Plant physiology or physiology simply refers to the study of the functioning of different plant systems.

A person studying physiology is called a physiologist. Julius Sachs, a physiologist, is known as the father of plant physiology.

Plants are essential for the lifecycle of every organism and the ecosystem, as they provide oxygen to other organisms and form the primary source of food. Therefore, understanding their functions is extremely important. Physiology also includes the functioning of different parts of plants, i.e., roots, leaves, stems, and flowers. Physiology also includes understanding the autotrophic functioning of plants and their respiration.

Components of Plant Physiology

Physiology deals with the different systems of plants. The basic systems under plant physiology are as follows:

Transportation in plants

Plants have a vascular system that comprises the xylem and phloem, which aid in the transport of water and minerals throughout the plant. Xylem is a non-living long tube-like structure that consists of tracheid cells. Phloem comprises sieve tubes, companion cells, phloem fibres, and phloem parenchyma. A suction force due to transpiration is responsible for the upward conduction of water and minerals through the xylem. On the other hand, the phloem circulates food prepared by the leaves to all parts of the plant. It functions in the same way as the circulatory system does in animals.

Photosynthesis

Plants are autotrophic, i.e., they have the ability to produce food using certain raw materials. Photosynthesis is the process by which plants capture sunlight and carbon dioxide in the presence of water to give out glucose and oxygen.

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

It is impossible to imagine life without photosynthesis, as every organism directly or indirectly depends upon it. Photosynthesis releases oxygen, which is a basic necessity for the survival of every living organism. Plants take up carbon dioxide from the atmosphere, thus helping purify the air.

Respiration

Respiration in plants is the process of the breakdown of sugars to release energy in the form of adenosine triphosphate (ATP). ATP is called the energy currency in both plants and animals. The respiration can be aerobic or anaerobic, based on the presence or absence of oxygen.

• Aerobic respiration: Aerobic respiration is the process of breaking down sugar molecules in the presence of oxygen. The glucose molecule breaks down in the presence of oxygen to release carbon dioxide and water and 38 units of ATP

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + 38 ATP

• Anaerobic respiration: Anaerobic respiration takes place in the absence of oxygen, and less amount of energy is liberated. It usually occurs due to the incomplete oxidation of food (in the form of sugars). Carbon dioxide is released along with ethyl alcohol as a by-product in anaerobic respiration

C₆H₁₂O₆ → CO₂ + alcohol + 2 ATP (energy)

Plant growth and development

Plant growth and development is the irreversible, permanent growth of plant cells. The meristematic cells continuously divide to give rise to new cells. They can be of two types: apical meristems and lateral meristems.

As the name suggests, the apical meristem is present on the apices of the stem or roots. It causes primary growth by increasing the length of the plant. On the other hand, the lateral meristems cause secondary growth by increasing the width and girth of the stem and roots.

Plants have naturally occurring chemical compounds that regulate their growth. These are called plant growth regulators. These regulators can have both promoting and inhibiting effects on the growth of plants.

Plant growth has three phases:

• Meristematic phase: During this phase, the cells continuously divide, forming new ones
• Elongation phase: In this phase, the cells elongate with an increase in vacuole formation and cell wall deposition
• Maturation phase: In the maturation phase, the cells attain their maximum size

Transpiration

Transpiration in plants is the loss of water in the form of water vapour due to the suction force from the aerial parts of plants. Most of the loss of water due to transpiration occurs from the leaf. Plants only use 3%-4% of water; the other 96%-97% of water is lost through transpiration.

There are three main types of transpiration based on the aerial part under influence:

• Stomatal transpiration: As the name suggests, stomatal transpiration is the loss of water from the stomatal opening of the leaf. The stomata are generally present on the underside of the leaves. The stomatal pores open in the day and close at night. Most of the plant transpiration (80%-90%) occurs through the stomata
• Cuticular transpiration: The loss of water through the cuticles of plants is called cuticular transpiration. The cuticle is the outermost covering of plant leaves, fruits, flowers, and other non-woody stems. It protects the plant from drought, UV rays, chemicals, and harsh temperatures. It also prevents excess loss of water from the aerial parts of the plants

Cuticular transpiration is approximately 3%-9% of the total transpiration

• Lenticular transpiration: Lenticels are the small openings on the old woody stems. Thus, the transpiration occurring through these lenticels is called lenticular transpiration. A minute amount of water (approximately 0.1%) is lost through lenticular transpiration

Conclusion

Plant physiology is the study of the functioning of plants. It includes all essential processes occurring in plants. Plants are the primary source of food and oxygen for every organism; therefore, it is necessary to understand their functioning. The processes that come under plant physiology are transportation in plants, mineral nutrition, respiration in plants, plant growth and development, photosynthesis, and sexual reproduction in flowering plants. Julius Sachs, a physiologist, is known as the father of plant physiology.