Introduction to Plant Physiology

Plant physiology is the study of the internal processes that take place inside the plant body, for example, photosynthesis and respiration. Two Greek words combine to make this word ‘physis’ meaning nature and ‘logos’ meaning discourse. So, plant physiology is a discourse about the plant. 

The following key can better make us understand the origin of physiology.

Why study plant physiology?

• Understanding the basic physiological functions for processes, for example, photosynthesis.
• Studying the integration(relationship) of these processes concerning productivity.
• To study the influence of different environmental factors, for example, soil environment, pH, temperature, texture, etc. 
• To test the effect of various processes on plant activities, for example, growth, development, and yield.
• Learning the sequence of events, for example, cell division, elongation, etc.

Water Potential 

Movement of water from one place to another because of energy i.e. from one soil zone to another soil zone. It relates the tendency to move or change state to energy level differences. 

There are different energies.

• Kinetic energy
• Potential energy
• Gravitational energy

Gibbs Free Energy is the sum of kinetic, potential and gravitational energy.

Because of this free energy, the molecules are in constant motion and move from a region of higher concentration to a region of lower concentration by diffusion. Water potential depends upon the number of water molecules. The water potential of pure water is zero. This serves as a reference position. Water potential is positive if it is over zero and negative if it is less than zero. Water potential is a measure of the free energy content of water molecules. A Greek letter ᴪ expresses water potential and its measuring unit bars. 

1 bars= 0.987atm

1 atm=14.7lbf/in2

 Some forces affect this energy

• Matric force
• Osmotic force
• Gravitational force

Matric force

The force of attraction between soil colloids and water molecules. It decreases the water potential.

Osmotic force

Osmotic force results from attraction between water and ions and some other solutes present in the soil. It also reduces water potential.

Gravitational force

Gravitation force is the downward pull of water. 

Components of water potential

Matric potential

ψm is the symbol that represents matric potential. It decreases water potential because of the affinity of water molecules to colloidal substances such as protoplasm and cell wall as colloidal particles tightly attach water molecules. The movement of water molecules becomes more difficult hence, water potential decreases. 

So, it is necessary to give attention to water potential while calculating the water potential of water in seeds or soils. 

Solute potential

ψs is the symbol that represents solutes potential which is the force of attraction between ions and water molecules. The free energy of water molecules decreases if some solutes are present in it. This is because of the collision of water molecules and solute molecules or ions. 

Pressure/turgor potential

ψp refers to the turgor pressure exerted by vacuoles. ψp is always positive. It means pressure potential increases the water potential of the cell. The pressure potential varies from +4 to +5bars during the day and +10 to +15bars during the night. ψp is zero in young non-vacuolated cells and plasmolyzed cells. Therefore water potential of the cell is a sum of component potential, 

 ψ=ψm + ψp + ψs

 In the cell, matric potential is negligible. So,

 ψ=ψp + ψs

This is was a brief introduction to plant physiology we shall be posting the full course of plant physiology soon.