2020 Solved Old Paper (BOT - 204) New

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.

>  The cohesive, adhesive forces and transpiration pull all together help in lifting up water through xylem elements.

Ans.

Use of Chelates:-

> Chelated micronutrients are fertilizers where the micronutrient ion (for example Fe or iron) is surrounded by a larger molecule called a chelator. 

> Chelator can be natural or synthetic chemicals. These compounds combined with a micronutrient forms a chelated micronutrient.

> Ethylenediaminetetraacetate (EDTA) is the most common chelating agent found in synthetic fertilizers. 

> Like other synthetic chelates, EDTA is an alien compound to the plant and is therefore not absorbed by the plant.

> Chelated trace elements help you achieve your production goals by providing the optimum nutrient availability. 

> The chelating process, with the assistance of chemical EDTA, forms a protective ring around the nutrient, which protects it from being tied up in the soil or by other nutrients.

> The chelating agents keep the metals soluble in the soil and are taken up by the plants with the metals. The two components may be separated in the roots but both can be transported to leaves.

> Examples:-

i. Arsenic Chelators:- Dimercaprol.

ii. Copper Chelators (for Wilson Disease):- Dimercaprol. Penicillamine. Trientine.

iii. Iron Chelators:- Deferasirox. Deferiprone. Deferoxamine.

iv. Lead Chelators:- Dimercaprol. EDTA.

v. Mercury Chelators:- Dimercaprol.

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

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Critical Photoperiod:- Plants require a certain day length to flower. This relative length of day and night needed for flowering is called as photoperiod. The length of the day and night above and below which a plant would not flower is called critical photoperiod. 

> For long day plants light period is critical for floweing.

> For short day plants dark period is critical for floweing.

Ans.

Chemical Structure 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.

> Indole-3-butyric acid (1H-indole-3-butanoic acid, IBA) is a white to light-yellow crystalline solid, with the molecular formula C12H13NO2. It melts at 125°C in atmospheric pressure and decomposes before boiling.

Ans.

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.

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.

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.

Ans.

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.

Physiological effects of Ethylene:- 

i. Fruit Ripening:- Its main function is to ripen fruits. The most commonly used chemical is called ethephon (2-chloro ethylphosphonic acid). It penetrates into the fruit and decomposes ethylene.

ii. Triple Response:- Ethylene  causes plants to have – 

    i. Short shoots:- Inhibition of stem elongation.

    ii. Fat shoots:- Stimulation of radial swelling of stems.

    iii. Diageotropism:- Increased lateral root growth and horizon­tal growth of stems with respect to gravity.

iii. Formation of Adventitious Roots and Root Hairs:- Ethylene induces formation of adventitious roots in plants from different plant parts such as leaf, stem, peduncle and even other roots. In many plants especially Arabidopsis, ethylene treatment promotes initiation of root hairs.

iv. Inhibition of Root Growth:- Ethylene is known to inhibit linear growth of roots of dicotyledonous plants.

v. Leaf Epinasty:- When upper side (adaxial side) of the petiole of the leaf grows faster than the lower side (abaxial side), the leaf curves downward. This is called as epinasty. Ethylene causes leaf epinasty in tomato and other dicot plants such as potato, pea and sunflower. Young leaves are more sensitive than the older leaves. However, monocots do not exhibit this response.

vi. Flowering:- Ethylene is known to inhibit flowering in plants.

vii. Sex Expression:- In monoecious species especially some cucurbits like cucumber, pumpkin, squash and melon; ethylene strongly promotes formation of female flowers thereby suppressing the number of male flowers considerably.

viii. Senescence:- Ethylene enhances senescence of leaves and flowers in plants. In senescence, concentra­tion of endogenous ethylene increases with decrease in conc. of cytokinins and it is now generally held that a balance between these two phytohormones controls senescence.

ix. Abscission of Leaves:- Ethylene promotes abscission of leaves in plants. Older leaves are more sensitive than the younger ones.

x. Breaking Dormancy of Seeds and Buds:- Ethylene is known to break dormancy and initiate germination of seeds in barley and other cereals. Seed dormancy is also overcome in strawberry, apple and other plants by treatment with ethylene. Non-dormant varieties of seeds produce more ethylene than those of dormant varieties.

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.

Stomatal Regulation of Transpiration:-

General Introduction:-

> The loss of water in the vapour form from the exposed parts of a plant is called transpiration. 

> The loss of water due to transpiration is quite high:

i. 2 litres per day in Sunflower.

ii. 36 - 45 litres in Apple.

iii. Up to 1 tonne per day in Elm tree. 

> Rather 98-99% of the water absorbed by a plant is lost in transpiration. Hardly 0.2% is used in photosynthesis while the remaining is retained in the plant during growth.

