle. Plants may require one or more inductive cycles for flowering.
* The phenomenon of conversion of leaf primordia into flower primordia under the influence of suitable inductive cycles is called photoperiodic induction. Example: Xanthium (SDP) -1 inductive cycle and Plantago (LDP) -25 inductive cycles.
Site of photoconductive perception:
* Leaves are the parts that receive photoperiodic stimulus (PPS), again it is only leaves that synthesize floral hormones and translocate them to the apical tip to promote flowering.
* This can be demonstrated by experiments conducted in the Cocklebur plant. Which is an SD plant. The nature of flower-producing stimulus has been elusive so far. It is believed by physiologists that a hormone is responsible for it, Chailakyan (1936) named it as Florigen It is not possible to isolate it.
Procedure
Observation
Inference
1. Take potted plant A and defoliate the plant subject it to SD – a condition
There is no induction of flowering
No leaf to receive stimulus or induction of flowering
2. Take potted plant B – and defoliate all, except one leaf subject it to SD – condition.
There is the induction of flowering
One leaf is enough to receive stimulus or induction of flowering.
3. Take potted plant C – and defoliate it and subject it to LD condition
There is no induction of flowering
no leaf to receive stimulus or induction of flowering
4. Take potted plant D and subject all leaves to LD but one leaf to SD
There is the induction of flowering
One leaf is enough to receive an induction in the SD condition

Senescence is controlled by plants’ own genetic program and the death of the plant or plants part consequent to senescence is called Programmed Cell Death. In short senescence of an individual cell is called PCD. The proteolytic enzymes involving PCD in plants are phytases and in animals are caspases. The nutrients and other substrates from senescing cells and tissues are remobilized and reallocated to other parts of the plant that survives.
The protoplasts of developing xylem vessels and tracheids die and disappear at maturity to make them functionally efficient to conduct water for transport. In aquatic plants, aerenchyma is normally formed in different parts of the plant such as roots and stems which enclose large air spaces that are created through PCD. In the development of unisexual flowers, male and female flowers are present in earlier stages, but only one of these two completes its development while the other aborts through PCD.
Part-II.
11th Bio Botany Guide Plant Growth and Development Additional Important Questions and Answers
I. Choose The Correct Answer

• It shortens the vegetative period and induces the plant to flower earlier.
• It increases the cold resistance of the plants.
• It increases the resistance of plants to fungal disease.
• Plant breeding can be accelerated.

The viability of seeds refers to their ability to germinate and produce a normal seedling under suitable conditions. It essentially indicates the living condition of the seed and its capacity to remain dormant yet alive. The shelf life of a seed, after which it loses its ability to germinate, is known as its viable period. This period varies significantly among different plant species, ranging from a few days to several centuries. For instance, Oxalis seeds have a very short viability, lasting only a few days. In contrast, Lotus seeds are known for their remarkable longevity, with some specimens germinating after more than 1000 years. The Judean Date palm, specifically the 'Methuselah' variety, has demonstrated even longer viability, with successful germination reported from seeds over 2000 years old. Factors such as storage conditions (temperature, moisture, oxygen), genetic makeup, and seed coat impermeability influence seed viability.

Climacteric and non-climacteric fruits differ significantly in their ripening patterns and response to ethylene. Climacteric fruits exhibit a sharp and dramatic rise in respiration rate near the end of the developmental phase, accompanied by increased ethylene production. The ripening process in these fruits can be induced on demand by exposing them to ethylene gas, typically at concentrations around 1 ppm, allowing for controlled ripening during storage and transport. Examples of climacteric fruits include apples, bananas, mangoes, and avocados, which continue to ripen after being harvested from the plant. Non-climacteric fruits, in contrast, do not show a sharp increase in respiration rate during ripening and cannot be artificially ripened by exposure to ethylene gas, hence they are known as non-climacteric fruits. These fruits must be allowed to ripen fully on the plant before harvesting, as their ripening process is not ethylene-dependent and cannot be induced after harvest. Examples of non-climacteric fruits include grapes, watermelons, oranges, and strawberries. This fundamental difference in ripening physiology has important implications for fruit storage, transportation, and commercial handling practices.

