2021 Solved Old Paper (BOT - 204) New

Ans.

Role of Nitrogenase Enzyme:-

- Nodule contains nitrogenase and leghaemoglobin. 

- The nitrogenase has 2 components:

i. Molybdoferredoxin (Mo-Fe protein)

ii. Azoferredoxin (Fe-protein)

- The free di-nitrogen first bound to MoFe protein and is not released until completely reduced to ammonia. 

- In this process ferredoxin serves as an electron donor to Fe-protein (nitrogenase reductase) which in turn hydrolyzes ATP and reduce Mo-Fe protein, the Mo-Fe protein in Turn reduce the substrate N2. The electrons and ATP are provided by photosynthesis and respiration of the host cells.

- Many intermediates are formed to form ammonia (NH3).

Dinitrogen → Hydrazine → Diamine → Ammonia

- Ammonia (NH3) is immediately protonated at physiological pH to form ammonium ion (NH4+). As NH4+ is toxic to plants, it is rapidly used near the site of generation to synthesize amino acids.

Ans.

Chemical nature of Cytokinin:- All the naturally-occurring cytokinins are substituted purines. The usual way of naming a cytokinin is to express it as a substituted 6-amino purine or as N6-substituted adenine.

Ans.

Vernalization:- It is the artificial exposure of plants or seeds to low temperatures in order to stimulate flowering or to enhance seed production. Gibberellin is a hormone that replaces vernalization. The metabolically active apical meristems are the sites of perception of temperature to initiate flowering. The younger leaves are more susceptible to the process of vernalization. The shoot apex of mature stems or embryo of seeds receives low temperature stimulus.

Mechanism of Vernalization:- Through vernalization, there is an advancement in the process of blooming as a result of the delayed period of low temperatures, for instance, that which is attained in winter. To describe the mechanism of vernalization, there are two main hypotheses –

a. Phasic development theory

b. Hormonal theories

a. Phasic Development Theory:- As per this hypothesis, there is organization of stages in the plant’s improvement. Each stage is under the impact of environmental elements such as light, temperature etc. Here, in turn, there are two main stages –

i.Thermostage:- It depends on temperature, wherein vernalization accelerates thermostat. Thermostage is the vegetative phase requiring low heat, aeration and enough dampness

ii. Photostage:- It necessitates high temperature. Here, vernalin assists in producing florigen.

b. Hormonal theories:- As per this hypothesis, the freezing treatment propels the development of a floral hormone referred to as vernalin. Such a hormone is imparted to various parts of the plant. The vernalin hormone diffuses from the vernalized plants to the unvernalized plants, prompting blooming.

Ans.

Bulk movement of water (Ascent of Sap):- The upward movement of water from roots to the aerial parts of a plant is known as an ascent of sap.

Transpiration Pull or Cohesion-Tension Theory:- 

- This theory was put forward by Dixon and Joly in 1894.

- But it was further improved by Dixon in 1914, so this theory is also known as Dixon’s theory of the ascent of sap.

- This is the most accepted theory for the study of the ascent of sap.

- In the water column, the water molecules remain attracted by the cohesive force and cannot be separated easily from one another.

- There is an attraction between water molecules and the inner wall of xylem ducts, due to which the water column cannot be pulled away from the walls of the xylem ducts.

- Hence, due to strong cohesive and adhesive forces, the continuity of the water column from roots to leaves is maintained.

- Water lost from the mesophyll cells of the leaves creates a strong negative water potential that leads to a negative pressure in the water column. This pressure exerts an upward pull over the water column, which is known as transpiration pull.

- This tension or pull is transmitted up to the roots in search of more water.

- The water column (formed in the xylem elements of roots) now moves upwards under the influence of transpiration pull.

- Thus, the cohesive, adhesive forces and transpiration pull all together help in lifting up water through xylem elements.

- Because of the critical role of cohesion, the transpiration pull is also called the cohesion-tension transpiration pull model of water transport.

- The water inside the xylem vessels forms a continuous column from top to bottom.

- The tension may cause a break in the water column but due to the adhesive and cohesive property of water, the continuous column of water does not break.

Soil Plant Atmosphere Continuum (SPAC):-

>  It is proposed by Huber in 1924.

