Plant Growth And Development - Class 11 Medical Biology - Extra Questions

Auxin is a plant growth hormone. This hormone is responsible for the growth and elongation of the shoot. There is apical dominance observed in presence of auxin. This results in the growth of the central stem which is dominant and suppresses the growth of the lateral buds such phenomena is called as apical dominance. The auxins are produced at the tips. The pruning of the tips is done where the branches are cut periodically in mariculture. This allows the lateral buds to grow and form branches. This helps to make the plant bushy. 

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

In animals, once a particular type of tissue is formed it cannot change to any other tissue. Moreover, growth in an the organs stop after a certain age of the animal. On the contrary, growth in plant is indeterminate because a plant part can grow into infinitely many numbers. A senescent part is usually replaced in plants. Differentiated cells undergo dedifferentiation in plants if a need arises. 
Sponges are exceptions in the animal kingdom. A sponge shows open type of growth.

Unlike multicellular organisms, unicellular organisms do not grow in size or volume. Growth in unicellular organisms is marked by division which produces daughter cells. Thus, asexual reproduction by amitosis shows growth in these organisms.

The roots of plants are the part which grows under the soil. The roots of the plants shows the movement called as geotropism. This is the property by which the roots move towards the soil. The other property of the roots is to move towards the water is known as hydrotropism. The shoot of the plant moves towards the light. 

Tropism is the directional growth of a plant in response to external stimuli, such as light, gravity, water, etc. Roots generally grow away from the light. Here, the stimulus is light. And the roots show negative phototropism i.e. growth away from the light.

Colchicine is an alkaloid which is obtained from the seeds of Colchicum autumnale. It inhibits the microtuble polymerization which is essential to mitosis. They have the property which can cause the disruption of the microtubules which are responsible for the formation of the spindle. This is considered as a toxic chemical which helps to induce polyploidy in the plants. 

Cytokinin is the plant growth regulator. There are many physiological effects of this hormone. The hormone causes delay in the process of ageing and senescence. This process is called as the Richmond - Lang effect. During this effect, there is prevention of the protein degradation and nutrient immobilisation. The molecules like the chlorophyll pigments in the leaf are not degraded. This prevents the ageing and the falling of the leaves. This effect is brought by the hormone cytokinin. 

Functions of Gibberellin: The main functions of it are as follows:
(1) It helps in the elongation of stem and the internodes increase in length.
(2) Gibberellin convert genetically dwarf plant into a plant of normal height.

In addition to auxin, cytokinin a must be supplied to culture medium to obtain a good callus in plant tissue cultures.

Growth and development in higher plants is referred to as being open. This is because various meristems, having the capacity for continuously dividing and producing new cells, are present at different locations in these plant bodies.

Phototropism is the growth towards the source of light. This type of movement is observed in plants. The cells on the plant that are far from the source of light produce a hormone called auxin. This chemical is stimulated and reacts during phototropism. This causes the plant cells on the side away from the source of light to elongate. 

$$\textbf{Correct option: }$$ C

$$\textbf{ Solution: }$$ 

$$\bullet$$Plant hormones are the chemical substances that control and coordinate one or the other aspect of the growth of the plant. 

$$\bullet$$Plant hormones are also known to be phytohormones or plant growth substances.

$$\bullet$$The functions of the plant growth hormone are to control all the growth and development activities of the plant such as cell division, cell enlargement, dormancy, flowering, seed germination, and abscission. 

$$\bullet$$The two different phytohormone used by plants are auxin and cytokinin. 

$$\textbf{Auxin: }$$

$$\bullet$$Auxin is the plant hormone that promotes the growth of the plant. 

$$\bullet$$It is also involved in promoting cell enlargement and cell differentiation in plants along with fruit growth. 

$$\textbf{Cytokinins: }$$

$$\bullet$$Cytokinins are the plant hormone that promotes the growth of the plant.

$$\bullet$$Cytokinins promote the ageing of leaves and also help in breaking the dormancy of seeds and buds.

Richmond Lang effect is the retardation of aging due to the application of the plant regulator cytokinins. The regulator acts by retarding the rate of destruction of chlorophyll and yellowing of leaves. 