Scarification and stratification are two distinct methods used to break seed dormancy. Scarification is a mechanical or chemical treatment applied to seeds to remove or weaken the seed coat. This process involves cutting, chipping, or abrading the seed coat, or using organic solvents to dissolve waxy or fatty compounds that prevent water and gas exchange. Scarification is commonly used for seeds with hard, impermeable seed coats. In contrast, stratification is a process where seeds are exposed to specific environmental conditions, particularly well-aerated, moist conditions combined with low temperatures ranging from 1°C to 10°C, for extended periods ranging from weeks to months. This treatment mimics the natural winter conditions that seeds experience in their native habitats. Stratification is particularly important for seeds of Rosaceous plants such as apple, plum, peach, and cherry, which require a period of cold exposure before they can germinate. While scarification addresses physical barriers to germination, stratification addresses physiological dormancy by satisfying the chilling requirements necessary for seed germination.

• Hard, tough seed coat causes barrier effect as impermeability of water, gas and restriction of the expansion of embryo prevents seed germination.
• Many species of seeds produce imperfectly developed embryos called rudimentary embryos which promotes dormancy.
• Lack of specific light requirement leads to seed dormancy.
• A range of temperatures either higher or lower cause dormancy.
• The presence of inhibitors like phenolic compounds which inhibits seed germination cause dormancy.

Senescence is the process of aging in plants, and it is influenced by various internal and external factors. Abscisic acid (ABA) and ethylene are hormones that accelerate senescence, promoting the breakdown of cellular components and the aging process. In contrast, auxin and cytokinin are growth-promoting hormones that reduce or retard senescence, helping to maintain plant vigor and delay aging. Nitrogen deficiency accelerates senescence as nitrogen is essential for protein synthesis and maintaining cellular functions, while adequate nitrogen supply retards the senescence process. High temperatures in vernalized seeds accelerate senescence, whereas low temperatures retard it, as cold conditions slow down metabolic processes. Water stress is another significant factor that accelerates senescence through the accumulation of abscisic acid, which triggers senescence-related changes in plant tissues. These factors work together to regulate the timing and rate of senescence, which is crucial for plant survival and reproduction.

• Abscission Zone: formed at the base of petiole
• Greenish grey in colour by rows of 2 to 15 cells thick primary wall and middle lamella
• The dissolution of by pectinase & Cellulase
• Formation tyloses – that block conduction of vessels
• Degradation of chlorophyll – Colour of leaves changes and leaves fall off.
• After Abscission – Suberization of outer layer of cells by the development of periderm.

Abscission is the process of shedding or separation of plant parts, and it holds several important biological significances. First, abscission separates dead or damaged parts of the plant, such as old leaves and ripe fruits, preventing the plant from wasting resources on non-functional tissues. Second, abscission facilitates the dispersal of fruits and seeds, enabling the plant to spread its offspring to new locations and continue its life cycle. Third, in deciduous plants, the abscission of leaves during autumn helps conserve water by reducing the surface area from which water can be lost through transpiration, which is particularly important during dry seasons or winter when water availability is limited. Fourth, abscission contributes to vegetative propagation in certain plants by allowing the shedding of specialized structures such as gemmae or plantlets, as seen in bryophytes. This process ensures that plant parts are shed at the appropriate time, maximizing the plant's chances of survival and reproduction.

