Essay on the Growth and Development of Plants

Carbohydrate Metabolism:

Carbohydrates are the storage materials that are stored in plants which upon its breakdown yield energy required for the vital processes in plants. The prominent carbohydrates in plants are sucrose and starch.

Image Source: genomebiology.com

Synthesis and Breakdown of Sucrose:

Synthesis: It may take place by three different ways:

1. From Glucose-1 phosphate and fructose in presence of enzyme sucrose phosphorylase. (eg.) in bacteria.

Glucose-1-phosphate + Fructose Sucrose Phosphorylase

Sucrose + inorganic phosphate.

2. From UDPG (uridine Diphosphate Glucose) and Fructose in the presence of sucrose synthetase, (eg) in higher plants.

UDPG + Fructose Sucrose Synthetase UDP + Sucrose.

3. From UDPG and Fructose-6-phosphate in the presence of the enzyme sucrose phosphate synthetase (eg) Higher Plants.

UDPG + Fructose Sucrose Phosphate Synthetase UDP + sucrose phosphate.

Sucrose phosphate thus produced is hydrolysed in the presence of the enzyme phosphatase to yield sucrose.

Sucrose phosphate Phosphate → +H₂O Sucrose + Phosphate

Break Down:

Sucrose is broken down or hydrolysed to yield glucose and fructose in the presence of enzyme invertase which is a reversible reaction.

Sucrose + H₂O ^Invertase→ Glucose + Fructose

Synthesis and Breakdown of Starch:

Synthesis of Amylose or α-1, 4 Glycosidic Linkages.

As the starch is made up of Amylose and Amylopectin, both the components are Synthesised Simultaneously with α-1,4 Glycosidic linkages and α-1, 6 linkages respectively.

Synthesis of amylose may take place by the following ways.

1. Amylose can be synthesised in the presence of the enzyme starch phosphorylase from glucose-1-phosphate and an acceptor molecule consists of 3-20 glucose units joined together by α-1, 4 glycosidic linkages.

n (Glucose-1-phosphate) + Acceptor Starch Phosphorylase Amylose + n (ip)

2. Formation of α-1, 4 glycosidic linkages may also take place in the presence of the enzyme UDPG-transglycosylase (amylose synthetase) by the transfer of glucose from UDPG to an acceptor molecule with 2-4 or more glucose units joined together to the α-1, 4 glycosidic linkages or even a starch molecule.

UDPG + Acceptor UDPG trans glycosylase UDP + α-1, 4 Glucose acceptor.

Synthesis of AmyloPectin or α-1, 6 Glycosidic Linkages:

Transfer of small chains of glucose units joined together by α -1,4 linkages to an acceptor molecule consisting of atleast four α -1, 4 linked glucose units in the presence of ‘Q’ enzyme. The a-1, 6 Glycosidic bond established between C-l of the terminal glucose units of donor molecule and C-6 of one of the glucose units of the acceptor molecule.

Break Down by Means of Two Ways:

1. By the enzyme Diastase

Starch Diastase → +H₂O Glucose

Diastase is a complex of many enzymes viz., α-amylase, β-amylase, R-enzyme and maltase.

2. By starch phosphorylase

Starch + Phosphate Starch Phosphorylase Glucose-1 -phosphate.

Glucose-1-phosphate Phoshpate → Glucose + Phoshpate

Fat Metabolism:

Lipids may be simple, compound or derived. Simple lipids are esters of fatty acids alcohol. If alcohol in the constitution is glycero they are called fats.

FAt metabolism consists of two parts

(a) Anabolism:

(a) Synthesis of fatty acid.

(b) Synthesis of glycerol.

(c) Condensition of fatty acids and glycerol into fats.

Glycerol + 3 Fatty acid COA → ATP Mg⁺⁺ lipid +3COA

(b) Catabolism:

3 Path ways

(a) P-oxidation

(b) a-oxidation

(c) Peroxidation

β-oxidation:

It is the chief process of fatty acid degradation in plants which is well established in saturated fatty acids not in unsaturated fatty acids. The place of occurence of this reaction is mitochondria and also in glyoxysomes which involves sequential removal of 2-C in the form of acetyl-СОА from the carboxyl and of the fatty acid. This is called β- oxidation, because β -с of the fatty acid is oxidised during this process.