(1) Apical dominance and dormancy: Presence of auxin at the tip of plant stimulates the growth of the apical bud but suppresses the growth of lateral buds, this phenomenon is called apical dominance. If the apical buds are removed and auxin is supplied to the decapitated shoot the lateral buds are suppressed and thus, it confirms that auxin is responsible for apical dominance.

(2) Cell Division and cell enlargement: Auxins stimulate cell division, cell enlargement and cell elongation in the apical region.

Gibberellin is one of the plant growth hormone which is required for the growth and development of the plants. The regulator is required for different processes.

Ethylene is one of the plant growth regulators. It has several applications which include:

The phenomenon in which there is involvement of two or more different type of sexes or mating types is considered as heterothallism. There are two different mating types. Blakeslee was the geneticist who discovered this phenomenon in the year 1903. The common example of this phenomenon is shown by Rhizopus spp. There are two different mating types as + and -. The mycelium of these mating types interact and thus, zygospore is formed which help in the development of heterothallic mycelium. 

Gibberellin is one of the plant growth hormone which is required for the growth and development of the plants. The regulator is required for different processes.

There are various different uses of the auxins in the field of agriculture and horticulture. 

There are various different uses of the auxins in the field of agriculture and horticulture. 

Gibberellin is a hormone that breaks dormancy in seeds and buds. It ends dormancy in those plants that require light for the induction of germination.

Auxins are growth hormones produced by plants. They are essential for proper growth of the plant and cell elongation. They also regulate phototropism, geotropism, and hydrotropism in plants.

Auxins are a class of plant hormones (or plant growth regulators) with some morphogen-like characteristics. Auxins have a cardinal role in the coordination of many growth factors.
Some of its function is:
    • When a plant cell comes into contact with auxin, it causes dramatic changes in gene expression, with many genes up- or down-regulated. It is the molecular mechanism of the Auxin.

    • Auxin response factors (ARFs) are a large group of transcription factors that act in auxin signaling.

    • On the cellular level, auxin is essential for cell growth, affecting both cell division and cellular expansion.

    • Growth and division of plant cells together result in the growth of tissue, and specific tissue growth contributes to the development of plant organs.

    • Local auxin maxima can be formed by active biosynthesis in certain cells of tissues, for example via tryptophan-dependent pathways, but auxins are not synthesized in all cells (even if cells retain the potential ability to do so, only under specific conditions will auxin synthesis).

    • As auxins contribute to organ shaping, they are also fundamentally required for proper development of the plant itself.

Auxins are plant hormones that stimulate elongation of stem and expansion of leaf. It also induces differentiation of xylem and phloem, cell division in cambium and delaying of fruit ripening.

Growth:

Growth:

The main difference between auxin and gibberellin is that the auxin promotes the growth of the shoot system whereas gibberellin promotes stem elongation, germination, and flowering. Furthermore, auxin plays a role in apical dominance whereas gibberellin has no role in apical dominance.

Abscisic acid (ABA) is the plant hormone that functions during drought. It is a stress hormone that induces partial closure of stomata to reduce water loss.

The physiological effects of the hormone auxin is :

Auxins are produced in the apex of stem and root tips and they promote apical elongation of stem along with inhibiting growth of axillary buds. This phenomenon is known as Apical Dominance.

1. Ethylene stimulates ripening of fruit such as tomatoes, lemons, oranges etc. It also increases the rate of respiration.
   
2. It also regulate abscission and senescence.
3. It is also responsible for breaking dormancy.

• Auxins are the plant hormone which is synthesized at the shoot tip.
• It helps the cells to grow at the root tips.
• When a tendril comes in contact with a support, auxin stimulates faster growth of the cells on the opposite side, as a result of the tendrils coil around the support.

a) Ethylene helps to ripen fruits and increase respiration as well. So the statement is false.

• Cell elongation: Auxin stimulates elongation of cells in the shoot. For example, in phototropic curvature, when unilateral light is unequally distributed in stem tips, the higher concentrations of auxin on the shaded side causes the cells of that side to elongate more rapidly and this results in bending of stem tip towards the unilateral light.
• Apical dominance: The apical bud inhibits the growth of axillary buds further down the stems and this phenomenon is called apical dominance. Auxin is produced by the shoot tip and thus, its concentration is highest there. It acts as a growth promoter for the apical bud and inhibits the growth of lateral buds.
• Root initiation: Higher concentration of auxin inhibits the elongation of roots but the number of lateral branches in roots is considerably increased.
• Prevention of abscission: Auxin prevents abscission of leaves, fruits etc.
• Parthenocarpy: Auxin induce the formation of parthenocarpic fruits. Normally fruits do not develop if fertilisation has not occurred but if auxins are applied, then the fruits will develop.
  