> All components of field environment i.e. soil, plant and atmosphere, form a physically unified and dynamics networking system in which various water flow processes occur independently. This unified system is called the soil-Plant-Atmosphere-Continuum or SPAC.

 > The fundamental principle of SPAC is water moves from higher total water potential to regions of lower total water potential at a rate depending on the hydraulic resistance of the medium.

Ans.

Mineral Absorption (Uptake of Minerals):-

·  Pathway:- Uptake of minerals takes place through apoplast as well as symplast pathway.

·  Mineral salts are translocated through xylem along with the ascending stream of water.

·  Mineral salts are absorbed from the soil solution in the form of ions. They are chiefly absorbed through the meristematic regions of the roots near the tips.

·  Plasma membrane of the root cells is not permeable to all the ions. It is selectively per­meable.

1. Passive Absorption (Passive Uptake):- It does not require expenditure of metabolic energy.

a. Simple Diffusion:- When the concentration of mineral salts is higher in the outer solution than in the cell sap of the root cells, the mineral salts are absorbed according to the concentration gradient by simple process of diffusion.

b. Ion Exchange Mechanism:- The ions adsorbed on the surface of the walls or membranes of root cells may be exchanged with the ions of same sign from external solution. For example, the cation K⁺ of the external soil solution may be exchanged with H⁺ ion adsorbed on the surface of the root cells. Simi­larly, an anion may be exchanged with OH^– ion. There are two theories regarding the mecha­nism of ion exchange:-

i. Carbonic Acid Exchange Theory:- According to this theory, the CO₂ released during respiration of root cells combines with water to form carbonic acid (H₂CO₃). Carbonic acid dissociates into H⁺ and an anion HCO₃^– in soil solution. These H⁺ ions may be exchanged for cations adsorbed on clay particles.

The cations thus released into the soil solution from the clay particles, may be adsorbed on root cells in exchange for H⁺ ions or as ion pairs with bicarbonate.

ii. Contact Exchange Theory:- According to this theory, the ions adsorbed on the surface of root cells and clay particles (or clay micelles) are not held tightly but oscillate within small volume of space. If the roots and clay particles are in close contact with each other, the ions adsorbed on clay particle may be exchanged with the ions adsorbed on root-surface di­rectly without first being dissolved in soil solution.

c. Donnan Equilibrium Theory:- The membrane is selectively permeable in nature. Some are permeable to ions and some are impermeable.

The ions for which the membrane is permeable enter the cell by diffusion. This disturbs the equilibrium. To re-establish the equilibrium, there is an exchange of cations and anions again.

d. Mass Flow Theory:- Ions are also transferred along with the flow of water. Therefore, the absorption of mineral salts from the soil occurs in a water-soluble state. Higher the rate of transpiration, greater is the absorption of mineral salts.

2. Active Absorption (Active Uptake):-

Ø It has often been observed that the cell sap in plants accumulates large quantities of min­eral salts ions

against the concentration gradient. It is an active process which involves the expenditure of metabolic energy through respiration.

Ø Following evidences favor this view:-

i. The factors like low temp., deficiency of O₂, metabolic inhibitors etc. which inhibit metabolic activities like respiration in plants also inhibit accumulation of ions.

ii. Rate of respiration is increased when a plant is transferred from water to salt solution. It is called salt respiration.

a. The Carrier Concept:- According to this theory the plasma membrane is impermeable to free ions. But some compounds present in it acts as carrier and combines with ions to form carrier-ion-complex which can move across the membrane. On the inner surface of the membrane this complex breaks releasing ions into the cell while the carrier goes back to the outer surface to pick up fresh ions.

b. Cytochrome Pump Theory:-

Ø Lundegardh and Burstrom (1933) believed that there was a definite correlation between respiration and anion absorption. Thus when a plant is transferred from water to a salt solu­tion the rate of respiration increases. This increase in rate of respiration over the normal res­piration has been called as anion respiration or salt respiration.

Ø The inhibition of salt respiration and the accompanying absorption of anions by CO and cyanides (which are known inhibitors of cytochrome oxidase of electron transport chain in mitochondria), later on led Lundegardh (1950, 54) to propose cytochrome pump theory. This is based on the following assumptions:-

i. The mechanism of anion and cation absorption is different.

ii. Anions are absorbed through cytochrome chain by an active process.

iii. Cations are absorbed passively.

