Transpiration is a loss of water from the plants surface/aerial parts of the plants in the form of vapor. Water move outside through stomata, cuticle layer and lenticels in the leave.
Types of Transpiration:
Based on sites for water loss it classified into following types.
- Stomatal transpiration
- Cuticular transpiration
- Lenticular transpiration
Stomata are elongated slit like pores in epidermis surrounded by kidney shaped guard cells. In monocots and in few dicots the guard cells are covered by some subsidiary cells which are differ from epidermal cells. They help the stomata opening and closing. Stomata are openings pores in plant structure that exchange gas.
Stomata are usually found in plant leaves however may also be found in stems. A specialized cells referred to as guard cells surround stomata and control open and closing of stomatal pores. Stomata permit a plant to require in greenhouse emission, that is required for chemical process.
Plants have thousands of stomata on the surfaces of their leaves. the bulk of stomata are placed on the side of plant leaves reducing their exposure to heat and weather condition. In aquatic plants, stomata are placed on the side of the leaves. A stomata is specialized plant cells. They consist of guard cells and subsidiary cells.
These cells enlarge and contract to open and shut stomatal pores. Guard cells additionally contain chloroplasts, the light-capturing organelles in plants.
Subsidiary cells surround and support guard cells. They act as a buffer between guard cells and stratum cells, protective stratum cells against guard cell growth. Subsidiary cells varied exist in various shapes and sizes. they’re additionally organized otherwise with relevance their positioning around guard cells.
These cells are thick walls and differ from some other leaf cells. These are more thickened as compare to dorsal wall. The thinking is due to the cellulose microfibrils in the guard cell and help in the closing and opening of stomata. Kidney shaped guard cell the cellulose microfibrils fan out radially from pores, thus the cell girth is reinforcing like a steel belted radial tire, and the guard cell curve out to widen the stomata pore.
Theories Explaining opening and closing of stomata:
- Photosynthesis in Guard Cells
- Starch Sugar Interconversion hypothesis of Stomata Movements
- Active K+ ion Transport and Hormonal Regulation Theory
Photosynthesis in Guard Cells:
These cells consist on chloroplasts, they synthesize glucose during day time that increase the osmatic potential in guard cells and increase turgor pressure. At night time the glucose converts into starch and osmotic potential of guard cells decrease and stomata become close. In guard cell protoplast lack of enzyme that use for reduction of carbon dioxide that lead to the formation of sugars. In CAM plant, during night stomata open and photosynthesis is absent.
Starch Sugar Interconversion hypothesis of Stomata Movements:
According to some scientist at night starch content increase at night time in guard cells so they close at night. Therefore, it was suggested that in the presence of light starch convert into glucose.
Similarly, stomata are very sensitive to pH. The low pH closer the stomata and high pH open the stomata. In the presence of light pH increase and starch convert into glucose. In dark this process is reverse. They pH change are brought by co2, carbon dioxide increases the pH and convert starch into glucose.
The presence of glucose 1- phosphate and phosphorylase enzyme in guard cell that convert starch into glucose is brought by phosphorylase. At pH 7 the starch is converting to glucose 1-phosphate and at about pH 5 starch is synthesis. The glucose 1-phosphate is converting into glucose 6-phosphate and then spilt into glucose and phosphate. The glucose dissolve and osmatic potential is increase in guard cell. At nigh this process is reverse.
Active K+ ion Transport and Hormonal Regulation Theory:
The accumulation of k+ and carbon dioxide increase in guard cell during day time in the presence of light and revers in night. At (400-800mM) the concentration of k+ is increase and stomata open and close at (100mM). The change in starch concentration increase the k+ ion concentration.
The starch concentration was found to decrease in the presence of light when stomata are open and increase in dark when stomata are close. It considers that light stimulates guard cell to accumulate potassium and become turgid.
This response triggered by illumination of a blue light receptor in the guard cell, perhaps built into the plasm membrane. Activation of these blue light receptors stimulates the activity of ATP powered proton pumps in the plasma membrane of guard cell, that increase the concentration of k+.
Process of Synthesis K+ Malateion:
Presence of light pH increase, carbon dioxide concentration decreased, and starch is converting into 3-C compound (phosphenolpyruvic acid) PEP. This 3-C compound combine with carbon dioxide and convert into oxalic acetic. Oxalic acetic is convert into malice acid. Malice acid is a week acid they break down into H+ and Malate ions. K+ and malate ions combinedly make potassium malate that increase solute in guard cell and lower water potential. Thus, water enter guard cells. At night this process become reverse.
Factor Affecting the rate of transpiration:
The rate of transpiration is affected by both external and internal factor. These factor effect on the stomata opening and closing.
Environmental Factor (External Factor):
- Vapour pressure (Humidity)
- Wind (Air Movement)
- Atmospheric Pressure
- Availability of soil water
Plant Factor (Internal factor):
- Ratio of Root and shoot
- Leaf surface area
- Leaf structure
- Type of cuticle layers
- Number of stomata on plants leaf