Notes-Part-2-Class-12-Biology-Chapter-7-Plant Growth and Mineral Nutrition-Maharashtra Board

Short day plants :

• Plants that flower under short day length conditions are called short day plants. Plants such as Dahlia, Xanthlum, Soybean, Aster, Tobacco and Chrysanthemum are short day plants or SDP
• Short day plants require a long uninterrupted dark period for flowering. Therefore, they are also called long night plants.
• Flowering in SDP is affected if the dark period is interrupted even by short duration (Flash of light).

Long day plants :

• Plants that flower only when they are exposed to light period longer than their critical photoperiod are called long day plants or LDR
• Long day plants require a short dark or night period for flowering. Hence, they are also called short night plants.
• Plants such as radish, spinach, wheat, poppy, cabbage, pea, sugar beet, etc. are long day plants.

Day neutral plants :

• Plants in which the flowering is not affected by the day length period are called day neutral plants or DNP or photoneutral plants.
• Plants such as cucumber, sunflower, cotton, balsam, maize, tomato, etc. are day neutral plants.
• Flowering is observed throughout the year.

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Vernalization :

• It is influence of low temperature on flowering in plants. The term vernalization was coined by Lysenko (1928).
• Temperature influences several physiological processes and reproductory growth i.e. flowering.
• Klippart (1918) observed low temperature or chilling treatment is responsible for stimulus of early flowering.
• The seeds or seedlings are exposed to low temperatures of 1 - 6° C for about a month’s duration.
• The shoot apical meristem receives stimulus in seedlings.
• Effective in seed stage (embryo) for annual plants. Cereals and crucifers show response to low temperature pretreatments.
• The stimulus is in the form of chemical substance which is proved by grafting experiment by Melcher. It is Vernalin.
• Devernalization : It is reversal of vernalization by high temperature treatment.

Advantages of vernalization :

• Crops can be produced earlier.
• Cultivation of crop possible where they do not occur naturally.

Classification of minerals :

On the basis of their requirement minerals were classified as essential and non-essential.

Essential minerals :

• Without these life cycle of plants cannot be completed.
• Important structural and functional (physiological role)
• Their unavailability causes major deficiency symptoms. e.g. C,H,O,N,P

Non-essential minerals :

• Not indispensable for completion of life cycle.
• Do not produce or cause major deficiency.
• Needed only at specific time during growth E.g. Bo, Si, Al

Based on the quantity requirement, minerals are classified as minor or microelements and major or macroelements.

Microelements :

• Microelements are required in traces as they mainly have catalytic role as co-factors or activators of enzymes.
• Microelements may be needed for certain activity in life cycle of plant e.g. B for pollen germination, Si has protective role during stress conditions and fungal attacks, Al enhances availability of phosphorus.
• The important micronutrients for plant growth are Mn, B, Cu, Zn, Cl.

Macroelements :

• Major elements
• Macroelements are required in large amounts, as they play nutritive and structural roles e.g. C, H, O, P, Mg, N, K, S and Ca. — Ca pectate cell wall component,
• Mg component of chlorophyll. C, H, O are non-mineral major elements obtained from air and water e.g. CO₂ is source of carbon, Hydrogen from water.

Recent classification is based on their functional role, i.e. on the basis of biochemical functions.

Roles of Mineral Elements in Plants :

1) Nitrogen NO^-₂ or NO^-₃ or NH⁺₄ :

• Region of plant in which required : Everywhere particularly in meristematic tissues
• Functions : Constituent of proteins, nucleic acids, vitamins, hormones, coenzymes, ATP, chlorophyll.
• Deficiency symptom : Stunted growth, chlorosis.

2) Phosporus H2PO^-₄ or HPO^2-₄ :

• Region of plant in which required : Younger tissues, obtains from older, metabolically less active cells
• Functions : Constituent of cell membrane, certain proteins, all nucleic acids and nucleotides required for all phosphorylation reactions.
• Deficiency symptom : Poor growth, leaves dull green.