Ø According to this theory:-

i. ehydrogenase reactions on inner side of the membrane give rise to protons (H⁺) and electrons (e^–).

ii. The electron travels over the cytochrome chain towards outside the membrane, so that the Fe of the cytochrome becomes reduced (Fe⁺⁺) on the outer surface and oxidized (Fe⁺⁺⁺) on the inner surface.

iii. On the outer surface, the reduced cytochrome is oxidized by oxygen releasing the electron (e^–) and taking an anion (A^–).

iv. The electron thus released unites with H⁺ and oxygen to form water.

v. The anion (A^–) travels over the cytochrome chain towards inside.

vi. On the inner surface the oxidized cytochrome becomes reduced by taking an elec­tron produced through the dehydrogenase reactions, and the anion (A^–) is released.

vii. As a result of anion absorption, a cation (M⁺) moves passively from outside to inside to balance the anion.

Ø Main defects of the above theory are:-

i. It envisages active absorption of only anions.

ii. It does not explain selective uptake of ions.

iii. It has been found that cations also stimulate respiration.

c. Protein-Lecithin Theory:-

Ø In 1956, Bennet-Clark suggested that because the cell membranes chiefly consist of phos­pholipids and proteins and certain enzymes seem to be located on them, the carrier could be a protein associated with the phosphatide called as lecithin.

Ø He also assumed the presence of different phosphatides to correspond with the number of known competitive groups of cations and anions (which will be taken inside the cell).

Ø According to this theory:-

i. The phosphate group in the phosphatide is regarded as the active centre binding the cations, and the basic choline group as the anion binding center.

ii. The ions are liberated on the inner surface of the membrane by decomposition of the lecithin by the enzyme lecithinase.

iii. The regeneration of the carrier lecithin from phosphatidic acid and choline takes place in the presence of the enzymes choline acetylase and choline esterase and ATP. The ATP acts as a source of energy.

Ans.

Fate of Ammonia:-

> What happens to the ammonia generated after nitrogen-fixation? It is protonated to form ammonium ion (NH4+) at physiological pH. Although plants can accumulate nitrate and NH4+ ions, NH4+ ions are toxic to them. Thus, it is in turn, used to synthesize amino acids in plants as follows:

(i) Reductive amination:- Here, ammonia reacts with α-ketoglutaric acid to form glutamic acid in the presence of the enzyme – glutamate dehydrogenase.

(ii) Transamination:- Here, the amino group of one amino acid is transferred to the keto group of a keto acid in the presence of the enzyme – transaminase. Asparagine and glutamine – the two most important amides in plants arise from two amino acids – aspartic acid and glutamic acid, respectively. Another NH2– radicle replaces the hydroxyl group of the acid to give an amide. The xylem then transports these amides that contain more nitrogen to different parts of the plant.

Ans.

Chemical Structure of Gibberellins:- Gibberellins are tetracyclic diterpenes with an ent-gibberellane ring structure.

Physiological Effects of Gibberellins:- 

i. Elongation of intact stems:- Many plants respond to application of GA by a marked increase in stem length; the effect is primarily one of internode elongation.

ii. Dwarf shoots:- Besides general increase in stem length, gibberellins specifically induce inter­nodal growth in some genetically dwarf varieties of plants like Pea and Maize. It appears that dwarf- ness of such varieties is due to internal deficiency of gibberellins.

iii. Bolting:- Gibberellins induce sub-apical meristem to develop faster. This causes elongation of reduced stem or bolting in case of rosette plants (e.g., Henbane, Cabbage) and root crops (e.g., Radish).

iv. Dormancy:- Gibberellins overcome the natural dormancy of buds, tubers, seeds etc., and allow them to grow. In this function they are antagonistic to abscisic acid (ABA).

v. Seed Germination:- During seed germination, especially of cereals, gibberellins stimulate the production of some messenger RNAs and then hydrolytic enzymes like amylases, lipases and pro­teases. The enzymes solubilise the reserve food of the seed. The same is transferred to embryo axis for its growth.

vi. Fruit Development:- Along with auxin, gibberellins control fruit growth and development. They can induce parthenocarpy or development of seedless fruits from unfertilized pistils, especially in case of pomes (e.g., Apple, Pear).

vii. Flowering:- They promote flowering in long day plants during noninductive periods.

viii. Vernalization:- Vernalization or low temperature requirement of some plants can be replaced by gibberellins.

ix. Application of gibberellins increases the number and size of several fruits, e.g., Grapes, To­mato; induce parthenocarpy in many species; and delay ripening of citrus fruits thus making storage safe.

