2018 Solved Old Paper (BOT - 204) New

Ans.
Synergistic effect:- When two or more hormones together produce an effect which is more than the individual effect of the hormone, it is called synergistic effect. 
Example:- Ethylene and auxin have synergistic effects on flowering; Auxin acts synergistically with cytokinin to control the shoot stem‐cell niche. 
Ans.
Apoplast:- When absorbed water moves through the cell wall and inter-cellular spaces. In this route, water does not enter the cytoplasm inside the cell.
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.

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.
Foliar Nutrition:- It is a technique of feeding plants by applying liquid fertilizer directly to the leaves. Plants are able to absorb essential elements through their leaves.
Importance of Foliar Nutrition:-
1. It is the most efficient way to feed your crop:-
> Soil applications are often ineffective and inefficient because the applied fertilizers interact negatively with the soil itself. As a result, the nutritional elements can be limited and sometimes are not available at all. This applies to all nutrients, both main and trace elements.
> We estimate that of the soil fertilization with main elements, only 5 to 15% reaches the plant. For the trace elements this percentage is often even lower, sometimes not even 1%.
>The following factors prevent nutrient elements from reaching the plant:
i. Acidity of the soil
ii. Physicochemical properties of the soil
iii. Texture of the soil
iv. Mutual balances between the different elements
v. Weather conditions
> Making soil fertilization more efficient is hardly achievable because we don’t have any influence on these factors. Foliar fertilization is not dependent on the soil and therefore offers a viable alternative.
2. Foliar nutrition stimulates root development:-
> Our own test results and those referenced in respected expert literature, both indicate that foliar nutrition improves root growth. 
> The total absorption capacity of the root system therefore increases. 
> The plant will be able to absorb more water and nutrients because it reaches a larger volume of soil. As a result, the plant uses the reserves in the soil and the fertilizers more efficiently. 
> The nutrition of your plants is more balanced and therefore increases your chances of a good harvest.
> Dr Nino Rossi, from the University of Bologna, says the following: "The treatments with foliar fertilizers often stimulate the formation of additional root systems, longer and deeper compared to untreated plants. This better development will, in itself, result in a utilization of a larger soil volume, as a result of which, a greater absorption of water and nutrients from the soil is possible. These two types of absorption, via the leaf and the root, therefore work together in a positive way. "
3. Compensate low activity of the roots:-
> There are all kinds of causes of poor root functioning:
i. Asphyxia
ii. Soil infections, nematoda
iii. Soil compaction
iv. Diseases
v. High salt levels
As a result, they absorb no, or an insignificant quantity, of nutrients and the crop develops difficulties.
> With foliar applications, we compensate the defective functioning of the roots. After all, fertilization through the leaf does not depend on the roots. In this way, the plant can still grow, recover and become stronger.
4. Foliar fertilization eliminates many common deficiencies:-
> Because soil fertilization is not efficient, deficiencies often occur during the growth cycle of the plant. 
> Foliar applications compensate the erratic and irregular soil absorption, which provokes these deficiencies. 
> The CEC or cation exchange capacity of the leaves is just as strong as that of the root. In fact, both organs, leaf and root, have a similar absorption potential.
> Leaves can therefore absorb large amounts of nutrients. This means that foliar applications are a more interesting manner to administer not only trace elements but also key elements. This way you avoid deficiencies and stunting of growth.
5. Foliar nutrition will immediately solve a deficiency:-
> The consequences of stress due to deficiencies are drastic:
i. The plant does not function optimally
ii. Photosynthesis is not occurring efficiently
iii. There is even a risk of a growth interruption
> All of these affect productivity and your final yield. It is, therefore, very important to treat the deficiency as quickly as possible and to ensure the maximum output of your crop.
> Rapid intervention is a distinct advantage of foliar nutrition: the effect is much faster than that obtained by soil applications. The plant absorbs the nutrients immediately. They almost instantaneously end up in the metabolism of the leaf, where the plant needs these nutrients the most.
> With soil fertilization, it can take days to sometimes weeks, to tackle and eventually eliminate a deficiency, which is often detrimental to your yield.
> In foliar applications there is obviously no risk of fixation of the nutrients in the soil. One more reason to prefer foliar nutrition to soil fertilization at crucial moments. If the plant has deficiencies, it is very important that the nutrients reach the affected areas, as quickly as possible.
6. With foliar nutrition you better control the growth of your crop:-
> By using different nutritional balances in our foliar fertilization, we can steer the vigor of our plant.
> If the vigor is too low, we can stimulate growth more strongly with products rich in nitrogen.
> On the other hand, if the plant's vigor is too strong (which has a negative effect on the fruits) we can slow down growth. How? By foliar nutrition with products low in, or without, nitrogen and rich in potassium. This inhibits the growth strength without harming the plant and stimulates the plant to channel its energy to the fruits. This, ultimately, improves the quality of the fruits.
> By applying certain elements at specific moments, we stimulate certain processes in the plant. For example, we can stimulate flowering by giving boron at the right time.
> With foliar nutrition, you take the control of the crop development into your own hands.
7. Foliar fertilization increases the resistance of your plant:-
> Well-fed plants are healthier! They are stronger, less sensitive to diseases and have a higher natural resistance to diseases.
> To further increase this natural resistance, we recommend foliar applications with specific elements such as calcium, copper, silicon. These nutritional elements strengthen the plant.
> We also recommend foliar fertilization for plants that recover from a disease or a pest attack. The recovery goes faster after a pesticide treatment because they are armed with the necessary nutrients.
8. Foliar nutrition improves the quality of your yield:-
> With foliar fertilization we ensure a more balanced diet for the plant. We ensure that the nutritional elements that the plant needs are available at the right time, independent of their variable and uncertain availability in the soil. This prevents a series of disadvantages such as:
i. Too high nitrogen availability, which harms the fruit quality.
ii. Phosphorus fertilization, which causes a zinc deficiency and therefore reduces production.
iii. Boron deficiency that affects the uniformity of the fruits etc.
> Another important element for high-quality production is calcium.
Rarely, or never, is there an absolute deficiency of calcium in the soil and neither in the plant. But calcium does not always end up where it has to be: in the fruits. 
Solution? Calcium treatments with a good absorbable calcium chelate with a high mobility within the plant. This type of product allows you to increase the calcium concentration in the phloem, the plant juice that nourishes the fruits, resulting in a better quality of the fruits.
9. You’ll save time with foliar nutrition:-
> Products in which the trace elements are well chelated can easily be mixed with many plant protection products. Of course, the quality and purity of the applied products plays an important role.
> The products of BMS MN are easily combined and mixed with the most commonly used pesticides. Always consult our mixability list for your applications.
> You can take advantage from each pesticide application to nourish your crop with the elements it needs at a given time. So, rarely or never will you have to enter the field only to treat your plants with foliar nutrition. Time is money!
10. You’ll save money with foliar nutrition:-
> Because you supply nutrients more efficiently with foliar fertilization, you’ll need less soil fertilization. This saves you a lot of money.
> You reduce not only the amount of fertilizer, but also you manage your crop in a more cost-effective way.
11. Foliar nutrition is good for the environment:-
> Environmental legislation is becoming increasingly demanding and complex. Farmers must, therefore, follow stricter rules, not only for pesticides, but also for fertilizers. 
> Pollution of rivers and groundwater is often linked to the excessive use of fertilizers. Governments - both national and European - therefore impose limits on the quantity of specific nutritional elements that you can administer directly to the soil. These quantities are often much lower than what is needed to obtain a high yield. Foliar nutrition can help here. 
> With a well-thought-through foliar fertilization program, you can fill up the gaps of the reduced soil fertilization applications and keep the production up to standard. 
> BMS MN has already proven this in various tests on hazelnut, wine grape, corn, pear,...: check out the results of field trials to your benefit.
> The stricter regulations also have financial consequences. Your entitlement to European support (subsidy) sometimes depends on respecting these limits to the letter of the law. 
> With foliar nutrition, you comply with these limits without loss of yield and, therefore, loss of income, and you continue to qualify for European support.
Ans.
1. Auxins:-
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.
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.