Here in this process, huge amount of energy is generated in the form of ATP molecules by the mitochandrial oxidation of fatty acids thro P-oxidation spiral and TCA cycle. Acetyl-COA may enter into TCA cycle or in case of germination of fatty seeds, they are converted to soluble sucrose through glyoxylic acid cycle.

Glyoxylate Cycle (or) Glyoxylic acid cycle:

In this cycle, the fats could be converted into sucrose during the germination of fatty seeds in plants. This cycle is to occurs in many other bacteria, yeasts, molds, and higher plants or is completed in Glyoxysomes, mitochondria and Cytosol.

Significance:

i. During Germination of fatty seeds, insoluble fats are converted into soluble sucrose, which is then available to the different growing regions by Glyoxylate cycle.

ii. Micro organisms that can grow in C₂H₅OH/acetate as a sole source use this cycle to produce longer carbon chains.

iii. It’s an example of Gluconeogeneris process.

Nitrogen Metabolism and Nitrogen Fixation:

Role of Nitrogen in Plants:

i. Proteins are the building blocks of protoplasm which are made up of Nitrogenous substances called aminoacids, which are synthesised when inorganic ‘N’ in the environment is converted to organic ‘N’ inside the plants.

ii. Proteins are the major substances in cell next only to water and mineral salts.

iii. ‘N’ is die constituent element of important organic compounds like chlorophylls, cytochromes, alkaloids, vitamins and Nucleic acids.

iv. Thus ‘N’ plays an important and fundamental role in metabolism, growth, reproduction and heredity.

Sources of ‘N’:

1. Atmospheric/molecular Nitrogen: Only some bacteria, some BGA, leguminous plants having root nodules can fix atmospheric ‘N’.

2. Nitrates, Nitrites, Ammonia in soil (In organic ‘N’): Nitrates is the chief form of ‘N’ taken up by the plants in the soil.

3. Amino acids (Organic Nitrogen) in Soil.

4. Ammonium Compounds of which NH4 ions are absorbed directly by the roots of plants along with nitrate ions.

5. Organic Nitrogenous Compounds in the bodies of Insects.

‘N’ Fixation in Plants: It can be achieved by the conversion of Nitrate in the soil to Ammonia by the plants which occurs as a two step process.

(i) Reduction of Nitrate to Nitrite

(ii) Reduction of Nitrite to Ammonia

The process of conversion is basically a reductive process, as the Nitrate (NO^–₃) which is a highly oxidised form gets converted into the reduced form (i. e.) Ammonia (NH₄) under the two major steps, there are many intermediate steps involved in the conversion, which are mediated by specific enzymes.

At each step R electrons are added and ultimately NO₃ in which ‘N’ has 5 positive charges converted to Ammonia (NH₃) with 3 negative charges. The electrons are supplied by NADH and NADPH.

I. Reduction of Nitrate to Nitrite:

Takes place in green leaves and roots and the enzyme is found in cytoplasm.

NO₃ + NADH + H⁺ Nitrate reductase → NO₂ + NAD⁺ +H₂O

The enzyme is the molybdoflavoprotein which contains FAD (Flavin Adenine Dinucleotide) as its Prosthetic Group associated with molybdenium.

Electrons are transferred from reduced coenzyme to FAD which becomes reduced FADH₂. From FADH₂, electrons are finally transferred to NO^–₃ thro molybdenium so that NO^–₂ and H₂O are formed.

II. Reduction of Nitrite to Ammonia:

The reduction takes place in the presence of Nitrite Reductase which catalyses by-reduction from NO^–₂ to NH⁺4 with the formation of Nitroxyl/ Hypo Nitrite and Hydroxyl amine as inter mediates.

Electron donors for this reduction are, reduced ferredoxian and also reduced pyridine nucleotides.

As against the presence of Nitrate reductase in cytoplasm, this enzyme is found in chloroplasts. Sirohaem, an iron-porphyrin is known to be associated with this enzyme and probably mediates the transfer.

The involvement of two intermediate compounds in the process is doubtful because.

1. Hyponitrite is unstable

2. Hydroxyl amine is toxic.

3. These two, they never occur in free state in the cells. It’s now generally believed that hyponitrite and hydroxyalamine may be formed at the surface of the enzyme and leave the surface only when they are completely reduced to further intermediate or ammonia.

