2.Plant Physiology
2.2 Transpiration
Transpiration
Dr V Malathi
Transpiration is the loss of water from plant in the form of water vapor. About 97-99% of water absorbed by a plant is lost through transpiration ,mainly from the stomata in leaves and also through evaporation from the surfaces of leaves, flowers, and stems.
Stomata
The are apertures in the epidermis, each bounded by two guard cells. In Greek, stoma means “mouth”, The plural of stoma is stomata. Stomata occur in Vascular plants, gymnosperms and angiosperms.
The stomata are commonly seen on green aerial parts of plants, particularly on the upper and lower sides of leaves, on flower petals, on stems, and on roots. Stomata are also found on stamens and gynoecia.
The stomata connect the inner air space of the plant cells with the atmosphere. The stomata is the major route for gaseous exchange, The stomata helps to bypass the impermeable cuticle that forms on the outer epidermal surface.. The stomatal aperture leads into a substomatal intercellular space . This is called the substomatal chamber and is continuous with the intercellular spaces in the mesophyll.
Stomata respond both to the environmental and as well as endogenous signals such as the chemical and hydraulic signals by opening and closing the pore.
It thereby regulates both photosynthesis , regulating the CO2 entry as well as transpiration by limiting the water loss.
“Stomata” by CNX OpenStax, is licensed under CC BY 4.0
Stomatal Opening and Closure
Stomatal Opening
The guard cells are a pair of bean or kidney-shape control the size of the stomatal aperture by changing their shape.
During extreme weather conditions such as high CO2 levels, O3, low air humidity, and drought, the turgor pressure of the guard cells exhibits an appropriate response against these stresses and closes the stomata. This phenomenon involves a complex network of ion channels and their regulation.
When turgor pressure increases in guard cells, the cells swell but the thickened inner walls of the guard cells near the stoma cannot expand and as a result they curve to accommodate the expanding outer walls. This curving of the guard cells opens the stoma
What is the role of light in the opening of stomata?
Light-induced stomatal opening can be divided into two different pathways, including the red light responses and blue light responses.
Distinct wavelengths of solar irradiance are detected by multiple plant photoreceptors
These include :
- blue/ultraviolet-A absorbing cryptochromes (CRY1/2) and phototropins (PHOT1/2),
- red/far-red absorbing phytochromes (PhyA-E), and
- ultraviolet-B absorbing photoreceptor UVRESISTANCE LOCUS 8
PHOT1 and PHOT2 are the major blue light-specific photoreceptors in guard cells modulating stomatal movement.
Guard cells use ATP, generated by light reaction s of photosynthesis to drive H+ out of the cell via H+-ATPases,
This generates a membrane voltage, negative inside, and an electrochemical potential difference
The cytosol usual