Plant Growth And Development

Ethylene is a gaseous plant growth hormone. Ethylene is highly effective in fruit ripening and in turn it increases the respiration rate during ripening of the fruits. It aids in breaking seed dormancy thus initiating germination. Thus ethylene acts as a ripening agent.

Ripening describes the change from physiological maturity of a plant to its senescence (ageing). During ripening, there's a change in the skin colour of the fruit, the internal flesh softens, the fruit sweetens, etc.

In 1941, Overbeek discovered that the milky endosperm of the coconut (bearing coconut water) had the ability to stimulate cell division - indicating presence of cytokinins. Cytokinins are synthesised naturally in regions where rapid cell division would occur like the young fruits, apices of roots etc.

Auxins and cytokinins are seen act antagonistically in relation to apical dominance so the action of one is overcome by the action of another. When cytokinins are present in more concentrations, it increases supply of required water and minerals to the lateral buds than the apical bud allowing growth of lateral buds.

A) IAA - iv) Weed free lawns

Auxin growth regulator herbicides are used for controlling annual, simple perennial etc. They are selective and is widely used to kill dicotyledonous weeds whereas not affecting mature monocotyledonous plants.

B) ABA - iii) Stomatal closure

ABA stimulates the closure of stomata increasing the tolerance of plants when in various stress conditions - stress hormone.

C) Ethylene - v) ripening of fruits

Ethylene is used in fruit ripening. It promotes the respiration rate during ripening of the fruits.

D) GA - ii) Bolting

Gibberellins ( Gibberellic acid or GA) promotes bolting which is the elongation of internode before flowering.

E) Cytokinins - i) Herring sperm DNA

Cytokinins were discovered as kinetin (a modified form of the purine nucleotide called adenine) from the autoclaved herring sperm DNA.

Apples are generally wrapped in wax paper to prevent aerobic respiration by checking the entry of O₂.

There is a loss of carbohydrate, acid, and materials of the cell wall followed by production of carbon dioxide and alcohol when apple undergoes aerobic respiration and gets rotten.

All of the above parameters can be used to measure growth.

Without increase in cell size or cell enlargement, no organism can continue to grow.

Without increase in cell number, the cells would eventually die out and growth will not continue.

Without increase in length and weight, metabolic processes cannot undergo since the cell is increasing in size too.

So, the above parameters all measure growth.

The term synergistic action of hormones refers to when two hormones act together and contribute to the same function.

Auxin acts synergistically with cytokinin to control the shoot stem‐cell meristem. The action of both together helps in callus formation during plant tissue culture. Thus, leading to an additive effect.

Plants follow various pathways when it responds to various environmental changes or phases of life and forms different structures which are not very similar to one another. This is called plasticity.

Gibberellins are responsible for adjusting the physiological function of sugarcane which is to improve the carbon oxidation of plant. It improves the photosynthesis efficiency and increases the length of the sugarcane stalks and hence of the sugar production (sugarcane stores carbohydrate as a sugar form) when it is sprayed on sugarcane.

ABA acts antagonistic to Gibberellic acid. The antagonistic relation between abscisic acid (ABA) and gibberellic acid (GA) plays an important role in controlling seed germination. Both ABA and GA may be involved in the regulation of nucleic acid and protein metabolism during seed dormancy thus controlling seed germination.

These plants are generally annual or biennial and very rarely perennial. Annual plants flower in one year, biennials take two years to flower and perennials takes many years to flower. Example: Rice (usually annual), Sugarbeet, cabbages, carrots (biennial), Talipot palm (perennial).

This can be shown by experiment on leaf Xanthium pennsylvanicum, which is a short day plant. If the plant is defoliated and then allowed to flower under short day condition, it will not flower.

a) A phase of growth which is maximum and fastest is the log phase or exponential phase.

In this phase, the cells are not dormant. Here cell numbers increase in a logarithmic pattern. It is a period characterized by cell doubling.

b) Apical dominance as expressed in dicotyledonous plants is due to the presence of more auxins in the apical bud than in the lateral ones.

