2019 Solved Old Paper (BOT

Ans.

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.

Ans.

Asymbionts (Free living):-

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

> Azotobacter (Aerobic, Saprophytic)

> Beijerinckia (Aerobic, Saprophytic)

Ans.

Bioassay:- 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:-

> Auxins are one of the major phytohormones essential for plant growth and development. Normally, in plants, auxins are synthesized in the growing apex and then transported to various regions for physiological activities.

> In plants, auxins are present in small quantities, thus chemical test of plant materials does not always provide desirable results. As such, bioassay methods are often used for detection of auxin in unknown phytohormone extracts. There are several bioassay techniques for auxin: Avena coleoptile curvature test, Root inhibition test, Wheat coleoptile test, etc.

Gibberellin Bioassay:-

The Gibberellins (GAs) are one of the major groups of growth-promoting hormones which play essential roles in the regulation of growth and development of seed plants. By comparing the response to the extract obtained from plants with a standard response curve obtained by treating dwarf peas with a range of known dosages of GA3, one can assay the quantity of GA3 present in the unknown extract.

Ans.

Biosynthesis of Gibberellins:- Synthesis of gibberellins begins with acetate molecule. Acetate is esterified with coenzyme (CoA) to form three acetyl coenzyme A (acetyl Co A) molecules which undergoes a series of condensing reactions to β-hydroxyl-β-methyl glutaryl CoA (BOG-CoA).

Biosynthesis of Ethylene:- Ethylene is known to be synthesized in plant tissues from the amino acid methionine. A non-protein amino acid, 1-amino cyclopropane-l-carboxylic acid (ACC) is an important intermediate and also immediate precursor of ethylene biosynthesis. The two carbons of ethylene molecule are derived from carbon no. 3 and 4 of methionine.

Ans.

Concept of water potential (Ψw):-

- Osmotic movement of water takes place by a driving force which is the difference between free energies of water on two sides of the semi-permeable membrane.

- Free energy for per mole of non-electrolyte is known as chemical potential. It is denoted by greek letter Psi (Ψ).

- For water, chemical potential is known as water potential and denoted by Ψw. Free energy of water increases with increase in temperature.

Ψw = Ψs + Ψ_P + Ψm + Ψg

Ψw = Water potential

Ψs = Solute potential

Ψ_P = Pressure potential

Ψm = Metric potential

Ψg = Gravity potential

Note:- Ψs is always negative.

The total water potential is the sum of four different components:

Metric Potential (Ψm):- The binding of water to surfaces.

Osmotic Potential (Ψs):- Binding to solutes in the water.

Gravity potential (Ψg):- The position of water in a gravitational field.

Pressure potential (Ψ_P):- Hydrostatic or pneumatic pressure on the water.

But at the cell level Ψm and Ψg are insignificant. Hence –

Ψw = Ψ_P + Ψs

Ψw = Water potential

Ψ_P = Pressure potential

Ψs = Solute potential

- Water potential decreases with increase in concentration of solution.

- Water potential of pure water is zero which is maximum.

- In osmosis water moves from high Ψw to low Ψw solution.

Conditions:-

i. Fully plasmolysed cell or Fully flaccid cell:- Ψ_P = 0

Ψw = Ψs

ii. Fully turgid cell:- Ψ_P = Ψs

Ψw = 0

Osmotic Adjustment:- The process of lowering of osmotic potential by net solute accumulation in response to water stress, is called osmotic adjustment. It has been considered to be a beneficial drought tolerance mechanism in some crop species.

Ans.

Internal Water Deficit:-

1. Internal Water deficit:- It is defined as being when plant water status is reduced sufficiently to affect normal plant functioning.

2. Causes of Water deficit:-

i. A combination of limited water absorption and high evaporative demand.

ii. Generally, plant water deficits may be considered as being induced by either insufficient available soil water or a high atmospheric evaporative demand.

3. Physiological implications of water deficit:-

i. Water deficit has an adverse effect on plant growth. Therefore, drought stress is the most severe environmental stress for plant growth and crop production.

ii. Water deficit reduces photosynthesis by closing stomata, decreasing the efficiency of the carbon fixation process, suppressing leaf formation and expansion, and inducing the shedding of leaves.

iii. Response of leaves to water stress is by inward curling or exhibiting a wilted appearance as a result of an absence of turgor in leaves.

iv. A notable response to the deficit of water is stomata closing themselves to scale back water loss by transpiration.

v. However, it increases the leaves’ internal temperature leading to heat stress.

vi. Closure of stomata decreases greenhouse emission diffusion, finally reducing the expansion of the plant.

