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Plant Physiology

Plant Physiology

Table of Content

Mineral Nutrition

Macronutrients

These elements are needed by the plants in large quantities. It includes carbon, hydrogen, oxygen, nitrogen, phosphorous, sulphur, potassium, calcium and magnesium.

Micronutrients

These are the nutrients that are needed by the plants in small quantity. It includes iron, manganese, copper, molybdenum, zinc, boron, chlorine, and nickel.

Role of different macronutrient and micronutrients

Element

Absorbed form

Function

Disease

Nitrogen

nitrate ions (NO3)- and some plants also absorb in the form of nitrite ions(NO2)- or ammonium ions(NH4)+Present in meristematic tissues, essential for photosynthesis, component of vitaminYellowing of leaves

Phosphorous

phosphate ions (PO4)3-component of cell membranes, proteins, nucleic acids and nucleotides, Involved in energy storage and transferLeaf tips appear burnt

Potassium

potassium ions (K+)Maintaining osmotic potential in a cell, abundant in actively growing tissuesInterveinal chlorosis

Calcium

calcium ions(Ca2+)Present in the middle lamella, activation of the certain enzymesBlossom end rot

Magnesium

magnesium ions(Mg2+)Synthesis of RNA and DNA, Key component of chlorophyllChlorosis of leaf

Sulphur

sulphate ions(SO4)2-

Component of amino acids such as methionine, Required for chlorophyll formation

 

Young leaves appear yellow first

Iron

ferric ions(Fe3+)Activates an enzyme catalase, involve in electron transferDieback disease

Manganese

manganese ions(Mn2+)Photolysis of water during non-cyclic photophosphorylationReduce plant parts with dead spots

Zinc

zinc ions (Zn2+).Required for chlorophyll formationRosette formation

Copper

cupric ions (Cu2+)Participates in reproductive and vegetative stages of plantsDieback of stems and twigs

Boron

BO33− or B4O72-.Pollen tube formation, cell differentiationDeath of meristem

Molybdenum

molybdate ions (MoO2)2-Component of nitrogenase enzyme,Stunted growth, leaves appear pale

Chlorine

chloride ions (Cl1-).Involved in photolysis of waterwilting, chlorosis,

 

Biological Nitrogen Fixation

  • Nitrogen fixing bacteria such as Rhizobium fixes nitrogen biologically.
  • Rhizobium lives in symbiotic association with the roots of the leguminous plants.
  • Nitrogenase enzyme catalyzes nitrogen fixation.
  • Leghemoglobin belongs to hemoglobin family serves as oxygen scavenger during nitrogen fixation as nitrogenase is sensitive towards oxygen.

Nitrogen fixation

Fig.1. Nitrogen fixation

Note: For detailed study kindly refer to the content “Mineral Nutrition”.

Transport in Plants

There are 3 types of transport that occurs in plants-

  • Diffusion is the movement of molecules from the region of high concentration to the region of low concentration
  • Facilitated diffusion is brought about by proteins known as permeases.
  • Active transport is an energy dependent process. It is a specific mode of transport of solutes

Types of transport

Fig.2. Types of transport

  • Water potential is the measure of the potential energy of water. It is also defined as the sum of the solute potential and pressure potential. In pure water as there is no solute, water molecules are free to move, thus potential energy of pure water is high.
  • Osmosis is the movement of solvent across the semi-permeable membrane from less concentrated solution to more concentrated solution.
  • Plasmolysis is the shrinkage of cell wall, when kept in hypertonic solution.

Behavior of plant cells in different solutions

Fig.3. Behavior of plant cells in different solutions

Different pathways of water absorption

  • Apoplast pathway involves transport via intercellular spaces and cell wall.
  • Symplast pathway is the movement via protoplasts with the help of the cell to cell connections known as plasmodesmata.

Water movement up the plant

  • Root pressure drives the upward movement of water. When rate of evaporation is low, excess water in the form of water droplets is observed near the tips of the leaves. This is known as guttation.
  • Cohesion-tension theory was given by Dixon and Jolly, to explain the upward movement of water in the plants. According to this theory, transpiration pull drives the upward movement of water.

Uptake and transport of mineral nutrients

The pressure flow hypothesis or mass flow hypothesis or Munch hypothesis was proposed for translocation of sugars from source to sink.

Munch hypothesis

Fig.4. Munch hypothesis

Note: For detailed study kindly refer to the content “Transport in Plants

Plant Growth and Development

Growth is the irreversible, permanent increase in the size of organism. Growth is measurable.

Phases of growth

  • Phase of cell division
  • Phase of cell enlargement
  • Phase of cell differentiation or phase of cell maturation

The increase of growth with time is known as growth rates. There are two types of growth- geometric growth and arithmetic growth.

The changes that occurs during the life of an organism from birth to death includes development.

Plant growth regulators

HormonePrecursorFunctions
AuxinTryptophanPromotes apical dominance, root formation
GibberellinTerpenesPromotes flowering, mobilization of alpha-amylase
CytokininPurineInvolved in cell division, delay in senescence
EthyleneMethioninePromotes fruit ripening, promote senescence
Abscisic acidTerpenesPromote dormancy, wilting. Closure of stomata

Photoperiodism

Long day plants require the exposure of light more than the critical period of light. For example, oat.

Short day plants require the exposure of light less than the critical period of light. For example, cotton.

Note: For detailed study kindly refer to the content “Plant Growth and Development”.

Photosynthesis in Higher Plants

It is defined as the process of conversion of light energy into chemical energy. The chemical energy is stored in the form of carbohydrates such as sucrose, starch etc.

In higher plants, photosynthesis occurs in chloroplast. It is a double membrane structure and an autonomous cell organelle containing DNA and ribosomes. Pigments present in chloroplast is chlorophyll. Other pigments observed during chromatographic separation are – chlorophyll a, chlorophyll b, carotenoids, and xanthophyll

The process of formation of ATP from ADP in presence of light is known as photophosphorylation.

Difference between cyclic and non-cyclic photophosphorylation

Plant Kingdom

Chemiosmotic hypothesis explains how ATP synthesis occurs. It is based on the hypothesis that. Proton gradient drives ATP synthesis. FO-F1 ATPase is involved in the synthesis of ATP.

Calvin cycle

It is a light independent reaction that takes place in the stroma of the chloroplast. It includes three steps- carbon-fixation, reduction, and regeneration. The primary electron acceptor is RuBP.

Calvin cycle

Fig.5. Calvin cycle

C4 pathway

Plants which are adapted for dry tropical regions undergo C4 pathway for carbon fixation. For example, maize. Sorghum. The characteristic feature of this pathway is Kranz anatomy. The primary carbon-dioxide acceptor is phosphoenol pyruvate (PEP).

C4 Cycle

Fig.6. C4 Cycle

Photorespiration

It is a waste process that occurs when RuBP binds with oxygen instead of carbon-dioxide. Chloroplast, peroxisomes and mitochondria are involved in photorespiration.

Photorespiration

Fig.7. Photorespiration

Crassulacean acid metabolism (CAM)

This pathway is adapted for arid conditions.

CAM pathway

Fig.8. CAM pathway

Note: For detailed study kindly refer to the content “Photosynthesis in Higher Plants”.


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