Explanation: Auxins are responsible for apical dominance. In most plants, the growing apical bud inhibits the growth of the lateral buds. This is called apical dominance.

c) Cytokinin- kinetin, zeatin, and 6-benzylaminopurine

Explanation: Auxin helps cells elongate, but it doesn't work alone. Cytokinins promote cell division (cytokinesis) and are produced in roots, embryos and fruits, or wherever there is actively growing tissue. However, cytokinins need auxin to induce cell division.

d) Leaves of a vegetative plants are the sites of photoperiodic perception.

This can be shown by experiment on leaf Xanthium pennsylvanicum, which is a short day plant. If the plant is defoliated and then allowed to flower under short day condition, it will not flower.

Plant growth substances are small molecules having diverse chemical composition. PGS of one group called plant growth promoters is involved in growth promotion like cell division or cell enlargement, flowering, fruiting, etc. Example: Auxins, Gibberellins. The other group has a role to play in plant responses to different stresses it is exposed to and also participates in growth inhibiting activities like dormancy. Eg: Ethylene.

a. Increase yield of sugar cane: Giberellins (Spraying sugarcane crop with gibberellins increases the length of the stem and hence increases the yield of sugarcane by about 20 tonnes per acre).

b. Promote lateral shoot growth: Cytokinins (These are generated in regions where rapid cell division occurs).

c. Cause sprouting of potato tuber: Ethylene (It breaks seed and bud dormancy and aids in initiation of sprouting of potato tubers)

d. Inhibit seed germination: Abscisic acid ( It acts as a growth inhibitor hormone. It inhibits seed germination and promotes closure of stomata thus increasing the tolerance of plants in different stress conditions hence it is also known as stress hormone).

L_t = L₀ + rt ... (i)

where L_t = length at time ‘t’ , L₀ = length at time ‘zero’ , r = growth rate / elongation per unit time.

Here, L_t = 19 cm

L₀ = 5 cm

t = 1 week

Putting these values in equation (i)

Relative growth rate =

=

Hence the growth rate is 14 cm per week while the relative growth rate is 280%.

a) Two functions of the phytohormone (plant hormones that are responsible for signalling in plants and occur in small concentrations) Gibberellin are:

i) Gibberellins cause fruits to elongate and improve in its shape. It is also responsible to delay sensescence.

ii) Spraying sugarcane crop with gibberellins increases the length of the stem, thus increasing the sugar production that increases the yield by almost 20 tonnes per acre.

iii) GA₃speeds up the malting process in brewing industry (GA₃ is one of the kinds of gibberellins).

b) The property of Gibberellin that causes foolish seedling disease in rice is the elongation of internodes. E. Kurosawa reported the symptoms of the disease in uninfected rice seedlings when they were treated with gibberellic acid.

Gibberellins promote the formation of male flowers on genetically female plants in Cannabis whereas ethylene promotes formation of female flowers on genetically male plants.

GA does not directly promote stamen differentiation in the bud but suppress the formation of female flowers due to which male flowers eventually develop.

Ethylene induces flowering so can be used to increase the number of female flowers in the cucumber plants. It selectively induces flowering of female plants suggesting a link between ethylene signalling and primordial anther specific DNA damage in female cucumber flowers.

Photoperiodism is the reaction of living organisms to the length of day or night.

Short Day Plants (SDP): Short-day plants can only flower when the night lengths exceed their critical photoperiod.

Long-Day Plants (LDP): Long-day plants can flower when the night length falls below their critical photoperiod.

Day Neutral Plants (DNP) : They do not initiate flowering based on photoperiodism.

Classification of the following plants into LDP, SDP and DNP are as follows:

Xanthium:

Henbane: Long Day Plant (LDP)

Spinach: Long Day Plant (LDP)

Rice: Short Day Plant (SDP)

Strawberry: Short Day Plant (SDP)

Bryophyllum: Long Short Day plant (LSDP)

Sunflower: Day Neutral Plant (DNP)

Tomato: Day Neutral Plant (DNP)

Maize:Day Neutral Plant (DNP)

The plant growth regulator that can be applied to achieve this is ethylene (C₂H₄).