Ans.

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.

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.

Ans.

Reduction of Nitrates (Denitrification):-

1. Denitrification:-

> Denitrification is the process during which the nitrogen compound is released back into the atmosphere by converting nitrate (NO3-) into gaseous nitrogen (N).

> The process of denitrification is carried out during the absence of oxygen by Thiobacillus species and Pseudomonas bacteria present in the soil. In this process, the genus of Gram-negative bacteria degrades nitrate compounds present in the soil and aquatic systems into nitrous oxide (N2O)and nitrogen gas, which are eventually released into the atmosphere.

> In this process, a large range of microorganisms is involved; therefore, it is also called the microbial process.

> This biogeochemical process is one of the main responses to changes in the oxygen (O2) concentration in the environment. Denitrification is a universal process for both terrestrial and aquatic ecosystems, which occurs naturally under the extreme concentrations in managed ecosystems – marine and freshwater environments, tropical and temperate soils, wastewater treatment plants, aquifers, manure stores, etc.

2. Mechanism of Denitrification:-

> Denitrification is the last step in the nitrogen cycle. It is a naturally occurring, microbially mediated process, where nitrate is used as a form of energy for denitrifiers.

> In this process, soil bacteria convert plant-available soil nitrate (NO3–) into nitrogen (N) gases that are lost from the soil. Denitrification produces several gases: nitric oxide (NO), nitrous oxide (N2O) and dinitrogen (N2).

> The flowchart of the denitrification process is:

Nitrite → Nitric Oxide → Nitrous oxide → Nitrogen gas

3. Sit of Denitrification:-

> Denitrification is a microbial process of removing valuable nitrogen from the soil and releasing the greenhouse gas nitrous oxide (N2O), and the tropospheric pollutant nitric oxide (NO).

> The biological cycle of denitrification involves a cascade of different enzymes, which reduces nitrate to dinitrogen.

4. Reason of Denitrification:-

> When the oxygen (O2) supply in the soil becomes limited, a variety of bacteria use the oxygen instead of nitrate for respiration. Denitrification most commonly occurs in wet, moist or the soil flooded with water where the supply of oxygen for respiration is reduced or limited. Some fungi can denitrify, but they are not considered significant.

5. Time of Denitrification:-

> Denitrification is more active in the regions where water-filled pore space in the soil exceeds 60 per cent. The end-product gas depends on the soil conditions and the microbial community. As the deficiency of oxygen increases, microbes perform their functions by converting more of the nitrate to dinitrogen (N2) gas. For the purposes of nutrient management, denitrification results in a loss of valuable nitrogen (N), but the impact on the atmosphere will vary.

6. Factors Affecting the Denitrification Process:- The complete process of denitrification is influenced by the following factors:

The main factor which influences the process of denitrification is the organic content in the soil. The organic matter available within the soil is the only source of nutrition for the bacteria. Therefore, the soil bacteria require a source of readily available organic matter, either from the plants, from the soil or from other additional sources.

Other factors include:-

i. Soil pH

ii. Soil texture

iii. Temperature

iv. Oxygen content in the soil

v. Moisture content in the soil

vi. The concentrate of nitrate in the soil

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.

Biosynthesis of Auxins:-

> It is biosynthesized from the amino acid tryptophan. Zinc ions are required for the synthesis of auxin. IAA or Indole acetic acid is structurally related to amino acid tryptophan.

Tryptophan dependent pathway:- Since the 1930s, when K.V. Thimann first observed the synthesis of IAA in the mold Rhizopus suinus, which had been fed the tryptophan, the conversion of tryptophan to IAA has been studied in vivo in more than 20 different plant species and in vitro with atleast 10 different cell free enzyme preparations.

> IPA pathway is found in most higher plants.

> Species generally not having IPA pathway have TAM pathway (here, deamination and decarboxylation reactions are reversed). Tomato has both IPA and TAM pathways.

> IAN pathway is mostly present in members of the families Brassicaceae, Poaceae, and

Musaceae. Recently, nitrilase like genes are identified in Cucurbitaceae, Solanaceae, Fabaceae,

and Rosaceae.

> IAM pathway is present in pathogenic bacteria.

Ans.

