TRANSPIRATION

TRANSPIRATION 

• Plant absorbs a large quantity of water from soil,and it is translocated through the plant and eventually lost to the surrounding atmosphere.
• The process by which water is lost from the plant into the surrounding atmosphere in the form of water vapor,is called TRANSPIRATION.
• All of the water absorbed by the plants,less than 5% actually retained for growth and even less is used biochemically.
• Although a small amount of water vapor may be lost through small openings  in the bark of young twigs and branches,called Lenticles , the largest portion by far (>90%) escapes through Stomata (sing -Stoma).
• Stomata are small pores occassionally present between the epidermis and overlying cuticle.
• Each   of the stomatal pore is surrounded by a pair of Guard cells . These guard cells function as hydraulically operated valves that control the size of pore.
• Stomata are located such that,when they open they provide a route for exchange of gases (carbondioxide, oxygen & water vapor) between the internal air space and the bulk atmosphere surrounding leaf.

TYPES OF TRANSPIRATION 

1. Cuticular transpiration -Cuticle is a multilayered  waxy deposit  covering on the epidermis of leaves and herbaceous stems. It is meant to check transpiration . However some water may be lost through it. The loss of water in the form of water vapor through the cuticle is called as cuticular transpiration. It accounts for about 5-10% of total transpiration by plants.
2. Lenticular transpiration- Loss of water in the form of water vapor through the lenticles is called lenticular transpiration. It accounts for only 1-5% of the total water loss by the plants. 
3. Stomatal transpiration- The primary function of stomata is gas exchange between plant's internal tissues and the atmosphere. When water is lost through stomata in the form of water vapor,  it is called stomata transpiration. It accounts for ~90% of total transpiration by the plant.

TRANSPIRATION IS A TWO STAGE PROCESS 

1. 1)Diffusion  of water from the moist cell walls into the substomatal air space

• It is assumed that water is pulled out from the xylem and evaporation occurs primarily at the surfaces of those mesophyll cells that borders substomatal air  spaces. 
• However some investigators have suggested that most of the water evaporates from the inner surface of epidermal cells in the vicinity of stomata known as Peristomal evaporation.

1. 2) Diffusion of water vapor from substomatal space into air

• It is relatively straightforward. Once the water vapor has left the cell's surfaces, it diffuses through the substomatal space and exits the leaf through the stomatal pore.

THE DRIVING FORCE FOR TRANSPIRATION IS THE DIFFERENCE IN WATER VAPOR CONCENTRATION

• The water vapor concentration is the driving force behind transpiration ,the  water vapor concentration difference  is expressed as Cwv (leaf) -Cwv (air).
• The water vapor concentration of bulk air (Cwv(air)) can easily be measured  ,but that of the leaf (Cwv(leaf)) is more difficult to assess.
• The air space in leaf is close to water potential equilibrium with the cell wall surfaces from which water is evaporating.
• Within the range of water potentials experienced by transpiring leaves (generally <2.0 MPa) the equilibrium water vapor concentration is within few percentage points of the saturation water vapor concentration. This allows to calculate water vapor concentration within a  leaf.

• The concentration of water vapor (Cwv)changes at various points along the transpiration pathway.

• From figure, we see that Cwv changes at each step of the pathway from the cell wall surfaces to the bulk air outside leaf.
• The important points to remember 

1. That the driving force for water loss from the leaf is the absolute concentration difference 
2. That this difference depends on leaf temperature. 

WATER LOSS IS ALSO REGULATED BY THE PATHWAY RESISTANCE 

• The second important factor governing water loss from the leaf is the Diffusional Resistance of the transpiration Pathway, which consists of two varying components-

1. The resistance associated with diffusion through the stomatal pore, the Leaf stomatal resistance (rs).
2. The resistance due to the layer of unstirred air next to the leaf surface through which  water vapor must diffuse to reach the turbulent air of the atmosphere. This second resistance (rb)is called the Leaf boundary layer resistance. 

GUARD CELLS ACTION MECHANISM 

• Guard cells are found in leaves of all vascular plants, and they are also present in organs from primitive plants such as liverworts and mosses 
• The cell wall of  guard  cells have specialized features. They are substantially thickened at portions. 
• Morphologically guard cells can be distinguished into two main types-

1. Dumbbell shaped guard cells- They are found in grasses and other monocots, such  as palms. They have a characteristic dumbbell shape with bulbous ends. These guard cells are always flanked by a pair of differential epidermal cells called Subsidiary cells, which helps the guard cell control stomatal pore. The guard cell, the subsidiary cells and pore are collectively called Stomatal complex. 
2. Kidney shaped guard cells- They are found in dicot plant  and non grass monocots.  They have an elliptical contour with the pore at its center. Subsidiary cells are not uncommon, but mostly absent. In that case the guard cells are surrounded by ordinary epidermal cells. 

Figure_(A)-Kidney shaped guard cells 

                  (B)Dumbbell shaped guard cells 

AN INCREASE IN GUARD CELLS TURGOR PRESSURE OPENS THE STOMATA

• Guard cells functions as multisensory hydraulic valves. 
• Environmental factors such as light intensity, temperature, relative humidity and intracellular CO2 concentrations are sensed by guard cells and these signals are integrated into well defined stomatal responses. 
• The early processes are ion uptake and other metabolic changes in the guard cells. Here we note that decrease in Osmotic pressure resulting from ion uptake and from biosynthesis of organic molecules in the guard cells. 
• As Osmotic pressure decreases, the water potential decreases and water subsequently moves to the guard cells. 
• As water enters the guard cells, Turgor pressure increases. Because of the elastic properties of their walls, guard cells can reversibly increase in their volume by 40-100% depending on  the species. 
• Because of the differential thickening of guard cell walls, such changes in volume lead to opening and closing of stomata.

FACTORS INFLUENCING TRANSPIRATION

• Several factors influence transpiration these factors can be external as well as internal.
• External factors that affect transpiration are-

1. Light -plant transpire  more in light than in the dark. Because  light stimulates the opening of stomata.
2. Temperature-plant transpire more rapidly at higher temperatures
3. Humidity- Increased humidity decreases transpiration because air is already saturated with water vapor.
4. Soil water - A plant cannot continue transpiration , if its water loss is not made up by replacement from the soil. Decrease in soil water slows down the transpiration.

• Internal factors-Internal factors that affect the transpiration are number of stomata ,thickness of cuticle ,leaf area .distribution of stomata etc.