Absolute growth rate and relative growth rate are two different ways of measuring and expressing plant growth. Absolute growth rate refers to the total increase in growth of an organ or organism measured and compared per unit time, expressed in absolute units such as millimeters or grams per unit time. This measurement gives the actual amount of growth that has occurred. Relative growth rate, on the other hand, expresses the growth of a given system per unit time in relation to the initial size or parameter of that system. It is calculated by dividing the absolute growth by the initial size, resulting in a dimensionless ratio or percentage. Relative growth rate is particularly useful for comparing the growth rates of organisms or organs of different initial sizes, as it accounts for the starting size and provides a standardized measure of growth efficiency. While absolute growth rate indicates the actual amount of growth, relative growth rate indicates the rate of growth relative to the existing biomass.

Absolute growth rate and relative growth rate are two distinct methods of quantifying plant growth. Absolute growth rate is defined as the total increase in growth of an organ or organism measured and compared per unit time. It represents the actual amount of growth that occurs, expressed in absolute units such as centimeters or grams per unit time. This measurement is straightforward and directly indicates how much an organ has grown in a given time period. Relative growth rate, by contrast, is the growth of a given system per unit time expressed per unit of the initial parameter or starting size. It is a standardized measure calculated by dividing the absolute growth by the initial size, resulting in a ratio that is independent of the initial size of the organ. Relative growth rate is particularly valuable when comparing growth rates between different organs or organisms of varying initial sizes, as it normalizes the data and allows for meaningful comparisons. While absolute growth rate provides information about the actual amount of growth, relative growth rate provides information about the efficiency or rate of growth relative to the existing biomass.

The Grand period of growth refers to the entire duration from the initial stage of growth to its final stage, encompassing all developmental changes in an organism or a specific organ. When the total growth (e.g., increase in size, weight, or cell number) is plotted against time, the resulting curve typically exhibits an 'S' shape, also known as a sigmoid curve or Grand Period curve. This curve illustrates the characteristic pattern of growth over time and is divided into four distinct phases: the Lag phase, where initial growth is slow; the Log or exponential phase, characterized by rapid and accelerating growth; the Decelerating phase, where the growth rate begins to slow down; and finally, the Maturation or stationary phase, where growth ceases or reaches a plateau, and the organism or organ attains its maximum size.

Phytohormones are chemical substances that are synthesized naturally within plants and regulate various physiological processes and developmental activities. These are organic compounds produced in small quantities that have profound effects on plant growth, development, and responses to environmental stimuli. The major classes of phytohormones include auxins, gibberellins, cytokinins, abscisic acid, and ethylene, each with distinct roles in plant physiology. In addition to these classical hormones, two other groups of compounds, brassinosteroids and polyamines, have been discovered to behave like hormones in regulating plant growth and development. Phytohormones work by binding to specific receptors in plant cells and triggering cellular responses that affect processes such as cell elongation, cell division, flowering, fruit ripening, and stress responses. Unlike animal hormones, phytohormones are often produced in one part of the plant and transported to other parts where they exert their effects.

• They are produced in root tips and stem tips and leaves (do not have specialized cells or organs for secretion)
• The transfer of hormones takes place through the conducting system (xylem and phloem)
• They are required in trace quantities
• They either promote, inhibit or modify growth.

• Mixture of two phenoxy herbicides – 2.4. D and 2.4.5 T together known as Agent orange.
• This Agent orange, was used by USA in Vietnam war as chemical warfare weapon to defoliate forests in Vietnam.

• Yes, trimming of plants removes apical buds and hence apical dominance is prevented the lateral buds sprout and give a beautiful bushy appearance and aesthetic value.
• Also in tea estates, this trimming develops more lateral branches and more tea leaves thus it has commercial significance.

The distribution of cytokinin in plants is not as wide as those of auxin and gibberellins but found mostly in roots. Cytokinins appear to be translocated through xylem.