Physiological effects of Cytokinins:-

i. Cell Division:- Cytokinins are essential for cytokinesis though chromosome doubling can occur in their absence. In the presence of auxin, cytokinins bring about division even in permanent cells. Cell division in callus is found to require both the hormones.

ii. Morphogenesis:- Both auxin and cytokinins are essential for morphogenesis or differentiation of tissues and organs. Buds develop when cytokinins are in excess while roots are formed when their ratios are reversed.

iii. Differentiation:- Cytokinins induce formation of new leaves, chloroplasts in leaves, lateral shoot formation and adventitious shoot formation. They also bring about lignification and differentiation of inter-fascicular cambium.

iv. Senescence (Richmond-Lang Effect):- Cytokinins delay the senescence of leaves and other organs by mobilisation of nutrients.

v. Apical Dominance:- Presence of cytokinin in an area causes preferential movement of nutrients towards it. When applied to lateral buds, they help in their growth despite the presence of apical bud. They thus act antagonistically to auxin which promotes apical dominance.

vi. Seed Dormancy:- Like gibberellins, they overcome seed dormancy of various types, including red light requirement of Lettuce and Tobacco seeds.

vii. Resistance:- Cytokinins increase resistance to high or low temperature and disease.

viii. Phloem Transport:- They help in phloem transport.

ix. Accumulation of Salts:- Cytokinins induce accumulation of salts inside the cells.

x. Flowering:- Cytokinins can replace photoperiodic requirement of flowering in certain cases.

xi. Sex Expression:- Like auxins and ethylene, cytokinins promote femaleness in flowers.

xii. Parthenocarpy:- Crane (1965) has reported induction of parthenocarpy through cytokinin treatment.

xiii. Stomatal opening:- It has been shown that an increased cytokinin concentration in xylem sap promotes stomatal opening.

Ans.

Reduction of Nitrates (Denitrification):-

1. Denitrification:-

> Denitrification is the process during which the nitrogen compound is released back into the atmosphere by converting nitrate (NO3-) into gaseous nitrogen (N).

> In this process, a large range of microorganisms is involved; therefore, it is also called the microbial process.

2. Mechanism of Denitrification:-

> Denitrification is the last step in the nitrogen cycle. It is a naturally occurring, microbially mediated process, where nitrate is used as a form of energy for denitrifiers.

> The flowchart of the denitrification process is:

Nitrite → Nitric Oxide → Nitrous oxide → Nitrogen gas

3. Sit of Denitrification:-

> Denitrification is a microbial process of removing valuable nitrogen from the soil and releasing the greenhouse gas nitrous oxide (N2O), and the tropospheric pollutant nitric oxide (NO).

> The biological cycle of denitrification involves a cascade of different enzymes, which reduces nitrate to dinitrogen.

4. Reason of Denitrification:-

5. Time of Denitrification:-

6. Factors Affecting the Denitrification Process:- The complete process of denitrification is influenced by the following factors:

Other factors include:-

i. Soil pH

ii. Soil texture

iii. Temperature

iv. Oxygen content in the soil

v. Moisture content in the soil

vi. The concentrate of nitrate in the soil

Ans.

Abscisic Acid (ABA):-

> It is found in older leaves, apical buds and seeds.

> Both GA and ABA hormones work opposite to each other.

> ABA inhibits the biosynthesis of the hormone GA.

1. Chemical nature of Abscisic Acid:- This phytohormone is a 15-carbon sesquiterpene and its synonyms are dormin and abscisin-II.

3. Physiological effects of Abscisic Acid:-

i. Seed and bud dormancy:- Its main function is to maintain seed dormancy. Abscisic acid induces dormancy of buds towards the approach of winter. Abscisic acid accumulates in many seeds during maturation and apparently contributes to seed dormancy.

ii. Senescence:- ABA acts as a general inducer of senescence (Thimann). The onset of senescence is correlated with stomatal closure. The ABA content of aging leaves increases markedly as senescence is initiated.

iii. Abscission:- It is the process of shedding old or unwanted organs such as leaves, flower, floral organs, and fruits. ABA promotes abscission through ethylene.

iv. Flowering:- In long-day plants, the effect of gibberellins on flowering is counteracted by ABA, which accumulated in the leaves during the short winter days. This ABA acts as inhibitor of flowering in long-day plants. On the other hand ABA induces flowering in short-day plants.

v. Starch hydrolysis:- The GA-induced synthesis of a-amylase and other hydrolytic enzymes in barley aleurone cells is inhibited by abscisic acid. This inhibition can be reversed by increasing the amount of GA supplied.

vi. Geotropism:- ABA controls geotropic responses of roots. It stimulates positive geotropism in roots.

vii. Stress response:- It also close the stomata of the leaves in dry conditions. Hence it is also called stress hormone.