Auxin is the hormone that promotes the growth of tendril around a support as it is synthesized in the tip of the shoot and stimulates the growth of the cells on the opposite side which causes coiling of the tendril around the support.

Auxins are the plant hormones which are used in horticulture and food industries. Auxins like IAA (Indole Acetic Acid) and IBA (Indole Butyric Acid) have been isolated from plants. NAA (Naphthalene Acetic Acid) and 2,4-D(2,4-dicholorophenoxyacetic) are synthetic auxins.

$$\textbf{ Solution: }$$ 

$$\bullet$$Differentiated parenchymatous cells  get dedifferentiated into cork cambium. 

$$\textbf{ Re-differentiation:}$$

$$\bullet$$It is the process in which de-differentiated cells become mature again and lose their capacity to divide.

$$\bullet$$The example of the process of re-differentiation in plants is the formation of  secondary phloem, secondary xylem, secondary cortex, cork, etc. 

$$\bullet$$This is redifferentiated from interfascicular cambium and cork cambium.

a) 'Cytokinin' is the plant hormone capable of the growth of plant tissue by cell division promoting action.

Onion plants can be grown from seeds or from bulbs. The process of growing onions from bulb is known as vegetative propagation (a form of asexual reproduction in which the new plant grows from a fragment of the parent plant). Bulb grows by the growth of the radicle and flag leaves after which the true leaves emerge.

Plant hormones are plant chemicals that control the procedures of cells. They are generally discovered throughout the plant in tiny quantities. These hormones are very essential because they determine a plant's many distinct characteristics. Plant hormones can be synthetic or natural. Plant hormones are also referred to as phytohormones

(a) The raw materials required for initial synthesis of auxins are called auxin precursors. Tryptophan is the precursor of Auxin.
(b) Compounds which inhibit the action of auxins are called anti-auxins. e.g.2,3,5-triiodobenzoic acid (TIBA) and naphthylphthalamic acid (NPA) (c) Auxins which are made artificially are called synthetic auxins, e. g. $$2,4-D, Naphthalene Acetic Acid $$.

Maximum growth is observe in Exponential  phase.

The  physiological effects of cytokinins are

Cell division: Induction of cell division is considered as a major function of cytokinins in plants.

Cell expansion: In Soya bean leaf discs and radish cotyledons, the addition of cytokinins results in a remarkable increase in size due to cell elongation.

Organ formation or Morphogenesis: Cytokinins in association with auxins, induce differentiation of organs. Skoog and his associates found that if a higher cytokinin to auxin ratio is maintained in the culture media, tissue differentiation takes place resulting in the formation of buds, stems, and leaves. On the other hand a low ratio of cytokinin favours root formation.

Delay in senescence or Richmond – Land effect: The ability of cytokinin to delay the process of senescence in plants is called Richmond-Lang effect. Senescence in leaves is maintained by yellowing.It is due to the breakdown or decreased synthesis of chlorophylls, RNA and proteins. Senescence is followed by the death of organs or whole plant. If a proper concentration of cytokinins are applied to the plants, plants remain green and healthy.

Opening of stomata: Cytokinins increase the potassium ion concentration in guard cells and help in the opening of stomata.

Increased growth per unit time is termed as growth rate.

Auxin : Enlargements of cells :: cytokinins : growth of shoot buds. Auxin is a plant hormone formed in the stem-tip that indorses cell elongation.

Phytohormone responsible for internode elongation Auxin is the phytohormone responsible for internode elongation. Auxin promotes stem elongation, inhibit the growth of lateral buds. 

Interphase is known as the period of growth. This phase is divided into three phases, namely

Auxins are the plant growth hormones that control the growing parts of the plant, stimulate cell elongation, differentiation of xylem and phloem, fruit development, apical dominance etc. They are synthesized at the tips of roots and shoot in the plant body. Auxin also influence plant responses to phototropism and geotropism.