3) Potassium K⁺ :

• Region of plant in which required : Meristematic tissues, buds, leaves, root tips
• Functions : Helps in determining anion- cation balance in cells involved in protein synthesis, involved in formation of cell memberane and in opening and closing of stomta; increases hardness; activates enzymes and helps in maintenance of turgidity of cells.
• Deficiency symptom : Yellow edges to leaves, premature death.

4) Calcium Ca²⁺ :

• Region of plant in which required : Meristematic and differentiating tissues, accumulates in older leaves
• Functions : Involved in selective permeability of cell membranes, activates certain enzymes required for development of stem and root apex and as calcium pectate in the middle lamella of the cell wall.
• Deficiency symptom : stunted growth.

5) Magnesium Mg²⁺ :

• Region of plant in which required : Leaves, withdrawn from ageing leaves and exported to developing seeds
• Functions : Activates enzymes in phosphate metabolism, constituent of chlorophyll, maintains ribosome structure.
• Deficiency symptom : Chlorosis

6) Sulphur SO₄^2- :

• Region of plant in which required : Stem and root tips; young leaves remobilised during senescence
• Functions : Constitutent of certain proteins, vitamins (thaimine, biotin CoA) and Ferredoxin.
• Deficiency symptom : Chlorosis

7) Iron Fe³⁺ :

• Region of plant in which required : Everywhere carries along leaf veins.
• Functions : Constituents of ferredoxin and cytochrome, activates catalase required for synthesis of chlorophyll.
• Deficiency symptom : Chlorosis

8) Manganese (trace) Mn²⁺ :

• Region of plant in which required : Leaves and seeds
• Functions : Activates certain enzymes (carboxylases)
• Deficiency symptom : Chlorosis, grey spots on leaves.

9) Molybdenum (Trace) MoO₂²⁺ :

• Region of plant in which required : Everywhere, MO³⁺ particularly in roots
• Functions : Activates certain enzymes in the nitrogen metabolism.
• Deficiency symptom : Slight retardation of growth.

10) Boron (trace) BO^3-₃ or B₄O₇^2- :

• Region of plant in which required : Leaves and seeds
• Functions : Required for uptake and utilisation of Ca²⁺, pollen germination and cell differentiation, carbohydrate translocation.
• Deficiency symptom : Brown heart disease.

11) Copper (trace) Cu²⁺ :

• Region of plant in which required : Everywhere
• Functions : Activates certain enzymes.
• Deficiency symptom : Die-back of shoots.

12) Zinc (trace) Zn²⁺ :

• Region of plant in which required : Everywhere
• Functions : Activates various enzymes especially carboxylases, part of carbonic anhydrase and various dehydrogenases needed for auxin synthesis
• Deficiency symptom : Malfomred leaves

13) Chlorine Cl^- :

• Region of plant in which required : Everywhere
• Functions : With Na+ and K+ helps to determine solute concentration and anion-cation balance in cells, essential for oxygen evolution in photosynthesis.
• Deficiency symptom : Poor growth of the plant

(i) Passive Absorption : The movement of mineral ions into root cells as a result of diffusion is without expenditure of energy is called passive absorption.

Passive absorption can take place by,

• direct ion-exchange,
• in direct ion-exchange
• mass flow
• Donnan equilibrium.

Donnan equilibrium :

• Some anions after their entry inside the cell get accumulated on inner side of cell membrane.
• Additional cations are needed to balance these accumulated anions, thus the cation concentration becomes more as they get accumulated.
• This transport from exterior against their own concentration gradient for either cations or anions is Donnan equilibrium, which is for neutralizing the effect of accumulated cations/anions.

(ii) Active absorption:

The absorption of minerals against the concentration gradient which requires expenditure of metabolic energy is called active absorption.

• The ATP energy derived from respiration in root cells is utilized for active absorption.
• Hence when there is scarcity of oxygen available to roots there is less absorption of minerals.
• Ions get accumulated in the root hair against the concentration gradient.
• These ions pass into cortical cells and finally reach xylem of roots.
• They are assimilated in organic molecules and carried further with phloem to other parts i.e. redistribution.
• A carrier concept, where membrane proteins of root cell membrane may pump these ions into cytoplasm is suggested.