Photoperiodism is defined as the physiological response of plants to the relative lengths of light and darkness, specifically the photoperiod, which influences various developmental processes, most notably flowering. Plants use this mechanism to synchronize their life cycles with seasonal changes. Critical day length, on the other hand, is the specific duration of light or darkness that is required to induce flowering in a particular plant species. It is not necessarily the optimal length but rather the threshold above or below which flowering will or will not occur. For instance, the Maryland Mammoth tobacco variety is a short-day plant that requires a photoperiod of less than 12 hours to flower. Conversely, Cocklebur, another short-day plant, requires a photoperiod shorter than 15.05 hours to induce flowering. Understanding photoperiodism and critical day length is crucial for agricultural practices and plant breeding, as it allows for the manipulation of flowering times.

• The knowledge of photoperiodism plays an important role in hybridisation experiments.
• Photoperiodism is an excellent example of physiological pre-conditioning that is using an external factor to induce physiological changes in the plant.

• The knowledge of photoperiodism an important role in hybridization experiments.
• It is an excellent example of physiological preconditioning that is using an external factor to induce physiological changes in the plant.

Epigeal and hypogeal germination are two distinct types of seed germination that differ in the position of cotyledons relative to the soil surface. In epigeal germination, the cotyledons are pushed out of the soil due to rapid elongation of the hypocotyl, which lifts the cotyledons above the soil surface. Examples include castor and bean seeds. In hypogeal germination, the cotyledons remain below the soil due to rapid elongation of the epicotyl instead, which pushes the plumule upward while the cotyledons stay embedded in the soil. Maize is a typical example of hypogeal germination. The key distinction lies in which part of the embryo elongates during germination and whether the cotyledons emerge from the soil or remain underground.

Seed dormancy is the physiological condition in which a viable seed fails to germinate even when exposed to suitable environmental conditions such as adequate moisture, appropriate temperature, and oxygen. This is a natural mechanism that allows seeds to survive unfavorable periods and germinate only when conditions are optimal for seedling survival and growth. Seed dormancy is classified into two main types: innate dormancy, which is inherent to the seed due to internal factors such as immature embryos, hard seed coats, or the presence of germination inhibitors; and imposed dormancy, also called exogenous dormancy, which results from external environmental conditions that prevent germination even though the seed is physiologically capable of germinating. Understanding these types is crucial for agricultural practices and seed storage.

• By mechanical and chemical treatments like cutting or chipping of hard tough sed coat and use of organic solvents to remove waxy or fatty compounds are called scarification.
• It is a method of breaking dormancy of the seeds.

Redifferentiation and devernalization are two distinct physiological processes in plants. Redifferentiation refers to the process in which differentiated cells, after undergoing multiplication and division, lose their ability to divide further and mature to perform specific specialized functions. Examples include the formation of secondary xylem and secondary phloem in plants, where cambial cells divide and then differentiate into specialized vascular tissues. Devernalization, on the other hand, is the reversal of the effects of vernalization, which is the process of flowering induction by prolonged exposure to cold temperatures. Devernalization occurs when plants that have been vernalized are exposed to high temperatures, which can reverse the vernalization effect and prevent flowering. These processes represent opposite physiological responses in plant development and reproduction.

• Ageing or getting old is called senescence.
• It refers to all collective, progressive and deteriorative processes which ultimately lead to complete loss of organization and function (Eg. leaves turn yellow and fall off from plant).

• In some seeds water and oxygen are unable to penetrate micropyle due to blockage by cork cells.
• These seeds are shaken vigorously to remove the plug
• The process of removing the plug or block is called impactation.

• The break dormancy, some plant seeds have to be exposed to well aerated, moist conditions under low temperature (0°c to 10°c) for weeks to months.
• This kind of seed dormancy breaking treatment is known as stratification.
• The stratified soil layers should be given a low-temperature treatment for a certain period so as to induce germination.
• Eg. the Seeds of Rosaceae plants Apple, Plum, Peach, etc.