Stomatal Transpiration:-

- It is the most important type of transpiration. 

- Stomatal transpiration constitutes about 50-97% of the total transpiration. 

- It occurs through the stomata. 

- The stomata are found mostly on the leaves. A few of them occur on the young stems, flowers and fruits.

- The stomata expose the wet interior of the plant to the dry atmosphere. Water vapours, therefore, pass outwardly through stomata by diffusion. 

- The stomatal transpiration continues till the stomata are kept open.

Structure of Stomata:- 

> Stomata are tiny pore complexes found in the epidermis of leaves and other soft aerial parts. 

> The size is 10- 14 µm (range 7-38 pm) in length and 3-12 µm in breadth. 

> The number of stomata per cm2 of leaf surface varies from 1000- 60,000 or 10-600/mm2.

> In mesophytic plants, stomata occur both on the upper (adaxial) and lower (abaxial) surfaces.

> Their number is roughly equal on the two surfaces in grasses and other monocot leaves. 

> In dicot leaves, the number of stomata on the upper surface is usually smaller, even absent in several

cases.

> Stomata are meant for the gaseous exchange but are also the main source of transpiration. 

> Each stomate or stoma is surrounded by two small but specialized green epidermal cells called guard cells. Because of their small size, they are rapidly influenced by turgor changes.

> The guard cells are connected with the adjacent epidermal cells through plasmodesmata. They contain a few small chloroplasts with peripheral reticulum characteristic of chloroplasts showing C4 photosynthesis. 

> The guard cells also possess small vacuoles and micro bodies. They store starch with the exception of a few. 

> The walls are differentially thickened and elastic. They have folds for expansion. Micro fibrils of these walls are oriented specifically to help in opening and closing of stomata.

> In most of the plants the guard cells are kidney shaped in outline. They are joined at their ends. The concavo-convex curvature of two guard cells is variable and causes stomatal pore to open and close. 

- The walls of these guard cells are thickened on inner side. They have one or two pairs of wall extensions or ledges to prevent entry of water drops into stomata.

- The walls are thinner and more elastic on the outer side. 

> When the stomata are to open, these guard cells swell up on the outer side by the development of a high turgor pressure. The inner concave sides also bend out slightly so as to create a pore in between two guard cells.

> During closure movement, reverse changes occur. 

> In cereals, members of cyperaceae and some plams the guard cells are dumb-bell shaped in outline. Their expanded ends are thin-walled while middle portions are highly thick-walled. In such cases opening and closing of the stomatal pore is caused by expansion and contraction of thin-walled ends of the guard cells.

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.

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

3. Biosynthesis of Gibberellins:- Gibberellins are synthesized inside the plastids of immature seeds, young leaves and even the roots. They are synthesized from acetate units of acetyl coenzyme A by the mevalonic pathway. The following steps are involved in the 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). Then BOG-Co A is reduced in two successive NADPH-requiring steps to form mevalonic acid.

> Mevalonic acid is then phosphorylated by mevalonic acid kinase ( mevalonate kinase) in the presence of 2ATP molecules to form mevalonic acid pyrophosphate.

> Then decarboxylation of mevalonic acid pyrophosphate in the presence of ATP which yields Isopentenyl pyrophosphate (IpPP)

> IpPP is converted into dimethylallyl pyrophosphate (DMAPP) which is an isomer of IpPP, by enzyme IpPP isomerase.

> One molecule of dimethylallyl pyrophosphate then serve as an acceptor of one IpPP molecule with elimation of pyrophosphate and formation of one molecule of di-isoprenoid alcohol pyrophosphate or gereniol pyrophosphate(GPP).

> GPP accepts a molecule of IpPP to form farnesol pyrophosphate which also accepts another IpPP molecule to form geranyl geraniol pyrophosphate (GGPP).

> Then geranyl gereniol pyrophosphate (GGPP) is folded on various ways and then converted into a partially cyclized compound, copalyl pyrophosphate (CPP in the presence of ent- copalyl diphosphate synthase. Then it is finally transformed into a fully cyclic compound, ent-kaurene by ent- kaurene synthase.

> ent-kaurene is oxidized step-wise at C-19 to form ent-Kaurenol, ent-kaurenal and entKaurenoic acid. The latter is hydroxylated to ent-7α-hydroxy Kaurenoic acid.

> Now the contraction of β-ring and β-hydroxylation occurs. The conversion of ent-7α-hydroxy Kaurenoic acid to a 20 carbon GA12 -aldehyde involves loss of 6β-hydrogen, a shift of 7, 8 bond to 6, 8-positionand loss of a proton from the extruded C-7.

> GA12-aldehyde is converted into GA12 by ent- kaurene acid oxidase (KAO).

> Loss of one carbon must occur to give rise to C-19 GAs such as GA3.