ABA stands for Abscisic Acid, it is a naturally occurring plant hormone. 

(a) Feedback mechanism - In feedback mechanism the amount of hormone released by an endocrine gland is regulated by the amount of hormone in the blood.

Ethylene

(a)Increase yield of sugarcane - Gibberellic acid
(b)  Promote lateral shoot growth - Cytokinin
(c) Cause sprouting of potato tuber - Ethylene
(d)  Inhibit seed germination  - Abscisic acid

Following examples show that ethylene and abscissic acid have both positive and negative roles. 
Positive Effects of Ethylene: 
Promotes horizontal growth.
Promotes positive geotropism in stem.
Promotes apical dominance.
Breaks seed and bud dormancy.
Promotes fruit ripening and root initiation. 

Negative Effects of Ethylene: 
Inhibits longitudinal growth.
Enhances senescence of leaves, flowers and fruits.
Prolongs dormancy of lateral buds. 

Positive Effects of Abscisic Acid: 
Helps in reducing transpiration and thus in preventing water loss.
Induces root initiation and flowering in some plants.
Promotes seed dormancy and bud dormancy.
Helps plants develop resistance against cold and other types of stress. 
Negative Effects of Abscissic Acid: 
Promotes senescence of leaves.
Retards cambial activity.
Inhibits the function of growth hormones.
Inhibits seed germination.

 A phase of growth which is maximum and fastest is exponential phase.

When de-differentiated cells become mature and assume specialized function once again, this is called redifferentiation. For example: cambium redifferentiates to form the cortex.

Annual plants have shorter jevenile phase compare to perennial plans. Moreover, it also depends on number of flowering seasons in a given year. Some perennial plants produce flower only once in a year. Some others may produce flowers once in 50 years or even 100 years. In annual plants like Pea, Watermelon etc the plants undergoes juvenile phase followed by reproductive phase within one growing season, so, it has a shorter juvenile phase.

The age of a living tree can be determined by comparing the diameter of the trunk with an average growth factor of that species. The average growth factor of a species can be found from records. Diameter can be calculated by measuring the girth of the trunk.

The auxins which can be easily isolated from cells are called free auxins and those which can be isolated with difficulty with the help of organic solvents are called bound auxins.

Zeatin is the first naturally occurring cytokinin. Letham and Miller (1964) isolated this from the endosperm of Maize.

• NAA (Naphthalene Acetic Acid).
• IBA (Indole-3 Butyric Acid).
• are two artificially synthesized Auxins.

During development of an embryo, cell differentiation is necessary to produce different kinds of tissues which could later develop into various organs and organ systems. If cell differentiation does not happen, then different tissues would not be produced and the embryo would only develop into a mass of identical cells; unable to develop into a new individual. This can be compared with differentiation of meristematic cell in plants. In vascular plants, it is differentiation which ultimately results in formation of specialized tissues which performs various functions.

Different contrasting characters are stem length, colour of flower, seed colour, seed shape, pod colour, pod shape, position of flower.

Movement in sensitive plants	Movement in legs
1. Movement in sensitive plants occurs in response to external or internal stimulus and is categorized as tropic or nastic movement; it is an involuntary action.	1. Movement in legs is under the regulation of nervous system and is a voluntary response, i.e., conscious response.
2. It does not involve any special tissues for the relay of information.	2. Leg movement is assisted by complex network of nervous system that includes CNS and PNS
3. Movement in the sensitive plant does not involve any specific proteins.	3. Contraction and relaxation of leg muscles to bring about leg movement is assisted by specific proteins like actin, myosin etc.

$$\textbf{ Solution: }$$ 

$$\textbf{ Growth: }$$ 

$$\bullet$$Growth is an irreversible and permanent process which can be seen by the increase in the length, size of the plant cells and tissues.
$$\bullet$$Few of the examples showing the growth in plants can be depicted as the lengthening of the roots and stem. 

$$\bullet$$The increase in the thickness of the woody plants  during their life span is also an example of growth process in plants. 

 $$\textbf{Differentiation : }$$ 

$$\bullet$$It is a process in which the cells derived from the apical meristem (root and shoot apex) and the cambium undergo structural changes in the cell wall and the protoplasm. 

$$\bullet$$These structural changes result in the achievement of the specific function.