There is yet another type of formation of Ammonia, through the Reduction of Hydroxylamine.

The reaction is catalysed by hydroxylamine reductase which requires manganese for its activity.

NH₂OH + NADH + H⁺ → mn NH₃ + NAD+ H₂O

The ammonia combines with organic acids to produce amino acids.

Biological Nitrogen Fixation:

It can be defined as the conversion of molecular nitrogen of the atmosphere into inorganic nitrogenous compounds through the activity of some living organisms.

The process of Biological Nitrogen Fixation can be divided into two types.

Non Symbiotic Nitrogen Fixation:

In case of non-symbiotic fixation, some of the components are necessary in the cell system of ‘N’ fixing bacteria.

1. Presence of hvdrogenase and nitrogenase enzyme systems. Hydrogenase reversibly catalyses the reduction of hydrogen ion to molecular hydrogen while, nitrogenase reduces molecular nitrogen to NH₃ in the presence of molecular hydrogen.

2. Presence of Ferredoxin as an electron carrier.

3. Presence of pyruvate which acts as e’donor and energy source.

4. Presence of co-factors like TPP. COA, ip and Mg²⁺ required in the metabolic utilization of pyruvate, that forms the energy [ATP).

Symbiotic Nitrogen Fixation:

It is carried out by bacteria which is frequently found in the root nodules of leguminous plants. The most common bacterium is Rhizobium which has two types of strains.

1. Fast Grower (Eg) R. leguminosarum/ Bacillus radicicola and R. trifolii.

2. Slow growers (Eg) R. Japanicum and R. Lupinii.

Nodule Formation:

It is the principal event in this process, which is a complicated process. In the initial stage, host cells have to recognise the correct strain of symbiotic bacteria where a critical specificity is involved in the association of various species of root nodule bacteria and leguminous plants and this step is controlled by specific protein found in the host cell called lectin. Lectins are the key proteins involved in the Rhizobium-legume symbiotic association and in the process calcium plays a critical role in modifying the ability of root to absorb rhizobia.

Rhizobia accumulate in the soil near the roots of the legumes probably due to the secretion of some growth factors by their roots. The bacteria now penetrate the root through soft or injured root hairs whose tips become curved.

They now enter into the cells of the inner layers of the cortex through infection threads and cause cortical cells to multiply which ultimately result in the formation of nodules on the surface of the roots. The cells of the bacteria enlarge, assume pleomorphic shape and are called bacteriods.

The nodule contains a pink coloured leghaemoglobin, like true haemoglobin combine with 0₂ and C0₂ gets readily oxidised to brown form with trivalent ion. The development of leghaemoglobin and Nitrogen fixing capacity are out come of symbiotic interaction. The leghaemoglobin is located in the space between bacteriods and membrane enclosing them or it’s present outside the membrane and in the host cytoplasm.

Nitrogenase:

The Process of ‘N’ fixation, whether its symbiotic or non-symbiotic involves the reduction of atmospheric ‘N’ to ammonia (NH₃) by the enzyme Nitrogenease. The enzyme is madeup of two protein components one containing iron and molybdenum and the other containing only iron called Fe-protein or azoferredoxin. The enzyme is extremely sensitive to oxygen. The development of leghaemoglobin pigment in nodule cells and heterocysts in BGA for protecting nitrogenase from oxygen inactivation.

Thus the enzyme remains active under anaerobic conditions. The pigment leghaeomoglobin regulates the course of oxygen and provides suitable atmosphere to the enzyme. It combines rapidly with oxygen to avoid any inhibitory effect of it on enzyme, at the same time its able to make oxygen available to bacteriods for ATP production, required for ‘N’ fixation. Thus the process of ‘N’ fixation requires anaerobic conditions even though Rhizobium is aerobic.

Mechanism of Nitrogen Fixation:

It’s to be noted here, during the process of ‘N’ fixation, the free atmospheric nitrogen first bound to the enzyme surface and is not released until its completely reduced to ammonia. ‘N’ bound to the enzyme surface is reduced in step wise reactions before N-N bond is ruptured. Pathway of e-transport in nodule bacteriods.