Ans.

Photoperiodism:- The flowering in plants in response to the relative lengths of light and dark periods, is called photoperiodism. Garner and Allard introduced the terms photoperiod and photoperiodism and classified plants into the photoperiodic groups we use today.

i. Long Day Plants:- A plant that flowers only after being exposed to light periods longer than a certain critical length, as in summer. Here light period is critical for floweing. Examples:- Spinach, lettuce, and some varieties of wheat.

ii. Short Day Plants:- A plant that flowers only after being exposed to dark periods longer than a certain critical length, as in winter. Here dark period is critical for floweing. Examples: - Chrysanthemum, rice, soybean, onion.

iii. Day Neutral Plants:- A plant that flowers regardless of the length of the period of light it is exposed to, is called day neutral plant. Example:- Maize, Cucumber

Role of Phytochrome in Flowering:-

Phytochrome:- It is a photoreceptor, a pigment that plants, and some bacteria and fungi, use to detect light. It is sensitive to light in the red and far-red region of the visible spectrum.

a. Phytochromes role in Short-day Plants:-

> Short Day Plants are those that require less than 10 hours of daylight and more than 12 hours of darkness to begin flowering. 

> Pr is changed to Pfr form in many SDPs when the dark cycle is broken with a brief exposure (approximately 1 hr) to red light. 

> Flowering is inhibited because of the accumulation of Pfr. 

> Pfr is changed to Pr and the plant produces flowers if far-red light is supplied for a short time following red light treatment.

> During the winter months, far-red light is received on the earth’s surface in greater quantities than portions of red light reaching the ground. This changes a large portion of the Pfr form into Pr, causing SDPs to flower.

> In the summer, however, the ratio is reversed because more sunlight reaches the soil, preventing SDPs from flowering.

b. Phytochromes role in Long-day Plants:-

> Long Day Plants are those that require more than 14-16 hours of daylight and 8–10-hour dark periods to begin flowering. 

> LDPs will not flower if the photo-period is less than 14 hours of light and more than 8 hours of darkness. 

> Light is critical for flowering in LDPs.

> The role of phytochrome in LDPs is more complicated, and a blue-light photoreceptor is also necessary for flowering control.

> During the summer, more red light reaches the earth’s surface, compared to portions of far-red light reaching the ground. This changes a large portion of the Pr form into the Pfr form, causing LDPs to flower.

> However, in the winter, the ratio is reversed because more far-red light reaches the land, keeping LDPs from flowering.

Ans.

Mechanism of Stomatal Movement:-

> Stomata function as turgor-operated valves because their opening and closing movement is governed by turgor changes of the guard cells. 

> Whenever, the guard cells swell up due to increased turgor, a pore is created between them. With the loss of turgor the stomatal pores are closed.

> Stomata generally open during the day and close during the night with a few exceptions.

>The important factors which govern the stomatal opening are light, high pH or reduced CO2 and

availability of water. The opposite factors govern stomatal closure, viz., darkness, low pH or high CO2 and dehydration.

> There are three main theories about the mechanism of stomatal movements:

a. Hypothesis of Guard Cell Photosynthesis:-

- Guard cells contain chloroplasts. 

- During day the chloroplasts perform photosynthesis and produce sugar. 

- Sugar increases osmotic concentration of guard cells. It causes absorption of water from nearby epidermal cells. 

- The turgid guard cells bend outwardly and create a pore in between. 

- However, photosynthetic activity of guard cell chloroplasts seems to be negligible.

b. Classical Starch Hydrolysis Theory:-

- The main features of the theory were spelled out by Sayre (1923). 

- It was modified by Steward (1964). 

- The guard cells contain starch. 