$$\bullet$$One of the major example of differentiation in plants is  the formation of interfascicular and cork cambium which is derived from the fully differentiated Parenchyma cells. 

$$\textbf{ Development: }$$ 

$$\bullet$$Plant development is referred to as the various continuous changes occurring in the plant body. 

$$\bullet$$The process involves embryogenesis followed by formation of the primary plant body (embryonic root and embryonic shoot) and continuous production of new plant parts such as roots, leaves, branches, and flowers.

$$\bullet$$The end stage of plant development is senescence. 

$$\textbf{ De-differentiation: }$$ 

$$\bullet$$It is the process in which permanent plant cells regain the power to divide under certain conditions.

$$\bullet$$Few of the example of dedifferentiation in plants are as follows: 

$$\bullet$$Parenchymatous medullary rays get dedifferentiated into interfascicular cambium in dicotyledonous stem. 

$$\bullet$$Differentiated parenchymatous cells  get dedifferentiated into cork cambium. 

$$\textbf{ Re-differentiation: }$$ 

$$\bullet$$It is the process in which de-differentiated cells become mature again and lose their capacity to divide.

$$\bullet$$The example of the process of re-differentiation in plants is the formation of secondary phloem, secondary xylem, secondary cortex, cork, etc. 

$$\bullet$$This is redifferentiated from interfascicular cambium and cork cambium.

$$\textbf{ Determinate growth}$$ 

$$\bullet$$The type of plant growth which gets inhibited or limited at a certain stage. 

$$\bullet$$Examples of determinate growth is as follows:

$$\bullet$$The limitation of growth of the stem in an inflorescence or other reproductive structure. 

$$\bullet$$The branches of the stem stop growing continuously while it is deriving from the main branch. 

$$\textbf{ Meristem: }$$ 

$$\bullet$$Meristem is the specialised regions in the plant which is specialized in the divisions of the active cells. 

$$\bullet$$There are three types of meristems that are apical meristem, lateral meristem, and intercalary meristem.

$$\bullet$$Apical meristem is the tip of the plat root or stem which is capable of division and growth. 

$$\bullet$$The type of meristem located in the lateral region of the plants and responsible for increasing the lateral growth of the plants , is defined as the lateral meristem. 

$$\bullet$$The type of meristem located in the middle position of the plants and is associated with the growth in length in the middle position, is defined as the intercalary meristem.  

$$\textbf{ Growth rate: }$$ 

$$\bullet$$It can be defined as the increased growth in plants per unit time.

$$\bullet$$It can be calculated by taking the change in size and dividing it by the amount of time it has been growing.

There are three phases of growth - meristematic, elongation and maturation. We can understand this better by looking at a seed. We already know that the tips of roots and shoots exhibit continuous growth and hence are meristematic. The cells in this region are rich in protoplasm and have large nuclei. The cells next to this region represent the elongation growth phase. In this region, there is the formation of new cell walls, vacuoles etc. The region next to the elongation phase represents the maturation phase in which the cells attain their maximal size.

IAA is the most commonly occurring form of auxin. It is a potent growth stimulator in plants that cause significant root growth and shoot growth. It can also delay fruit ripening and causes cell elongation in plants.

A plant hormone can modulate transcription at the molecular level. Plant cells have to maintain a dynamic transcription sequence to regulate their development, communication, and response to the environment. These plant hormones can modulate such transcriptions according to environmental stress or physical needs.

Cytokinins are a class of plant growth substances that promote cell division, or cytokinesis, in plant roots and shoots. They are involved primarily in cell growth and differentiation, but also affect apical dominance, axillary bud growth, and leaf senescence.

Cytokinins are synthesized in the root apex of plants. The two functions of cytokinins include an increase in cell division and delay of senescence.

$$\textbf{ Solution: }$$ 

$$\bullet$$Milky endosperm of coconut is rich in cytokinin phytohormone. 

$$\bullet$$Cytokinins are the plant hormones which promote cell division and growth in plants. 

$$\bullet$$It also helps in breaking the dormancy of seeds and buds. 

$$\bullet$$Cytokinins promote the ageing of leaves and also help in breaking the dormancy of seeds and buds.

$$\bullet$$It also promotes the opening of the stomata.

$$\bullet$$This hormone acts upon the guard cells of the stomata which are responsible for the opening of the stomatal pore, controlling the rate of transpiration. 