Plant Growth Regulators are produced in various parts of the plant. They control differentiation and development of various parts of the plants as well as has physiological effects on plants too.

Ethylene induces flowering so can be used to increase the number of female flowers in the cucumber plants. It selectively induces flowering of female plants suggesting a link between ethylene signalling and primordial anther specific DNA damage in female cucumber flowers.

a. IAA : in the tips of shoot and apical buds from where they travel to the regions of their actions.

b. Gibberellins : in young leaves and tips of roots and later from the terpenoid pathway in plastids where the food is made, they travel to the ER where they are modified for further use.

c. Cytokinins: meristematic zones of the root like the root tip i.e. it is synthesized in regions where rapid cell division occurs.

In plants especially the higher plants, apical dominance or growth of apical buds forbids the growth of lateral buds. When the gardeners trim the plants regularly what they are doing is decapitation or removal of shoot tips (apical dominance is stopped) that results in growth of the lateral buds. This allows the plants to remain bushy in the botanical gardens and tea gardens.

Three physiological processes in plants which are affected by light are:

a) Photosynthesis: The process of photosynthesis occurs when green plants use the light energy to convert carbon dioxide (CO₂) from air and water (H₂O) absorbed from the soil via roots into carbohydrates. Light energy is absorbed by chlorophyll which is a photosynthetic pigment of the plant.

b) Photoperiodism: In photoperiodism, flowering is regulated in response to the day length exposure time or photoperiod. Short Day Plants flower when day length is below a certain threshold level (as in rice), while in long day plants when day length is above a certain threshold level, only then can flowering occur (like in spinach).

Pfr: active form of phytochrome is Pfr or phytochrome far red.

Pr: inactive form of phytochrome is Pr or phytochrome red.

These forms are interconvertible between each other.

c) Phototropism: Phototropism is the growth of plant towards or away from light. Growth towards the light source is called positive phototropism, while away from the light source is called negative phototropism. In phototropism, plant usually bends or grows in response to the light. Shoot usually directs itself towards light while roots away from it.

The segments labelled 1,2,3 are as follows:

1: Lag phase: During this period, the cells are active metabolically only increasing their cell size and synthesizing enzymes required for cell division under environmental conditions that are given.

2: Log phase: In this phase, the cell increases in number in a logarithmic pattern resulting in balanced increase of cell constituents. This phase shall continue unless nutrients are depleted and toxic materials accumulation reaches a threshold level.

3: Stationary phase: This phase occurs when the nutrients get depleted and toxic by-products accumulate thus not leading to anymore increase in cell number.

Characteristics of growth are increase in mass of the cell and increase in cell number.

Yes, unicellular organisms also grow. When it comes to a multicellular and unicellular organisms, it grows by dividing its cells or undergoing cell division by increase in cell mass as well as increase in number of cells. This can be observed by observing the cells under microscope and counting the increase in number of cells.

The property of Gibberellin that causes foolish seedling disease in rice is the elongation of internodes.

E. Kurosawa reported the symptoms of the disease in uninfected rice seedlings when they were treated with gibberellic acid.

Short day plants need a prolonged dark period to flower because during this time all the chemical transformations occur in the plant which aids in the flowering of the plant. It requires a short day exposure to light in order to flower but beyond a certain point when exposed to more than the critical period of light, short day plants will not be able to flower. So Nicotiana tabacum being a short day plant is unable to flower because of the above stated reasons.

Structural characteristics of

a) Meristematic cells near the root tip:

• large nucleus

• rich protoplasm

• vacuoles are less in number

• cell wall is thin and made up of cellulose

• No intercellular spaces

This is an area of cell division.

b) The cells in the elongation zone of the root

• more number of vacuoles

• new cellulosic walls are deposited

• there is an increase in size

This is an area of cell elongation.

The growth pattern in plants differs from that in animals.

Plants have parts that have the ability to divide all throughout life and grow which is absent in animals i.e. they do not have unlimited growth. Plants have this characteristic due to the presence of meristems at different locations of the plant which is a region of rapid cell division. So new cells keep on getting added while the old ones lose capacity to divide and constitutes the plant body.