Physical nitrogen fixation :

• Physical nitrogen fixation occurs in step-wise manner and it takes place in atmosphere and soil.
• Under the influence of electric discharge, lightning and thunder, atmospheric nitrogen combines with oxygen to form nitric oxide.

N₂ + O₂ \(\overset{\text{Electric discharge (Lightening)}}{\rightarrow}\) 2NO

• Nitric oxide is then oxidized to nitrogen peroxide in presence of oxygen.

2NO + O₂   \(\overset{\text{Oxidation}}{\rightarrow}\)   2NO₂ (Nitrogen peroxide)

• Nitrogen peroxide combines with rainwater to form nitrous and nitric acid which come on ground as acid rains.

2NO₂ + Rain water → HNO₂ + HNO₃

• On ground, alkali radicals (mainly of Ca, K) react with nitric acid to produce nitrites and nitrates which are absorbable forms for plants.
• Industrial nitrogen fixation : It occurs by Haber-Bosch nitrate process at high temperature and pressure.

N₂ + 3H₂  \(\frac{450^oC}{200\,atm}>\)  2NH₃ (Ammonia)

• Ammonia is then converted to urea as it is less toxic.
• Nearly 80% of nitrogen found in human tissues originate from the Haber-Bosch process.

Biological nitrogen fixation : When living organisms are involved in nitrogen fixation process it is known as biological nitrogen fixation.

• The process is mainly carried out by prokaryotic organisms, i.e. different kinds of bacteria present in soil.
• The nitrogen fixing organisms are known as diazotrophs or nitrogen fixers and about
• 70% nitrogen is fixed by them.
• The nitrogen fixers are either free living bacteria or symbiotic associated with other higher plants e.g. Rhizobium.
• The cyanobacteria have specialized cells heterocysts which help in process of nitrogen fixation.
• Nitrogen fixation is high energy requiring process and 16 ATP molecules are needed for fixation of one molecule of nitrogen to ammonia.

• Nitrification : Soil bacteria like Nitrosomonas, Nitrosococcus convert ammonia to nitrate and the Nitrobacter convert nitrite to nitrate. This is known as nitrification, biological oxidation. These bacteria are chemoautotrophic and utilize these processes for their metabolism.

2NH₃ + 3O₂  \(\frac{Nitrosomonas}{Nitrosococcus}>\)   2HNO₂ + 2H₂O

2HNO₂ + O₂  \(\overset{\text{Nitrobactor}}{\rightarrow}\)  2HNO₃ 

• Symbiotic N₂ fixation : The best known nitrogen fixing symbiotic bacterium is Rhizobium. This soil living/dwelling bacterium forms root nodules in plants belonging to family Fabaceae e.g. beans, gram, groundnut etc.

Amides :

Ammonia may be absorbed by amino acid to produce amides. The process is called amidation.

• The amides are the amino acids having two amino groups. Extra amino group is attached to acidic group (-COOH) in presence of ATP.
• Amides like asparagine and glutamine are formed from glutamic acid and aspartic acid respectively by addition of another amino group to each.

Glutamic acid + NH₄⁺ + ATP → alpha glutamine + ADP

Aspartic acid + NH₄⁺ + ATP → Aspargine + ADP

• Amides are transported to other parts of plants via xylem vessels.

De-nitrification :

• It is the process in which anaerobic bacteria can convert soil nitrates back into nitrogen gas.
• Denitrifying bacteria removes fixed nitrogen i.e. nitrates from the ecosystem and return it to the atmosphere in inert form.
• Denitrifying bacteria includes Bacillus spp., Paracoccus spp. and Pseudomonas denitrificans.
• They transform nitrates to nitrous and nitric oxides and ultimately to gaseous nitrogen.

2NO₃ → 2NO₂ → 2NO → N²