- At low carbon dioxide concentration (in the morning achieved through photosynthesis by mesophyll and guard cells), pH of guard cells rises. It stimulates enzyme phosphorylase. Phosphorylase converts starch into glucose 1- phosphate. The latter is changed to glucose 6-phosphate which undergoes hydrolysis to produce glucose and phosphoric acid. Glucose increases osmotic concentration of guard cells. On account of it, the guard cells absorb water from neighbouring cells, swell up and create a pore in between them.

- Evening closure of stomata is brought about by increased carbon dioxide content (due to stoppage of photosynthesis) of leaf. It decreases pH of guard cells and brings about phosphorylation of glucose. In the presence of phosphorylase, glucose 1-phosphate is changed into starch. As a result, osmotic concentration of guard cells falls. They lose water to adjacent epidermal cells. With the loss of turgidity, the guard cells shrink and close the pore in between them.

Objections:-

(i) Glucose is not found in guard cells at the time of stomatal opening.

(ii) Starch ↔ Sugar changes are chemically slow while opening and closing of stomata are quite

rapid.

(iii) Wide changes in pH of guard cells cannot be explained on the basis of carbon dioxide

concentration.

(iv) Onion and some of its relatives do not possess starch or related polysaccharide that can be

hydrolysed to the level of glucose.

(v) Blue light has been found to be more effective than other wavelengths for opening of

stomata. The same cannot be explained by starch hydrolysis theory.

(vi) Hydrolysis of starch theory cannot account for high rise in osmotic pressure found in guard

cells. 

c. Malate or K+ ion Pump Hypothesis (Modern Theory):- The main features of the theory were put forward by Levitt (1974). 

i. During Stomatal Opening:-

- According to this theory, pH of the guard cell can rise due to active H+ uptake by guard cell chloroplasts or mitochondria, CO2 assimilation by mesophyll and guard cells. 

- A rise in pH causes hydrolysis of starch to form organic acids, especially phosphoenol pyruvate. Starch → Hexose Phosphate → Phosphoenol Pyruvate.

- Phosphoenol pyruvate can also be formed by pyruvic acid of respiratory pathway. With

the help of PEP carboxylase (PEP case), it combines with available CO2 to produce oxalic acid

which gets changed into malic acid.

- Malic acid dissociates into H+ and malate. H+ ions pass out of the guard cells actively. In exchange, K+ ions pass inwardly. Same CI– ions may also enter guard cells along with K+ ions.

- Guard cells maintain their electroneutrality by balancing K+ with malate and Cl–. 

- In the combined state they pass into the small vacuoles and increase the osmotic concentration of the guard cells. As a result guard cells absorb water from the nearby epidermal cells through endosmosis, swell up and create a pore in between them.

ii. During Stomatal Closing:-

- The H+  ions diffuse out of the guard cell chloroplasts. It decreases pH of the guard cell cytoplasm. 

- Any malate present in the cytoplasm combines with H+  to form malic acid. 

- Excess of malic acid inhibits its own biosynthesis. High CO2 concentration also has a similar effect. 

- Un-dissociated malic acid promotes leakage of ions. As a result K+ ions dissociate from malate and pass out of the guard cells.

- Formation of abcisic acid (as during drought or midday) also promotes reversal of H+ = ↔ K+ pump and increases availability of H+ inside the guard cell cytoplasm. 

- Loss of K+ ions decreases osmotic concentration of guard cells as compared to adjacent epidermal cells. This causes exosmosis and hence turgidity of the guard cells decreases. It closes the pore

between the guard cells. Simultaneously the organic acids are metabolised to produce starch.

Ans.

Biological Nitrogen Fixation:-

The conversion of atmospheric nitrogen into the nitrogenous compounds by living organisms is called biological nitrogen fixation. Only prokaryotes can fix nitrogen. Nitrogen fixation require anaerobic conditions because oxygen inactivates nitrogenase enzyme. 

Hence for obligate anaerobes nitrogen fixation is easy, but in case of facultative anaerobes the nitrogen fixation occurs only in anaerobic conditions. In case of obligate aerobes the oxygen level inside the cell must be kept low for nitrogen fixation.