The abscission zone, also called the abscission layer, is a specialized anatomical region that forms internally at the base of the petiole where a leaf attaches to the stem. This zone consists of a few layers of thin-walled cells arranged transversely across the petiole. These cells are structurally distinct from surrounding tissues and are the site where separation occurs during leaf abscission. The abscission zone is formed in response to environmental stresses, seasonal changes, or developmental signals, and when activated, the thin-walled cells in this zone break down, leading to the detachment and fall of the leaf from the plant. This mechanism is an important adaptation that allows plants to shed leaves during unfavorable seasons or in response to damage.

• It is a bioassay technique by which Ethylene can be measured.
• It helps in the detection of the exact amount of ethylene from different plant tissues like lemon and orange.

Gibberellins are primarily produced in actively growing regions of the plant, such as young leaves near the tip, developing embryos, and roots. Immature seeds are particularly rich sources of gibberellins. The precursor for gibberellins is a 5-carbon isoprenoid unit called Isopentenyl Pyrophosphate (IPP), which is formed through the mevalonate pathway, with acetate being a primary precursor. Cytokinins, in contrast, are mainly synthesized in the root apex and shoot apex, similar to auxins, where cell division is highly active. They are also found in developing buds and young fruits. Cytokinins are derived from the purine adenine, specifically through the modification of adenine to form compounds like zeatin and kinetin, which are common forms of cytokinins. Both hormones play vital roles in plant growth and development, but their sites of synthesis and chemical origins differ significantly.

The Grand period of growth refers to the entire duration from the initial stage of growth to its final stage in an organism or a specific organ. When the total growth is plotted against time, the resulting graph is typically 'S' shaped, known as a sigmoid curve. This curve illustrates the characteristic pattern of growth over time and is divided into four distinct stages. The first is the Lag phase, where the initial growth rate is slow as the cells are adapting to the environment and preparing for division. This is followed by the Log or exponential phase, characterized by a rapid and accelerating rate of growth due to active cell division and enlargement. Next is the Decelerating phase, where the growth rate gradually slows down as resources become limited or internal factors begin to inhibit rapid growth. Finally, the Maturation or stationary phase is reached, where growth ceases or plateaus, and the organism or organ attains its maximum size and differentiation, often entering a state of maturity or senescence. This sigmoid curve is a fundamental representation of growth in biological systems.

• Eradicate weeds: Eg. 2.4 D and 2.4.5.7
• Formation of seedless fruits: (Parthenocarpic fruits) Eg. Synthetic Auxin.
• Break dormancy.
• Induction of flowering: In pineapple NAA induce flowering
• Increase the number of female flowers: Eg. Cucurbita.

• It is used to eradicate weeds, eg: 2,4 – D and 2,4,5 – T.
• Synthetic auxins are used in the formation of seedless fruits (Parthenocarpic fruit).
• It is used to break the dormancy in seeds.
• Induce flowering in Pineapple by NAA & 2,4 – D.
• Increase the number of female flowers and fruits in cucurbits.

• Gibberellins are chemically related to terpenoids (natural rubber, Carotenoids, and steroids) formed by 5-C precursors and an Isoprenoid unit called Iso Pentenyl Pyrophosphate (IPP) through a number of intermediates.
• The primary precursors are Acetate.

• Ethylene normally reduces flowering in plants except in Pineapple and Mango.
• It increases the number of female flowers and decreases the number of male flowers.
• Ethylene spray in the cucumber crop produces female flowers and increases the yield.