4. Mode of Action of Abscisic Acid:-

a. Regulation of Gene Expression and Enzyme Synthesis:-

> ABA has inhibitory effects on protein synthesis. The effect of ABA on protein synthesis appears to be selective since it has been shown to affect directly the synthesis of those proteins whose synthesis is under hormonal control.

> The best example of such translational control is the inhibition by ABA of the GA-promoted synthesis of α-amylase and other hydrolases like protease and ribonuclease in barley aleurone layers.

> Ho and Varner suggested that the inhibition of a-amylase synthesis by ABA is due to an effect on translation, since they found that ABA still inhibited the formation of α-amylase at 12 h, a time when RNA synthesis inhibitor cordycepin had no longer any effect.

> They have postulated that ABA might de-repress a regulator gene or interact with a regulator RNA protein species to inhibit the translation of α-amylase mRNA.

> ABA has regulatory effects at the levels of both transcription and translation. The expression of numerous genes is stimulated by ABA under various stress conditions. Such conditions include heat shock, low temperature and salinity stress as well as the period of seed maturation.

> It is well known that gene activation is mediated by DNA-binding proteins, which acts as transcription factors. ABA has been shown to stimulate the transcription of the genes, which encode these binding proteins.

> Distinct DNA segments have been identified that are involved in stimulation of transcription by ABA, thought to be ABA-response elements (ABAREs). Contrary to the stimulation of gene expression, ABA is also involved in repression of gene transcription.

> The common example is ABA-induced repression of barley a-amylase gene which is expressed by GA. In this case, a few DNA elements which mediate ABA-induced gene repression are quite similar to the gibberellin response elements (GAREs).

b. Stomatal Complexes and ABA:-

> Turgor changes within the guard cells regulate stomatal aperture. Such turgor changes are caused by movement of K+, H+, Cl– ions and the synthesis, metabolism and movement of organic anions, particularly malate. The role of ABA in these movements and synthesis is still a matter of conjecture.

> One important factor in regulating guard-cell turgor is thought to be the operation of an active H+/K+ exchange process. ABA inhibits K+ uptake into the guard cells and also proton release. The fungal toxin fusicoccin (FC) overcomes the effect of ABA on the stomata by stimulating H+/K+ exchange. ABA may also affect the distribution of malate.

> During stomatal closure in dark, malate leaks from the epidermal strips of Commelina communis, Vicia faba floating on water. Addition of ABA increases both the rate of closure and the rate of malate leakage.

> Malate serves as a source of protons for H+/K+ exchange process during opening and malate leakage from guard cells during closure is required to reduce turgor. ABA would inhibit H+/K+ exchange and promote specific leakage of malate, thus inhibiting opening and promoting closure.

c. Membrane Depolarization by Increasing Ca2+ and pH of Cytosol:-

> Although stomatal closure is mediated by a reduction in guard cell turgor pressure caused by massive outward movement of K+ and anions (CI– and malate) from the cell, ABA also induces a net influx of positive charge.

> ABA has been shown to induce an increase in cytosolic Ca2+ concentration by activation of calcium channels leading to membrane depolarization. Cytosolic calcium concentration is further increased by ABA by causing release of calcium from an internal store like vacuole, and this increase is sufficient to cause stomatal closing.

> In addition to increasing cytosolic Ca2+, ABA causes alkalization of the cytosol. The increase in cytosolic pH triggers the voltage-gated K+ efflux channels to open resulting in K+ loss and stomatal closure.

> The inhibition of plasma membrane H+-ATPase has also been held responsible for membrane depolarization. ABA inhibits the plasma membrane proton pump and favours depolarization in an indirect manner. It is presumed that the increase in Ca2+ concentration and pH of the cytosol in the presence of ABA results in the inhibition of H+ -ATPase.

> ABA not only causes stomatal closure by activating outward ion channels but also prevents stomatal opening. Under normal conditions, the inward K+ channels are open when the membrane is polarized by the proton pump and K+ ions get inside through H+/K+ exchange causing stomatal opening. In this case, ABA inhibits inward K+ channels through an increase in Ca2+ and pH, thus preventing stomatal opening.

Search This Blog

Translate

M.SC. BOTANY SEM - II MGSU, BIKANER

2019 Solved Old Paper (BOT - 204) New