No, all the parts of the plant does not grow indefinitely. Only the meristematic tissues have the ability to keep on dividing i.e. the meristems are the parts which grow indefinitely throughout the plant life.

a. Differentiation

The cells of the apices of roots, apices of shoot and cambium has the ability to differentiate and mature so that they can perform specific functions. This is known as differentiation. Example: When a tracheary element is formed, cells lose their protoplasm and develop a strong, elastic, lignocellulosic cell wall that would be responsible to carry water for long distances when conditions are extreme. Usually, the cell changes to a more specialized type during differentiation.

b. De-differentiation : A differentiated cells has the ability to regain its capacity for cell division when placed under certain conditions. This is called dedifferentiation. Example: During formation of meristems from fully differentiated parenchyma cells.

c. Redifferentiation: Redifferentiation is the phenomenon where the dedifferentiated cells lose the ability to divide. Example is formation of phloem and secondary xylem from the vascular bundles.

Auxins are plant growth hormone regulators which are generally produced in the growing apices of the stems and roots, from where they are translocated to the regions of their action by vascular bundles.

The meaning of the following terms related to auxins are as follows:

a) auxin precursors: Substances that are responsible for production of auxins are known as auxin precursors. For example, the amino acid tryptophan is an auxin precursor, biosynthesis of auxin requires this amino acid.

b) anti-auxins: Substances that inhibit auxin action by competing for the same receptor. It does not cause any growth response. An example of anti-auxin is PCIB (p-chloro phenoxyisobutyric acid).

c) synthetic auxins: Some chemical compounds that have been synthesized in the laboratories and function as auxins are called synthetic auxins. They are not synthesized naturally by the plant parts. For example: Phenoxyacetic acid.

Ethylene and abscisic acid are plant growth hormones which have both positive as well as negative effects on a plant.

Positive Roles of Ethylene :
(a) Ethylene acts as a ripening agent involved in ripening of fruits.

(b) When present in low concentration, it stimulates root meristem formation and promotes growth of lateral roots.

(c) It is responsible for breaking dormancy of seeds and hence has a role in seed germination.

(d) It increase the number of female flowers in the cucumber plant.

Negative Role of Ethylene :
(a) Ethylene is responsible for nullifying geotropism.
(b) Ethylene inhibits the growth of lateral buds thus stimulating growth of apical buds or leading to apical dominance.
(c) Ethylene promotes abscission of leaves, flowers and fruits thus leading to senescence.

Positive Role of Abscisic acid:
(a) During stress conditions, abscisic acid concentration increases in the leaves of the plant due to which stomata closes preventing water loss via transpiration. Thus it increases the tolerance. Hence, it aids during stress conditions because of which it is called stress hormone.

(b) It increases the resistance of plants to cold climatic conditions.

(c) It leads to tuber formation in potato.
(d) It is also responsible for initiation of roots in stem cuttings of some selected plants. root initiation in stem cuttings of some plants.

Negative Role of Abscissic acid:

(a) It inhibits growth of a plant and in turn induces dormancy of buds.

(b) It leads to abscission of flowers and fruits.

(c) It promotes senescence of leaves by destroying the photosynthetic pigment called chlorophyll and inhibiting the synthesis of RNA and proteins.

Thus we see how these two hormones have both positive and negative effects on a plant.

Hormones have synergistic and antagonistic effect with respect to one another. Hence it becomes difficult to assign any affect seen to any single hormone because it can be a synergistic or an antagonistic effect as well.

Synergistic effect occurs when two hormones come together to have the same effect and when their effect is combined, it amplifies the needed objective.

Antagonistic effect on the other hand is when two hormones counter the effect of each other.

For eg: Auxin acts synergistically with cytokinin to control the shoot stem‐cell meristem while both the hormones act antagonistically to maintain the root meristem.

The mechanism underlying the phenomenon by which the terminal/apical bud suppresses the growth of lateral buds is known as apical dominance where apical bud growth is dominant. This is mainly due to the presence of auxin hormones synthesized in the tips of the apex of bud.