Irreversible increase in size, usually accompanied by an increase in dry weight of an organism is called growth.

Overcome genetic dwarfism, breaking of dormancy flowering and sex expression.

Growth hormones:

Cytokinins are a group of plant growth regulators which are primarily involved in performing cell division in plant roots, shoot system. This hormone helps in promoting the cell's growth, development, differentiation, affecting apical dominance, leaf senescence, and axillary bud growth. Cytokinins are plant hormones that cause increased cell division by stimulating the process of mitosis. They are made naturally by plants but have been synthesized by humans. Increased mitosis results in plant growth and the formation of shoots and buds, as well as the development of fruits and seeds.

Auxin also elongates root cells down into the ground, while at the same time elongating cells in the stem upward. This phenomenon, called gravitropism helps the plant develop a strong underground support system and get nutrients from the ground, and it also helps it grow tall towards the sunlight.

When GA3 is applied to rice seedlings, the growth regulator stimulates elongation of the stem. Thus the plant will grow to be very tall and slender.

They can stimulate rapid stem and root growth, induce mitotic division in the leaves of some plants, and increase seed germination rate. In this way, it can overcome the dwarfism in plants.

Kurosawa, a Japanese botanist, discovered gibberellin while investigating the rice foolish seedling disease in which spindly seedlings are formed due to GA like compounds produced by the fungus ( Gibberella fujikuroi) infecting the plant. ( see text for more information) .
Experiment 1:
Several suitable seedlings of Vigna or Phaseolus are grown in two Petri dishes. One lot of seedlings is sprayed with 20 ppm gibberellic acid (GA3) solution with the help of an ato­miser or a sprayer. The other lot is sprayed with distilled water which serves as a control. Observations are made after one week by measuring the height of the seedlings.
Observation
It is observed that the height of the gibberellin-treated seedlings has increased considerably compared to that of control seedlings.

Experiment 2:
Three lots of Vigna or Phaseolus seedlings are grown in three Petri dishes. One lot is kept in dark and the other two lots are kept in light of which one lot is sprayed with 20 ppm gibberellic acid (GAg) solu­tion. The stem growth of the seedlings in each case is measured and com­pared.
Observation
Light has an inhibitory effect on stem elongation. Appli­cation of gibberellin to the plants growing in light greatly increases their stem growth. Reversal of light-induced inhibition of stem elongation by application of gibberellin suggests that endogenous gibberellin is the limiting factor in stem growth.
It may be concluded that light causes inhibition of stem growth by lowering the level of endogenous gibberellin and this inhibi­tion is erased by applying exogenous gibberellin to the plants.

Bolting is a process by which a plant quickly produces next-generation seeds before they die in a harsh environment. Bolting is done artificially in a rosette plant by gibberellins.

Gibberellins are a group of plant hormones responsible for growth and development. Chemically speaking, gibberellins are actually acids.  They have different functions in plant growth regulation :
    • Gibberellins are growth hormones that stimulate cell elongation and cause plants to grow taller.
    • Gibberellins also have a role in other plant processes, such as stem elongation, germination, flowering, and fruit ripening.

The physiological roles of auxin are as follows:

(a) Gibberellin induces bolting. i.e. growth of internode in rosette plants. Gibberellin also induces seed germination by breaking seed dormancy.
(b) The fungus produces excess amount of Gibberellin. This promotes too much growth In internodes and the plant becomes too tall to hold its own weight.

Auxins are the plant growth hormones that control the growing parts of the plant, stimulate cell elongation, differentiation of xylem and phloem, fruit development, apical dominance etc. Auxins influence gibberellins that promote cell elongation and this increases plant length. They increase the distance between nodes thus spacing the branch points. Auxins control seedling orientation and always move away from light and downward due to gravity. In areas, where auxin is highly concentrated, cell growth is more. Auxin when applied, also stimulate root branching by initiating roots at the cut stem.

From the theory of physics, we know that if a substance regains its original shape after the removal of deforming forces, the substance is said to exhibit elasticity. On the other hand, if a substance fails to regain its original shape after the removal of deforming forces, the substance is said to exhibit plasticity. In above examples; rubber band is showing elasticity. But bubble gum is showing plasticity.