Enzymes involved —

1. Glucose-6-P04 dehydrogenase

2. NADPH-Ferredoxin oxido reductase

3. ‘Fe’ protein compund of Nitrogenase.

4. Fe-MO protein compund of Nitrogenase

The reduction of ‘N’ into NH₃ by nitrogenase

in bacteroids depend upon the availability of ATP and reduced substrate capable of donating ‘H⁺ atoms to ‘N’ Glucose-6-PO₄ is the reduced substrate for the process and reduced NADP together with ferrodoxin function as electron carriers.

ATP interact with non-home iron (NHI) protein component of Nitrogenase and brings about confirmational change to convert it to a powerful reductant, which becomes capable of transferring electrons to reduce N₂, and NH₃.

Thus the reduction of one molecule of ‘N’ in to two molecules of ammonia requires twelve molecules of ATP because six electrons are required per molecule of ‘N’ reduced.

N₂ + 6H⁺ + 6e^– + 12 ATP →2 NH₃+12 ADP + 2 Pi

Photoperiodism:

The plants in order to flower require a certain day length (i.e.) relative day length and night is called photoperiod. The response of plants to the photo period expressed in the form of flowering is called as photoperiodism.

The observation of Garner and Allard (1920) that a mutant variety of tobacco, Maryland mammoth flowers when the relative length of the day was shorter than the length of the dark period, which led to the discovery of photoperiodism.

Depending upon the duration of photo period, they classified plants into three categories.

I. Short Pay Plants:

i. Qualitative (flowering occurs only in short days)

ii. Quantitative (Flowering accelerated by short days)

iii. The plants which require a relatively short day light period (8-10 hrs) and a continuos dark period about 14-16 hrs for subsequent flowering.

iv. Examples are, maryland mammoth variety of Tobacco, rice, datura, onion, etc.

i. In short day plants, dark period is critical and must be continuos. If this dark period is interrupted even with a brief exposure of red light, short day plant will not flower.

ii. Maximum inhibition of flowering with red light occurs at about the middle of the critical dark period.

iii. The inhibitory effect of redlight can be over come by a subsequent exposure with far-red light (730-735 nm).

iv. Interruption of the light period with redlight doesnot have inhibitory effect on flowering in short day plant.

v. Prolongation of the continuos dark period initiates early flowering in short day plants.

II. Long Day Plants (LDP):

i. Qualitative – flowers only in long days.

ii. Quantitative – Flowering accelerated by long days.

iii. These plants require a longer day light for a period of 14-16 hrs. in a 24 hrs cycle for subsequent flowering.

iv. Examples are spinach, lettuce, radish alfalfa, sugar beet, opium poppy, oats and wheat.

v. In the long day plants, the light period is critical.

vi. The best flowering of long day plant usually occurs in continuos light.

vii. For flowering they require either no dark period or a very short dark period.

viii. A flash of light in long dark periods can induce flowering in them even during short-day periods.

ix. Long day plants can flower even in short- day periods, if these short-day periods are accompained with still shorter dark periods.

x. Too long dark periods inhibit flowering and so these are called short-night plants.

xi. A brief exposure in the dark period the prolongation of the light period stimulates flowering in long day plants.

III. Day Neutral Plants (Ambiphotoperiodic species):

i. The plants that flower in all photo periods ranging from 5 hrs to 24 hrs continuous exposure.

ii. Examples are tomato, cotton, cucumber, pea, sunflower maize and dandelion etc.

iii. Their flowering is not affected by the length of the day and they can flower even if the light period provided ranges from few hrs to continuos illumination.

Vernalisation:

It can be defined as the method of inducing early flowering in plants by pretreatment of their seeds at very low temperatures. Lysonko gave the term vernalisation which means conversion of the winter variety into spring variety by low temperature or chilling treatment.

Physiological Pre conditioning:

The effect of the cold stimulus on plant is not immediately visible. It’s expressed only at a certain later stage in the form of flowering. Thus like the photo periodism, the phenomenon is an excellent example of physiological pre conditioning.

Perception of the Cold Stimulus:

The cold stimulus is perceived by the apical meristerms. And also all dividing cells including those in roots or leaves may be the potential sites of vernalization.