There are measures to overcome this phenomenon like: Decapitation or removal of the apical bud (shoot cutting) that inhibits growth of the apical bud and promotes the growth of the lateral branches. Cytokinin application can also be used in order to promote lateral bud growth and inhibit apical dominance.

Plant hormones are found in the different tissues like the tip of the shoots, the tip of the roots, meristematic tissues, apical buds etc. Hormones like cytokinins, auxins which are usually plant growth promoters usually are synthesized in regions of active cell division. Hormones like ethylene which are plant growth inhibitors are found almost throughout the plant body but specially in the parts which are old and in regions of ripened fruits.

These hormones are translocated to the site of activity by the vascular bundle tissues like xylem and phloem.

Many discoveries in science have been accidental which is true for plant hormones' discovery too. The five major groups of plant growth regulators have all been discovered accidentally.

• When Charles Darwin and his son Francis Darwin observed the coleoptiles of canary grass, they noticed that it responded to unilateral illumination by bending and growing towards the light source. Auxin was isolated by F.W. Went from tips of coleoptiles.

• Foolish seedling disease of rice was caused by the fungal pathogen Gibberella fujikuroi. The symptoms of which were first reported by Kurosawa who noticed that uninfected rice developed these symptoms when gibberellins were added.

• Only when auxins were added, the callus of stems proliferated. This was observed by F. Skoog and his co-workers .

• Skoog and Miller crystallised the cytokinesis promoting substance called kinetin.

• Scientists confirmed the release of a gaseous substance from ripened oranges - ethylene.

To get a carpet like grass, lawns are mowed regularly as mowing (process of cutting the grass) causes decapitation that promotes the growth of lateral buds. The process of Decapitation of plants leads to the inactivation of axillary buds and promotes growth of lateral buds giving a carpet like appearance.

In a slide showing different types of cells, it can be identified as to which cell is meristematic and which is incapable of dividing.

The meristematic cells which actively divide show the following characteristics which are not shown by cells that are non-dividing:

a) The nucleus of the cell is large.

b) Cell wall is thin and composed of cellulose.

c) It is rich in protoplasm.

d) Less vacuoles

e) Cells are differentiated.

A rubber band stretched and reverts back to its original position due to the property of elasticity where the form gets reversed. Bubble gum would not return to its original position after getting stretched because of the phenomenon of plasticity. It is irreversible.

Plants follows different types of pathways in response to different environmental conditions which leads to form different kinds of structures. Example is heterophylly (different leaf forms on the same plant) in plants like buttercup.

a) This is the shoot apex meristem of a dicotyledonous plant.

The shoot apical meristem is present at the tip of the plant stem, has two functions:

a) it is the point of source of new cells needed for growth stem

b) small cellular outgrowths occurs here called leaf primordia that later develop into leaves

The different parts labelled 1,2, 3 are:

1: Leaf primordium

2: Axillary bud: During the formation of leaves and elongation of stem, some cells which are left behind make the axillary bud: forms a branch or a flower.

3: Shoot apical meristem

b) If part 1 is removed from the plant, then the apical bud is inhibited while there is growth of lateral buds.

We say that growth and differentiation in plants is open and not so in animals despite both growing is because the plant growth and differentiation of certain parts like the meristematic tissues of the meristems present in different locations of the plant body is an active site of cell division where growth is indefinite. The cells keep on dividing at the meristems and hence growth occurs indefinitely. Because new cells are constantly added to the body of the plant it is called open growth.

Yes, this statement shall hold true for sponges as well.

Parthenocarpy is the natural or artificial induced development of fruit without the fertilisation of ovules. This process of development of fruit was introduced in the year 1902. Examples of fruits showing parthenocarpy naturally are grape, orange etc.

The plant hormone used to induce parthenocarpy is Gibberellin. Auxin is responsible for inducing parthenocarpy in tomatoes.

A method by which watermelons can be produced seedless is parthenocarpy. Parthenocarpy is the natural or artificial induced development of fruits without the fertilization of ovules or formation of seeds.

Growth hormones can induce parthenocarpy like Gibberellins and auxins inducing parthenocarpy in tomatoes.

To get rid of the weeds efficiently, it can be selectively killed using plant growth hormone regulator called auxins.