Experiment:
1. Arc auxanometer:
The increase in the length of the stem tip can easily by measured by an arc auxanometer. If consists of a small pulley to the axis of which is attached a long pointer sliding over a graduated arc. A thread one end of which is tied to the stem tip and another and to a weight passes over the pulley tightly. As soon as the stem tip increases in length, the pulley moves and the pointer slide over the graduated arc (Refer Figure) The reading is taken. The acutal increase in the lengthwm stem is then calculated by knowing the length of the pointer and the radius of the pulley. If the radius of the pulley is 4 inches and the length of pointer 20 inches the actual growth is measured as follows:
Actual growth in length\(=\frac{\text { Distance travelled by the pointer radius of the }}{\text { Length of the pointer }}\)
For example, actual growth in length \(=\frac{10 \times 4 \text { inches }}{20 \text { inches }}\) = 2 inches

Auxin is a plant hormone with multiple physiological effects on plant growth and development. Cell elongation is one of the primary effects, where auxin promotes cell elongation in stems and coleoptiles by increasing cell wall plasticity and water uptake. In root growth, auxin shows a dual effect: at extremely low concentrations it promotes root elongation, but at higher concentrations it inhibits root cell elongation while simultaneously inducing the formation of lateral roots. Apical dominance is maintained by auxin produced in the apical bud, which suppresses the growth of lateral buds below it. Auxin also prevents abscission by maintaining the integrity of cells in the abscission zone. In secondary growth, auxin promotes cell division in the vascular cambium, leading to the formation of secondary xylem and phloem. Additionally, auxin stimulates respiration in plant tissues and induces vascular tissue differentiation. Agricultural applications of auxin are extensive and economically important. Synthetic auxins such as 2,4-D and 2,4,5-T are used as selective herbicides or weedicides to control broad-leaved weeds in crops. Auxins are also used to induce parthenocarpy, the development of seedless fruits, which is commercially valuable for crops like tomatoes and cucumbers. They can break seed dormancy, promoting germination when seeds are treated with auxin solutions. Auxins are employed to induce flowering in certain plants such as pineapple. In crops like cucumber, auxin application increases the number of female flowers, which is desirable for fruit production. These applications demonstrate the practical importance of understanding auxin physiology in agriculture.

Auxins have numerous important agricultural applications that enhance crop productivity and quality. As weedicides, synthetic auxins such as 2,4-D (2,4-dichlorophenoxyacetic acid) and 2,4,5-T (2,4,5-trichlorophenoxyacetic acid) are used as selective herbicides to remove broad-leaved weeds from cereal crops without harming the crop plants themselves. These compounds mimic natural auxin activity at high concentrations, causing abnormal growth and death in susceptible weed species. Auxins are used to induce parthenocarpy, the development of seedless fruits without fertilization, which is commercially valuable as seedless fruits are preferred by consumers. Synthetic auxins such as NAA (naphthaleneacetic acid) and 2,4-D are applied to induce parthenocarpy in various crops. Auxins can break seed dormancy by promoting metabolic activity and weakening the seed coat, allowing seeds to germinate even under conditions that would normally maintain dormancy. In pineapple cultivation, auxins are used to induce flowering at desired times, allowing farmers to control harvest schedules and improve crop management. In crops such as cucumber, auxin application increases the number of female flowers relative to male flowers, which is desirable because female flowers develop into fruits, thereby increasing overall fruit yield. These diverse applications make auxins invaluable tools in modern agriculture for improving crop quality, yield, and management efficiency.

• Induction of cell division & cell elongation – Extraordinary stem elongation.
• Reversal of dwarfism & Bolting – Rosette (genetic dwarfism) plants when treated with Gibberellins exhibit excessive enter nodal growth – This sudden elongation of a stem followed by flowering is called Bolting.
• Breaks dormancy – in Potato tubers.
• Biennials flower in the 1 st year – Instead of cold exposure, if biennials treated with Gibberellins flower in the 1st year itself.