The perception of the cold stimulus results in the formation of a floral hormone, which is transmitted to other parts of the plant. It can also be possible, the cold stimulus may even by transmitted to another plant across a graft union and the floral hormone has been named as vemalin by melchers.

Conditions Necessary for Vernalization:

1. Age of the Plant: In cereals, effective only at germinating seed.

2. Appropriate Low temperature and Duration of exposure – 1 – 6°C, one and a half months or more.

3. Oxygen – Vernalisation an aerobic process and requires metabolic energy provided by oxygen.

4. Water: In dry seeds, vernalisation is not possible.

Mechanism of Vernalization:

Two theories are prevailing to deal about the mechanism of vernalization.

I. Phasic Development Theory:

According to this theory, the process of development of an annual seed plant consists of a series of phases which must occur in some predetermined sequence. An important condition is, commencement of any of these phases will take place only when the preceding phase has been completed.

Thermo Stage:

First stage conditions required are –

1. Temperature (0-20°C)

2. Moisture

3. Aeration

4. Varied time

Photo Stage:

When thermo stage is completed, the plant passes through next stage. There is pronounced effect of the relative length of day and night on the production of flowers, and this phenomenon is called photoperiodism.

Third Stage:

It’s quite necessary for seed germination as it has been connected with formation of sexual elements and is associated with garmetogeneris. The photoperiod requirement for the commencement of this stage is little shorter than required for the completion of photo stage.

II. Hormonal Theories:

Vernalization probably involves the formation of a floral hormone called as vemalin. The first hormonal theory proposed by long and melchers.

Process of Vernalisation:

Germination of seeds and

cold tretment at 0-5°C

↓

Drying and after that

sowing of seeds

After the cold treatment, seedlings are allowed to dry for some time and then down. The drying period also should not be so long as the vernalize response decreases and it continues till seedlings completely devemalised.

Significance:

1. Shortens the vegetative period of the plants.

2. Increases the cold resistance of the plants.

3. Increases resistance of plants to fungal diseases.

4. Crops can be produced earlier (i.e.) crop can be harvested much earlier than the control crop.

5. Crops can be grown in the regions where they donot naturally reproduce.

6. Plant breeding work can be accelerated.

7. In colder countries like Russia, the harmful effects of the cold can be overcome and thus improving crop production.

Dormancy of Seeds:

It may be defined as any phase in the lifecycle on plant in which active growth is temporarily suspended.

All the viable seeds have capacity to germinate if placed under suitable conditions necessary for germination. When seeds fail to germinate for some time even if placed under suitable conditions that are ordinarily favourable for germination. During this period the growth of the seeds, remain suspended and they are said to be in dormant stage.

Factors Causing Dormancy of Seeds:

1. Dormancy due to seed coat:

It’s of two types –

(a) Seed coats impermeable to water:

Seeds of families leguminoceae, malvaceae, chenopodiaceae, and solanaceae have hard seeds coats, that are impermeable to water. The seeds remain dormant until the impermeable layer of testas decay by soil microorganisms.

(b) Seedcoats impermeable to oxygen:

In cocklebur (xanthium), many grasses, compositae, the seeds coat is impermeable to oxygen. During the period of dormancy of seed coats gradually become more permeable to oxygen so that they may germinate afterwards.

2. Dormancy due to immature embryos:

Some seeds are shed, before the embryo is matured and they will not grow, which require an “after ripening period” during which certain changes occur with in the seed. The changes include acidity, enzyme activity and respiratory rate etc. and once the embryo developement is complete, the seed then germinates without any special treatment.

3. Mechanically resistant seed coats:

Seeds of certain weeds such as pigweed (Amaranthus), Sheperd’s Purse (capsella), water plantain (Alismas) etc. remain dormant because their hard seed coat prevent any appreciable expansion of the embryo.

4. Dormancy due to chemical inhibitors:

Seeds present in the juicy fruits such as orange, tomato donot germinate immediately. Para-ascorbic acid, Phenolic acids, Dehydro acetic acid in the embryo, endosperm or other tissues of the seed fruit is also the cause of seed dormancy.