1. Nitrogen Fixers (Diazotrophs):- Among the earth’s organisms, only some prokaryotes like bacteria and cyanobacteria can fix atmosphere nitrogen. They are called nitrogen fixers or diazotrophs. They fix about 95% of the total global nitrogen fixed annually by natural process.

a. Asymbionts (Free living)

b. Symbionts

a. Asymbionts (Free living):-

i. Bacteria:- They add up to 10-25 kg, of nitrogen/ha/annum.

> Azotobacter (Aerobic, Saprophytic)

> Beijerinckia (Aerobic, Saprophytic)

> Clostridium (Anaerobic, Saprophytic)

> Desulphovibrio (Chemotrophic)

> Rhodopseudomonas (Photoautotrophic)

> Rhodospirillum (Photoautotrophic)

> Chromatium (Photoautotrophic)

ii. Blue Green Algae (Cyanobacteria):- Heterocysts are the special cells that fix nitrogen. They add 20-30 kg Nitrogen/ha/annum.

> Nostoc

> Anabaena

> Aulosira:- A. fertilissima is the most active nitrogen fixer in Rice fields.

> Cylindrospermum:- It is active in sugarcane and maize fields.

> Trichodesmium

b. Symbionts:- Live in close symbiotic association with other plants.

i. Bacteria:- 

> Rhizobium:- It is aerobic, gram negative nitrogen fixing bacterial symbionts of legume roots. Sesbania rostrata has Rhizobium in root nodules and Aerorhizobium in stem nodules. 

> Frankia:- It is symbiont in root nodules of many non-leguminous plants like Casuarina and Alnus.

> Xanthomonas and Mycobacterium:- They occur as symbiont in the leaves of some members of the families Rubiaceae and Myrsinaceae (e.g., Ardisia). 

ii. Blue Green Algae (Cyanobacteria):-

> Nostoc and Anabaena:- They are common symbionts in lichens, Anthoceros, Azolla and Cycas roots. 

> Anabaena azollae:- It is found in fronds of Azolla pinnata (a water fern). It is often inoculated to Rice fields for nitrogen fixation.

2. Rhizobium Nitrogen Fixation:-

> Rhizobium bacteria:-

i. Free living

ii. Gram negative

iii. Aerobic

iv. Soil bacteria

> Rhizobium becomes anaerobic upon entry into roots. 

> Leghaemoglobin (legHb or symbiotic Hb):- 

- It is a pink coloured pigment.

- It occurs in the root nodules of leguminous plants. 

- It acts as an oxygen scavenger. It provides anaerobic conditions for the nitrogenase enzyme and protects the enzyme from inactivation.

> Two main steps:-

a. Nodule formation

b. Nitrogen fixation

a. Nodule formation:- Root nodule formation is initiated, when the soil contains a low level of nitrogen. Steps of nodulation are:

i. Aggregation:- Roots of legumes secrete flavonoids, which attracts rhizobia towards the root. Rhizobia aggregate around root hairs.

ii. Developmental changes:- Rhizobia secrete nod factors, which causes stimulate many developmental changes:

- Membrane depolarization

- Curling of root hairs 

- Cell division in the root cortex 

- Intracellular calcium movement

iii. Infection thread:- The nod factor attaches to receptors present on the plasma membrane of the root hairs, which leads to the formation of the infection thread. 

iv. Entry:- Infection thread provides the passage to bacteria to enter epidermal cells. Rhizobia then enter cortex cells, each bacterium gets surrounded by a plant-derived membrane known as symbiosome.

v. Nodulation:- Nodule formation is initiated by chemicals produced by rhizobia. It is a result of calcium dependent signal transduction pathway, which triggers biochemical changes leading to cell division and nodule formation. Cytokinin also plays an important role in nodules formation.

vi. Bacteroids:- Within nodules, bacteria get differentiated into bacteroids, which fix nitrogen. The Rhizobia stop dividing, loose cell wall and become nitrogen fixing cells as bacteroids . Vascular tissues are developed for nodules for exchange of nutrients.

b. Nitrogen fixation:-

- The nodule serves as site for N2 fixation. 

- Nodule contains nitrogenase and leghaemoglobin. 

- The nitrogenase has 2 components:

i. Molybdoferredoxin (Mo-Fe protein)

ii. Azoferredoxin (Fe-protein)

- The free di-nitrogen first bound to MoFe protein and is not released until completely reduced to ammonia. 

