Relationship between osmotic potential & pressure potential with water potential

Relationship between osmotic potential & pressure potential with water potential

Explain the relationship between osmotic potential & pressure potential with water potential.

·        Water potential mainly depends on concentration, pressure, and gravity. If the symbols of water potential, the effects of solutes, pressure, & gravity are denoted by Ψw, Ψs, Ψp & Ψg  , then water potential can be expressed as;

Water Potential (Ψw) = Ψs + Ψp+ Ψg 

·        In plants of small height (less than 5 meters), Ψg is negligible. So the equation becomes as;

                        Ψw = Ψs + Ψp

·        Pure water is usually defined as having osmotic potential (Ψs) of zero. As the solute is added solute potential or osmotic potential (Ψs) decreases. So, in this case solute potential can never be positive.

·        The pressure potential (turgor potential) on the other hand in living plant cell is usually positive. In plasmolysed cells & open system , Ψp = 0. Negative pressure potential occurs when water is pulled through an open system such as a plant xylem vessels.

Ø So, in the living cells,

·        If , Ψs = -ve  & Ψp = +ve (or, when pressure potential is less negative than the osmotic potential) then,  Ψw =  -ve.

·        If Ψs =  Ψp i.e., for e.g. Ψs = -1 &  Ψp = +1 ( or, when pressure potential equals to osmotic potential) then, Ψw =   0 (zero).

·        If the value of pressure potential exceeds the value of osmotic potential then, Ψw =  +ve. ( But this is not practically feasible because  it is considered that the value of water potential for pure water is zero).

Give the different parameters involved in the determination of water potential.

OR,

What are the factors involved in affecting the water potential?

·        Basically, there are three parameters involved in the determination of water potential (Ψw). They are:

1.     Solute concentration

2.     Pressure

3.     Gravity

·        Sometimes matrix potential of the system also affects the water potential.

a.     Solute concentration:

 In pure water the value of water potential is maximum i.e., it is zero. Addition of solutes reduces the free energy of water. The term Ψs is used for denoting the concentration of the solute and its effect on the water potential. It is termed solute potential or the osmotic potential.

b.    Pressure:

During osmosis the entry of water results in the development of hydrostatic or turgor pressure which is here called as pressure potential (Ψp). If the pressure potential is positive it will add to the water potential but if it is negative it reduces the value of water potential.

c.      Gravity:

The term Ψg termed gravity potential denotes the effect of gravity on the water potential of a water column in a vertically growing plant. It’s magnitude depends on the height of the plant from the ground level as well as on the density of water and the acceleration due to gravity. In plants of small height (less than 5 meters) the Ψg is negligible.

Ø Water potential is decreased by factors which reduce the relative water vapor viz., by addition of solutes, negative pressure or tensions, reduction in temperature and by matrix forces.

Ø Water potential is increased by factors which increase the negative vapor pressure, mechanical pressure and increase temperature.

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Plant Hormones: Ethylene

CHAPTER: 15(E)

Plant Hormones: Ethylene

Ethylene:       

•      The gas ethylene is also known to function as growth hormone.

•      Earlier it was considered to be exogenous substances which regulate a large number of physiological processes in plants but now it has been kept under the categories of plant hormones.

•      This is only gaseous hormones of plants known.

•      Ethylene is actually a natural product of ripening fruit. It is most powerful ripening hormone present in small quantity in plant tissues.

•      Ethylene is produced by fruits, flowers, seeds, leaves and roots.

•      Its chemical formula is CH₂ = CH₂ .

Biosynthesis of Ethylene:

•      Ethylene is produced from essentially all parts of higher plants, including leaves, stems, roots, flowers, fruits, tubers, and seeds. Ethylene production is regulated by a variety of developmental and environmental factors. During the life of the plant, ethylene production is induced during certain stages of growth such as germination, ripening of fruits, abscission of leaves, and senescence of flowers.

•      Ethylene production can also be induced by a variety of external aspects such as mechanical wounding, environmental stresses, and certain chemicals including auxin and other regulators.

•      Ethylene is biosynthesized from the amino acid methionine to S-adenosyl-Lmethionine (SAM, also called Adomet) by the enzyme Met Adenosyltransferase.

•      SAM is then converted to 1-aminocyclopropane-1-carboxylic acid (ACC) by the enzyme ACC synthase (ACS).

•      The activity of ACS determines the rate of ethylene production, therefore regulation of this enzyme is key factor for the ethylene biosynthesis.

•      The final step requires oxygen and involves the action of the enzyme ACC-oxidase (ACO), formerly known as the ethylene forming enzyme (EFE). 