Almost all plant tissues produce ethylene gas in minute quantities.
1. Discovery:
In 1924, Denny found that ethylene stimulates the ripening of lemons. In 1934, R. Gane found that ripe bananas contain abundant ethylene. In 1935, Cocken et al., identified ethylene as a natural plant hormone.
2. Occurrence:
Maximum synthesis occurs during climacteric ripening of fruits and tissues undergoing senescence. It is formed in almost all plant parts like roots, leaves, flowers, fruits and seeds.
3. Transport in plants:
Ethylene can easily diffuse inside the plant through intercellular spaces.
4. Precursor:
It is a derivative of amino acid methionine, linolenic acid and fumaric acid.
5. Bioassay (Gas Chromatography):
Ethylene can be measured by gas chromatography. This technique helps in the detection of exact amount of ethylene from different plant tissues like lemon and orange.
6. Physiological Effects:
* Ethylene stimulates respiration and ripening in fruits.
* It stimulates radial growth in sterft and roof and inhibits linear growth.
* It breaks the dormancy of buds, seeds and storage organs.
* It stimulates the formation of an abscission zone in leaves, flowers and fruits. This makes the leaves to shed prematurely.
* Inhibition of stem elongation (shortening the internode).
* In low concentration, ethylene helps in root initiation.
* Growth of lateral roots and root hairs. This increases the absorption surface of the plant roots.
* The growth of fruits is stimulated by ethylene in some plants. It is more marked in climacteric fruits.
* Ethylene causes epinasty.
7. Agricultural role:
* Ethylene normally reduces flowering in plants except in Pineapple and Mango.
* It increases the number of female flowers and decreases the number of male flowers.
* Ethylene spray in cucumber crops produces female flowers and increases the yield.

• With IAA – Promotes cell division With IAA & GA – Induces cell enlargement
• Breaks dormancy of light-sensitive seeds (tobacco) induces seed germination.
• Promotes growth of lateral
• buds even in the presence of apical bud.
• Delays the process of aging by nutrient mobilization known as Richmond Lang effect.
• Induces rate of protein synthesis.
• Induces the formation of interfascicular cambium Overcomes apical dominance
• Induces the formation of new leaves chloroplast and lateral shoots.
• Induces Accumulation of solutes.

• Stimulates respiration and thereby ripening of fruits
• Stimulates radial growth in stem and root and inhibits linear growth.
• breaks dormancy of
• buds
• Seeds
• Storage Organs
• Stimulates abscission 2 one formation in
• leaves
• flowers
• fruits (so leaves shed prematurely)
• Prevents stem elongation by preventing internodal growth
• Root growth in low concentration
• Stimulates growth of lateral roots and root hairs and increase the absorptive surface
• Ripening of fruits – Increases ripening in climacteric fruits (Mango, banana) etc.
• It causes epinasty

The dormancy of seeds can be broken by different methods. These are:
1. Scarification:
Mechanical and chemical treatments like cutting or chipping of hard tough seed coat and use of organic solvents to remove waxy or fatty compounds are called Scarification.
2. impaction:
in some seeds, water and oxygen are unable to penetrate micropyle due to blockage by cork cells. These seeds are shaken vigorously to remove the plug which is called Impaction.
3. Stratification:
Seeds of rosaceous plants (Apple, Plum, Peach, and Cherry) will not germinate until they have been exposed to well aerated, moist conditions under low temperature (0°C to 10°C) for weeks to months. Such treatment is called Stratification.
4. Alternating temperatures: Germination of some seeds is strongly promoted by alternating daily temperatures. An alternation of low and high temperature improves the germination of seeds.
5. Light:
The dormancy of photoelastic seeds can be broken by exposing them to red light.

a. The physiological change on flowering due to the relative length of light and darkness is called photoperiodism.
* Gamer and Allard (1920) coined the term
* They studied photoperiodism in Biloxi variety of soybean (Glycine max) and Many land mammoth varieties of tobacco.
b. Depending on photoperiodic responses plants are classified into several types.
1. L.D. Plants (Long Day) The photoperiod required to induce flowering is called critical day length depending on critical day length if it is long (more than 12 hours) and with short nights. Eg. Pea Barley and Oats
Short LD Plants: These are Long day plants but need short day length during the early period of growth for flowering Eg. Wheat, Rye
2. SD Plants: Plants requiring short critical day length for flowering or a long night.
Eg. Tobacco, Cocklebur, Soya, Rice, and Chrysanthemum.
Long SD Plants: Actually SD plants but need long days during the early period of growth for flowering Eg. Some SPS of Bryophyllum & Night Jasmine.
3. Intermediate day plants:
These require a photoperiod between a long day and a short day for flowering Eg. Sugarcane and coleus.
4. Day Neutral plants:
There are a number of plants which can flower in all possible photoperiods, known as photo neutral or hiterterminate plants. Eg. Potato, Rhododendron, Tomato & Cotton.