5. Need for after-ripening in dry storage:

In many-plants (Eg.) Barley, oats, wheat, seeds though containing fully developed embryos are dormant, when they are harvested they require no special treatment to overcome this dormancy and germinate if kept under dry storage conditions at normal temperatures for few weeks to several months. This type of dormancy can be overcome by removing the seed coats or temperature treatments. (35-40°C storage).

6. Chilling or Low Temperature Treatment:

Seeds of temperature species show dormancy which is over come by chilling (Eg) Apple, Rose, Peach, Grapes, etc., which will not germinate, if planted under moist conditions at 20°C, but will germinate if kept at 0-5°C for several weeks and then transferred to warmer conditions.

7. Dormancy due to light Sensitivity:

The germination of many seeds affected by light. Such seeds are called photo blastic. The seeds in which germination is stimulated by light are called positively photoblastic whereas those in which germination is inhibited by light are negatively photoblastic.

Positive Photo blastic: Amaranthus, Sheperd’s purse, , tobacco, and tomato etc.

Negative Photoblastic: Helleborus niger, Nigella damascena and Silene armenia.

Mechanism to Seed Dormancy:

I. Regulation of dormancy is primarily hormonal.

II. Importance of interference with gaseous exchange by seed coats and other structure.

The hormonal theory visualises that the prevention of growth may be either (1) because of deficiency of some essential growth promoting substance, (endogenous gibberelins) (2) because of the presence of some inhibitory substances. (Dormins/abscisins).

Methods of Break Dormancy:

1. Scarification:

The process of rupturing or weakening the seedcoats by mechanical or other means and is employed, where the dormancy is due to impermeable seed coats. Scarification can be done either mechanically or chemically. Chemical scarification is accomplished by treatment of the seeds with organic solvents, such as alcohol, acetone, ether or xylol; and strong acids.

2. Pressures:

When the dormant seeds are subjected to 2000 atm at 18°C for about 5-20 mins. and this works in cases, where the seed coats are impermeable to water.

3. Low temperature:

When the dormancy is due to chilling requirement, it can be overcome, if the seeds are treated in moist medium at low temperature (5-10°C) for sufficient period of time, which is called stratification.

4. Alternating Temperatures:

An alternation of low and high temperatures improves the germination of seeds.

5. Light Dormancy of Positively Photoblastic:

Seeds can be broken by exposing them to red light or white light. This promotion of germination by redlight and inhibition by far-red light probably involves physochrome.

6. Germination Stimulating Compounds:

Such as gibberellins induce germination in tobacco, lettuce etc., and chemicals such as KNO3, Thiourea and ethylene also involved.

Advantages of Seed Dormancy:

1. In temperate zones, to overcome the severe colds.

2. In tropical zones, seed dormancy due to hard seed coat improves the chances of survival.

3. Dormancy of Cereal Seeds, increase the availability of them as food and in otherwise, they may not be used as food grains.

Seed Germination:

The process of seed germination starts with the imbibition of water by seed coats and emergence of growing root tip of embryo. It ends when the embryo has developed into a seedling which is out of bounds of seed coats and has its own photosynthetic system.

Conditions for Germination:

(i) Water (ii) Aeration (iii) Temperature (iv) Light.

(i) Water:

It is absorbed initially by imbibition followed by absorbtion.

i. Period required for soaking of seeds in water varies depending upon the nature of seed coat.

ii. Seeds with impermeable seed coats are to be soaked longer than those with permeable seed coats.

iii. Prolonged Soaking affects, as the metabolites leached out which are essential for germination (e.g.) Barley when soaked for 2-3 days.

(ii) Aeration:

Oxygen requirement of different seeds vary considerably, but most seeds germinate at a lower oxygen level.

a. High requirement of oxygen may be due to the presence of permeablity barrier. (i.e.) seed coat.

b. Most seeds require active aerobic oxidation system for germination.

(iii) Temperature:

The seeds of temperate plants require relatively low temperature as compared to tropical plants.

(iv) Light:

The garmination of many seeds is affected by light. Such seeds are called photoblastic. The seeds in which garmination is stimulated by light are called positively photoblastic, those in which garmination is inhibited by light are negatively photoblastic.