Physiological roles of Ethylene:

•      Activate malic enzyme, pyruvate decarboxylase and breaking down protein in ripening.

•      Accelerates the abscission layer.

•      Help in RNA and DNA synthesis.

•      Responsible for epinastic and tropic movement

•      Prevent elongation of root and stem in longitudinal direction.

Practical Application of Ethylene:

•      It is used for synthesized flowering and accelerate the ripening of fruits like pineapple, banana, mango etc.

•      Breaks the dormancy.

•      Reduce the incidence of insect and pest diseases.

•      Increase the no. of male flowers.

•      Helps in degreening of many fruits.

•      Stimulates isodiametric growth of root and stem.

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Plant Hormones: Cytokinins

CHAPTER: 15(D)

Plant Hormones: Cytokinins

Cytokinins:

•      Cytokinins (CK) are a class of plant growth substances (phytohormones) that promote cell division, or cytokinesis, in plant roots and shoots.

•      They are involved primarily in cell growth and differentiation, but also affect apical dominance, axillary bud growth, and leaf senescence. 

•      There are two types of cytokinins: adenine-type cytokinins represented by kinetin, zeatin, and 6-benzylaminopurine, and phenylurea-type cytokinins like diphenylurea and thidiazuron (TDZ). 

•      Most adenine-type cytokinins are synthesized in roots Cambium and other actively dividing tissues also synthesize cytokinins.

•      No phenylurea cytokinins have been found in plants. 

•      Cytokinins participate in local and long-distance signaling, with the same transport mechanism as purines and nucleosides. 

•      Typically, cytokinins are transported in the xylem.

•      Cytokinins act in concert with auxin, another plant growth hormone.

Biosynthesis of Cytokinins:

•      Adenosine phosphate-isopentenyltransferase (IPT) catalyses the first reaction in the biosynthesis of isoprene cytokinins.

•      It may use ATP, ADP, or AMP as substrates and may use dimethylallyl diphosphate (DMAPP) or hydroxymethylbutenyldiphosphate (HMBDP) as prenyl donors. 

•      DMAPP and HMBDP used in cytokinin biosynthesis are produced by the methylerythritol phosphate pathway (MEP).

•      Cytokinins can also be produced by recycled tRNAs in plants and bacteria. The prenylation of these adenines is carried out by tRNA-isopentenyltransferase.

•      Auxin is known to regulate the biosynthesis of cytokinin.

Physiological roles of Cytokinins:

•      Accelerates cell division, cell elongation and morphogenesis.

•      Initiation in protein and nucleic acid metabolism.

•      Counteract the influence of apical dominance.

•      Helps to delay senescence.

•      Provide resistance to plant injured by high temperature and low temperature.

•      Can break seed dormancy and promotes germination.

Practical Applications of Cytokinins in Agriculture:

1. Accelerate the induction of flowering of Short-Day plants.
2. Accelerate the development of fruits.
3. Accelerate tissue culture.
4. Stimulates root initiation.
5. Breaking the dormancy of certain light sensitive crop seeds.
6. Delay in senescence in vegetables and keeps the cut flowers and vegetables fresh for a long period.

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Plant Hormones: Gibberellins

CHAPTER: 15(C)

Plant Hormones: Gibberellins

Gibberellins:

•      Gibberellins (GAs) are plant hormones that regulate growth and influence various developmental processes, including stem elongation, germination, dormancy, flowering, sex expression, enzyme induction, and leaf and fruit senescence.

•      Gibberellin was first recognized in 1926 by a Japanese scientist, Eiichi Kurosawa, studying bakanae, the "foolish seedling" disease in rice. It was first isolated in 1935 by Teijiro Yabuta and Sumuki, from fungal strains (Gibberella fujikuroi) provided by Kurosawa. Yabuta named the isolate as gibberellin.

•      They are found in several forms: GA-1, GA-2, GA-3……………………..GA-60.

•      The first gibberellins to be obtained were gibberellin GA-3. Now 52 gibberellins have been identified from different groups of plants.

Structure of Gibberellins:

The structure of gibberellins consists of diterpenoid acids. These are synthesized in plastids through terpenoid pathway. Their modification occurs in the endoplasmic reticulum & cytosol. There are two classes of gibberellins based on carbon number, one containing 19 carbons and another containing 0 carbons. In case of 19 carbon gibberellins, carbon 20 is replaced by five members Lactone Bridge. It links carbon 4 and 10 in the structure of gibberellins.

Physiological roles of Gibberellins:

1.      Induces cell elongation:

This hormone helps in the elongation of internodes. Gibberellins increase the stem of plant.