2. Cytokinins:-
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.
Mode of Action of Cytokinins:-
> Cytokinin moves from the roots into the shoots, eventually signaling lateral bud growth. 
> Simple experiments support this theory. 
> When the apical bud is removed, the axillary buds are uninhibited, lateral growth increases, and plants become bushier. 
> Applying auxin to the cut stem again inhibits lateral dominance.

3. Gibberellins:-
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.
Mode of Action of Gibberellins:-
> Gibberellins cause seed germination by breaking the seed's dormancy and acting as a chemical messenger. Its hormone binds to a receptor, and calcium activates the protein calmodulin, and the complex binds to DNA, producing an enzyme to stimulate growth in the embryo.
> Gibberellin appears to induce its effect on stem elongation by de-repressing negatively regulated genes Le., by deactivating or degrading the repressors of GA response so that GA induced genes are transcribed and stem elongation or growth occurs.

4. Abscisic Acid:-
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.

5. Ethylene:-
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.
Mode of Action of Ethylene:-
> At a cellular level, ethylene can inhibit or promote cell division. 
> It sometimes inhibits cell expansion. 
> In other circumstances, it stimulates lateral cell expansion. 
> The presence of ethylene is detected by transmembrane receptors in the endoplasmic reticulum (ER) of cells. 
> Binding of ethylene to these receptors unleashes a signaling cascade that leads to activation of transcription factors and the turning on of gene transcription.
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.

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.
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.