Dicot weeds grow by apical shoot meristem growth whereas the grasses usually have the presence of intercalary meristems. When plant growth hormones like auxins is applied, it causes damaging effect on the shoot meristem thus destroying the dicot weeds and has no effect on the lateral meristem keeping grasses intact without a damaging effect. Hence the weeds are selectively destroyed.

a) When a seed germinates, it first produces shoots with leaves. This means that the plant enters the vegetative growth phase. During this phase florigen is synthesized which is a flowering hormone. It induces flowering in plant.

b) It is advantageous to plants as the vegetative phase prepares the plants for the reproductive phase so that it can bear structures like fruits and seeds.

a. Maximum growth is observed in exponential phase.

- In this phase, the cells are not dormant. Here cell numbers increase in a logarithmic pattern. It is a period characterized by cell doubling.

b. Apical dominance is due to presence of auxins.

Explanation: Auxins are responsible for apical dominance. So apical dominance occurs when more auxins are available in apical bud than in the lateral buds. In most plants, the growing apical bud inhibits the growth of the lateral buds. This is called apical dominance.

c. Auxins hormone initiate rooting.

They help to initiate rooting in stem cuttings. This application is widely used for plant propagation. High concentrations of auxin inhibits root elongation and instead promotes adventitious root formation.

d. Pigment involved in Photoperception in flowering plants is phytochrome.

Phytochrome is a photoreceptor protein (homodimer) that senses seasonal changes in length of night- photoperiod. It is a crucial pigment that regulates different aspects of plant growth and development like seed germination, chloroplast development, flowering, etc.

Some plants are dependent on exposure to low temperature for flowering. This is called vernalisation. It allows plant to have adequate time to reach maturity. The ‘spring’ variety of wheat are sown in the spring and is harvested by the end of the same season while wWinter varieties if planted in spring would normally fail to flower or produce mature grain within a span of a the flowering season so they are sown in autumn. Then they take time to germinate and over winter they come out as seedlings. After that, they resume growth in the season of spring. Following which they are harvested around midsummer.

Winter wheat ^

^Spring wheat

a) Varieties of wheat which are the winter varieties would not normally flower in in a single flowering season available.

When they are planted in autumn, they take their time to germinate and over the winter period, seedlings are noticed which resumes its normal growth during the spring season so get harvested around the next midsummer.

b) Promotion of flowering under low temperature is called vernalization.

It is one of the important adaptations of plants growing at high altitudes to ensure that flowering occurs during the summer or warmer temperatures giving time for the development of seed in the plant.

c) The plant hormone that can replace the cold treatment is called gibberellin.

Plants that need cold treatment require photoperiodism to induce flowering in them. Without this treatment, vernalization has no effect. But it is observed that when these plants are treated with gibberellins (GA), they undergo flowering without subjecting the plants to cold environment and photoperiodism. Gibberellins are seen not only to substitute vernalization but also photoperiodism.

Vernalization- It is a process of cooling of seeds during its period of germination to induce faster flowering in them.

Photoperiodism: Physiological response of plants to day or night length or light exposure.

Hormone which

a. is gaseous in nature: ethylene

Ethylene is a small hydrocarbon gas which is an ageing hormone in different plants. It causes ripening of fruits as well as has growth inhibitory functions.

b. is responsible for phototropism: auxin

Presence of auxins is responsible for plant response to light intensities. Plant cells further away from the light has more auxin thus grows faster aiding bending of the stem towards the light source.

c. induces femaleness in flowers of cucumber: ethylene

Ethylene induces flowering so can be used to increase the number of female flowers in the cucumber plants. It selectively induces flowering of female plants suggesting a link between ethylene signalling and primordial anther specific DNA damage in female cucumber flowers.

d. is used for killing weeds (dicots): auxins

Auxin growth regulator herbicides are used for controlling annual, simple perennial etc. They are selective and is widely used to kill dicotyledonous weeds whereas not affecting mature monocotyledonous plants.

e. induces flowering in long day plants : Gibberellin

Usually after gibberellin is applied to long-dayplants, stem elongation occurs before flower primordia are formed.