Definition:
It is a bluish biliprotein responsible for the perception of light in the photophysiological process, existing in two different forms is mainly involved in flower induction, (i.e) Pr and PFr.
* Butler et al(1959) named the pigment.
* It exists in two interconvertible forms
Pr
PFr
1. red light absorbing form
2. Absorbs red lgiht of wavelength 660 nm
3. Biologically inactive form & stable found in the diffused state in cytoplasm
4. Promotes flowering in SD plants and inhibits flowering LD plants.
1. Far-red light absorbing form
2. Absorbs far-red light of wavelength 730 nm
3. Biologically active and it is unstable Associated with a hydrophobic area of the membrane system
4. Promotes flowering in LD plants and inhibit flowering in SD plants.
Mechanism:
Other functions:
Play a role in seed germination and changes in membrane conformation.

Definition:
* Many biennials and perennials are induced to flower by low-temperature exposure (O°c to 5°c) This process is called Vernalization.
* T.D. Lysenko – Coined the term.
Mechanism of Vernalization:
2 theories explain the mechanism of vernalization.
1. Hypothesis of Phasic development (T.D. Lysenko),
The development of the annual plant has 2 phases.
* Thermostate-Vegetatine stage requiring low temperature and suitable moisture.
* Photo stage -high temperature needs to synthesize florigen.
2. Hypothesis of hormonal involvement (Purvis 1961)
Vernalization has several steps
The technique of Vernalization:
* Seeds soaked in water
* Allowed to germinate at 10°C to 12°C
* Transferred to low temperature for few days to 30 days (3°C to 5°C).
* Germinated seeds after the low temp, treatment are allowed to dry & then sown.
* Quickened flowering than untreated (control seedling)
Devernalization:
The reversal of the effect of vernalization is called Devemalization.
* Practical Applications:
* Vernalization shortens the vegetative period and induces the plant to flower earlier
* It increases cold resistance
* It increases fungal resistance
* It accelerates Plant Breeding.

Definition:
Getting old or Ageing is call d senescence in plants.
It refers to all collective, progressive, and deteriorative processes which ultimately lead to complete loss of organization and function.
Types – 4 types (Leopold -1961)
* Overall senescence: When the entire plant gets affected and dies – Eg. Annuals – Wheat & Soybeans, Perennials – Agave & Bamboo
* Top senescence: Occur in aerial parts only Eg. Parrennials – Banana and Gladiolus
* Deciduous senescence: Occur only in leaves Eg. Decidual plants – Elm and Maple
* Progressive Senescence: Occur in Annuals occur in old leaves first followed by new leaves than stem and finally root system.

Change in the structure of cells
* Vacuoles act like lysosome-secrete hydrolytic enzymes.
* Reducation in photosynthetic rate (due to loss of chlorophyll & accumulation of anthocyanin)
* The decrease in Starch content, Protein content
* Decrease in …………. r RNA level due to increased activity of enzyme RNA ase
* Degeneration of DNA – by increased activity of enzyme DNA ase
Factors affecting senescence:
Name of the factor
Effect of senescence
ABA & Ethylene
Accelerates
Auxin & Cytokinin Nitrogen deficiency
reduces increases
Nitrogen supply
retards
High temperature in vernalized seeds
Accelerates
Low temperature
Retards
Water stress
Accumulation of ABA leading to senescence