2.      Stimulates enzyme activity.

Some enzyme in plants requires gibberellins to become activated.

3.      Able to overcome dwarfism by cell elongation:

Dwarfism of plants is prevented by this hormones by elongation the internodes.

4.      Promotes Germination, leaf expansion.

5.      Induce De-Novo synthesis of enzyme.

Practical Applications of GA in Agricultural:

1. Flowering

2. Germination
3. Apical Bud Dormancy
4. Rooting
5. Parthenocarpy
6. Fruit Growth
7. Increase fruit size and prevent cracking of fruits.
8. Induce internode elongation.
9. Prevent senescence in leaves.
10. Increase sugar content in sugarcane and induce alpha-amylase in barley brewing

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Plant Hormones: Auxin

CHAPTER: 15(B)

Plant Hormones: Auxin

Auxin:

•      Auxins were the first of the major plant hormones to be discovered.

•      These are a class of organic compounds which are mainly responsible for bringing about cell elongation in shoots.

•      They may be produced in plants as a result of metabolism or they may be of synthetic origin.

•      The principal natural auxin is indole acetic acid (IAA).

•      Auxins have a cardinal role in coordination of many growth and behavioral processes in the plant's life cycle and are essential for plant body development.

•      All auxins are occurred at the meristematic apices of root, shoot, buds, leaves, cotyledons, bacteria, yeast, fungi etc.

•      There are several forms of auxins: Auxin –a, auxin-b, hetero auxin and related auxin are: IAA, IBA, IPA, NAA, 2, 4-D etc.

Biosynthesis of Indole Acetic acid (IAA):

•      Tryptophan is the primary precursor of IAA in plants. The IAA can be formed from tryptophan by  different pathways.

1. Indole-3-Pyruvic acid pathway:

•      The amino acid tryptophan loses the amino group by deamination or transamination to form indole-3-acetic acid which then loses CO₂ to form indole -3-acetaldehyde. The oxidation of indole-3-acetaldehyde results in the formation of IAA.

2. Tryptamine pathway:

•      This is the next way to synthesis IAA. In this process the tryptophan is first of all decarboxylated to form tryptamine, which is then oxidised as well as deaminated to form indoleacetaldehyde. The oxidation of indole-3-acetaldehyde results in the formation of IAA.

3. Indole-3-aceto-nitrile pathway:

•      In this pathway, tryptophan is converted into indole-3-acetaldoxime and indole-3-acetonitrile. The enzyme nitrilase is involved in this pathway. Indole-3-acetonitrile is converted to indole-3-acetic acid.

4. Indole-3-acetamide (IAM) pathway:

•      It occurs only in some bacteria.

•      The tryptophan is first converted by the enzyme trp monoxygenase to Indole-3-acetamide which is then converted to IAA by the action of the enzyme IAM hydrolase.

Physiological Role of Auxin:

•      Auxin promotes nuclear activities in cell enlargement.

•      IAA increases the plasticity of cell wall.

•      Responsible for initiation and promotion of cell division.

•      Help to prevent apical dominance.

•      Promotes the cambial activity.

•      Useful in tissue culture.

•      Involve in gene expression

•      Help in protein synthesis.

Practical Application of Auxin in Agriculture:

1. Apical Dominance:

•      The influence of terminal bud in supressing the growth of laterals growing immediately below it is termed as apical dominance. If however, the apical bud is substituted by auxin , the axillary buds are inhibited which enhance its own growth.

2. Prevention of Abscission Layer:

•      The premature drop of fruits as apple, citrus, and pear can be prevented to a great extent by spraying the tree with a dilute solution of 2, 4-D, IAA, NAA or some realetd auxin.

3. Germination:

•      Auxin (IAA, IBA, and NAA) widely used to break seed dormancy and increase germination.

4. Root Initiation:

•      In vegetative propagation (cutting roots/shoots), auxins can be applied to cutting serves to induce more rooting.

5. Flowering:

•      Auxin generally inhibit flowering but enhances early flowering in pineapple, berries and barley by spraying NAA.

6. Parthenocarpy:

•      Parthenocarpy is the phenomenon of development of seedless fruits (without pollination & germination). If a flower bud is emasculated and auxin is applied to the stigma of the flower a seedless fruit develops.

7. Tissue Culture:

•      Auxin is one of the important components in tissue culture lab. It is requires for the media formulation which helps in rooting and shooting.

8. Weed Control:

•      Auxin like 2, 4-D spray can remove the weed.

9. Sex Expression:

•      The spray of auxins increases the number of female flowers in cucurbits. It helps to prevent sterility in plants.

10. Respiartion:

•      Auxin has been found to stimulate repiration.

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