Physiological, Biochemical Changes During Germination:

Water Uptake:

Seed coat imbibes water thr6ugh seed coat, which is purely a physical process various hydrophilic groups such as – NH₂, -OH, -COOH Proteins and CHO attract dipolar water molecules and swelling occurs. Due to imbibition of water. Seed coats become (i) more permeable to oxygen and water and (ii) less resistant to outward growth of the embryo.

Respiration:

Uptake is accompanied by rapid increase in respiration rate of embryo. Initially its anaerobic, which is soon replaced by aerobic due to the availability of oxygen.

Mobilization of Reserve Materials:

Occurs to provide,

(i) Building blocks for the development of embryo.

(ii) Energy for the biosynthetic processes.

(iii) Nucleic acids for control of protein synthesis and overall embryonic development changes in these components are as follows.

Nucleic Acids:

In monocots, during imbibition stage, there is a rapid decrease of DNA and RNA content in the endosperm. Appreciable amount of RNA appears in the aleurone layer after 16 hrs. Due to more cell division the DNA content increased.

Carbohydrates:

Starch are the important reserve materials of cereals in the endosperm. During germination starch is hydrolysed first into maltose in the presence of a-amylase and β- amylase and then maltose is converted into glucose by maltase. Glucose is absorbed by scutellum, converted into soluble sucrose and transported to the growing embroyonic axis.

Gibberellin is secreted during germination by embryonic axis into aleurone layer, causing denovo synthesis of a-amylase. This enzyme is not found in ungerminated seeds. GA induced synthesis of a-amylase is countered by ABA.

In contrast to a-amylase, β-amylase is already present in the seed in inactive form, which gets activated during germination. The activity of maltase is also regulated by GA.

Lipids:

During germination, mobilization of lipids is brought about by hydrolysis of fats to fatty acids and glycerol by lipases and P-oxidation of fatty acids to acetyl-СОА. Some of the acetyl COA is converted into sucrose via gly6xylate cycle and is transported to the growing embryonic axis.

Proteins:

Some plants store proteins as reserve food in their seeds in the form of aleurone grains. Mobilization of these proteins involves the hydrolytic cleavage into amion acids by peptidases.

Inorganic Materials:

Number of inorganic materials such as Phosphate, Ca, Mg, and Potassium are also stored in seeds in the form of Phytin.

Enzyme:

Enzyme has been called the “agents of life”. Since they control almost all life process. It is catalysts of life.

Terms co-factor is used to describe inorganic ions such as zinc, iron, copper and magnesium these ions chemically combine with enzyme and assist enzymes. If an organic molecule functions as co-factor it is called co-enzyme.

Cofactors: Fe⁺, Mg²⁺, Mn²⁺ (Inorganic in nature):

1. Cu – Cytochrome oxidase

2. Fe – Cytochrome oxidase, catalase, peroxidase

3. K⁺ – Pyruvate kinase

4. Mg²⁺– Hexokiuase, Pyruvate kinase, glucose-6-phosphate

5. Mn²⁺– Arginase, ribonuclease reductase.

6. Mo – Dinifrogenase

7. Ni – Urease

8. Se – Glutathione Peroxidase

9. Zn – Carboxianhydrase, alcohol dehydrogenase.

Coenzyme: Organic Group Transfer:

1. Biotin CO₂

2. Coenzyme A Acyl

3. Deoxy adenosolyl H⁺

4. FAD e^–

5. NAD H⁴

6. Pyridoxy phosphate amino

7. TPP (Thymine pyrophosphate)

Basic equation:

For enzyme kinetic V₀ = V_max(s) / km + [s]

i. Small value of km – maximal catalytic efficiency at low substrate concentration.

Enzyme substrate Interaction:

Weak interaction between enzyme and substrate

Turnover Number:

The number of molecules of substrate that a single molecule of enzyme can react with in a given unit of time.

Characterstics of enzyme:

1. Colloidal in nature

2. React with both acidic and alkaline substance

3. Show sensitivity

4. Inhibited

5. Activity can be accelerated

6. Size is greater than substrate

7. Specific in nature

8. pH sensitive

Mode of enzyme action:

Michaelis and Menton (1913)

i. Enzyme have active sites for the attachment of substrate molecule where as enzyme can form an intimate rehatible with the substrate.

Factors affecting enzyme activity:

1. Temperature

2. pH

3. Anaesthetics

4. Substrate + Enzyme concentration.