- In this process ferredoxin serves as an electron donor to Fe-protein (nitrogenase reductase) which in turn hydrolyzes ATP and reduce Mo-Fe protein, the Mo-Fe protein in Turn reduce the substrate N2. The electrons and ATP are provided by photosynthesis and respiration of the host cells.

- Many intermediates are formed to form ammonia (NH3).

Dinitrogen → Hydrazine → Diamine → Ammonia

- Ammonia (NH3) is immediately protonated at physiological pH to form ammonium ion (NH4+). As NH4+ is toxic to plants, it is rapidly used near the site of generation to synthesize amino acids.

Ans.

Chemical Nature of Auxins:- Auxin is an indole derived phytohormone which is weakly acidic. It has an unsaturated ring structure. Auxins belong to a group of hormones that have indole structure. Indole has a bicyclic structure, consisting of a six-membered benzene ring fused to a five-membered ring.

Physiological Effects of Auxins:-

a. Tropism:-  Response or orientation of a plant to a stimulus that acts with greater intensity from one direction than another. Growth due to tropism is mediated by changes in concentration of the plant hormone auxin within plant cells.

i. Phototropism (Heliotropism):- The orientation of a plant in response to light. 

Positive:- Orientation towards the source of light.

Negative:- Orientation away from the source of light.

ii. Geotropism (Gravitropism):- The orientation of a plant in response to gravity. 

Positive:- Orientation towards the gravity.

Negative:- Orientation away from the gravity.

b. Root initiation:- Auxin promotes growth of lateral and adventitious roots.

c. Flower initiation:- Auxin promotes flowering.

d. Apical dominance:- It is the dominance of shoot apex over its lateral branches. Here shoot apex inhibit the growth of lateral axillary buds. It is caused by the apical bud producing IAA (auxin) in abundance. The IAA causes the lateral buds to remain dormant.

e. Cell enlargement:- Its main function is of cell elongation. Far-red light induced increased internode elongation is a result of both increased cell elongation and increased cell division.

f. Prevention of Abscission:- Natural auxins have controlling influence on the abscission of leaves, fruits etc.

g. Respiration:- According to French and Beevers (1953), the auxin may increase the rate of respiration indirectly through increased supply of ADP (Adenosine diphosphate) by rapidly utilizing the ATP in the expanding cells.

h. Vascular Differentiation:- Auxin induces vascular differentiation in plants.

Bioassay of Auxins:-

It is testing of a biological activity like growth response of a substance by employing a living material like plant or plant part. Auxin bioassay is quantitative test as it measures concentration of auxin to produce the effect and the amount of effect.

a. Avena Curvature Test:-

> The test is based upon experiments of Went (1928). 10° curvature is produced by auxin concentration of 150 µg/litre at 25° C and 90% relative humidity. The test can measure auxin upto 300 pg/litre.

> Auxin from a shoot tip or any other plant organ is allowed to diffuse in a standard size agar block (generally 2 x 2 x 1 mm). Auxin can also be dissolved directly in agar. 15-30 mm long oat coleoptile grown in dark is held vertically over water. 1 mm tip of coleoptile is removed without injuring the primary leaf.

> After 3 hours a second decapitation is carried out for a distance of 4 mm. Primary leaf is now pulled loose and agar block supported against it at the tip of decapitated coleoptile. After 90-110 minutes, the coleoptile is found to have bent. The curvature is measured. It can also be photographed and the curvature known from shadow graph.

b. Root Growth Inhibition:-

Sterilized seeds of Cress are allowed to germinate on moist filter paper. As the roots reach a length of 1 cm or so, root lengths are mea­sured. 50% of the seedlings are placed in a test solution while the re­maining are allowed to grow over moist paper.

Lengths of the roots are measured after 48 hours. It is seen that the seedlings placed in test solution show very little root growth while root growth is normal in con­trol seedlings.

Mode of Action of Auxin:- Auxin causes Rapid increase in cell wall Extensibility. Cell wall enlargement in plants involves two steps:

i. Osmatic uptake of water across the plasma membrane resulting in increased turgor pressure of the cell.

ii. Extension of cell wall in response to increased turgor pressure.