Transpiration
Transpiration is the process of water movement through a plant and its evaporation from aerial parts, such as leaves, stems and flowers. Water is necessary for plants but only a small amount of water taken up by the roots is used for growth and metabolism. The remaining 97–99.5% is lost by transpiration and guttation. Leaf surfaces are dotted with pores called stomata,\
The stomata are bordered by guard cells and their stomatal accessory cells (together known as stomatal complex) that open and close the pore. Transpiration occurs through the stomatal apertures, and can be thought of as a necessary
Why do plants need to do transpiration?
The water, warmed by the sun, turns into vapor (evaporates), and passes out through thousands of tiny pores (stomata) mostly on the underside of the leaf surface. This is transpiration. It has two main functions: cooling the plant and pumping water and minerals to the leaves for photosynthesis
Transpiration serves three essential roles: Movement of water and nutrients – Moves minerals up from the root (in the xylem) and sugars (products of photosynthesis) throughout the plant (in the phloem). Cooling – 80% of the cooling effect of a shade tree is from the evaporative cooling effects of transpiration
Transpiration is the process of water movement through a plant and its evaporation from aerial parts, such as leaves, stems and flowers. Water is necessary for plants but only a small amount of water taken up by the roots is used for growth and metabolism. The remaining 97–99.5% is lost by transpiration and guttation. Leaf surfaces are dotted with pores called stomata,\
The stomata are bordered by guard cells and their stomatal accessory cells (together known as stomatal complex) that open and close the pore. Transpiration occurs through the stomatal apertures, and can be thought of as a necessary
Why do plants need to do transpiration?
The water, warmed by the sun, turns into vapor (evaporates), and passes out through thousands of tiny pores (stomata) mostly on the underside of the leaf surface. This is transpiration. It has two main functions: cooling the plant and pumping water and minerals to the leaves for photosynthesis
Transpiration serves three essential roles: Movement of water and nutrients – Moves minerals up from the root (in the xylem) and sugars (products of photosynthesis) throughout the plant (in the phloem). Cooling – 80% of the cooling effect of a shade tree is from the evaporative cooling effects of transpiration
Transpiration Types: Stomatal, Lenticular and Cuticular Transpiration | PlantS
Type # 1. Stomatal Transpiration:
Water vapour diffuses out through minute pore (stomata) present in soft aerial part of plant is known as Stomatal Transpiration. Of the total water loosed, near about 85 – 90% of water loosed by the stomatal transpiration.
Type # 2. Lenticular Transpiration:
Sometimes water may evaporate through certain other openings present on the older stems. These openings are called Lenticels and the transpiration that takes place through term is known as Lenticular Transpiration.
Huber observed in some plants that water lost by lenticular transpiration was about 0.1% of the total transpiration loss. He further noted that coating the bark of the trees reduced the total loss by 20% from total bark surface, showing that some water loss was taking place through general surface of the bark also.
Cuticular Transpiration:
Loss of water may also take place through cuticle, but the amount so lost is relatively small and make up only about 5 to 10 percent of the total transpiration. This type of transpiration depends upon the thickness of the cuticle and presence or absence of wax coating on the surface of the leaves. Xerophytic plants generally have very thick cuticle and wax coating on the leaves and stem in order to check cuticular transpiration.
The phenomenon of transpiration can be demonstrated by a simple experiment (see Fig. 4.1) with a small plant, such as Geranium. In order to eliminate evaporation, the surface of the pot and soil is covered with polythene sheath or an oil cloth. The potted plant is then put under a glass bell-jar. After sometime drops of water appear on the inner side of the bell-jar wall.
Mechanism of Stomatal Transpiration:
Mechanism of stomatal transpiration involves following steps:
i. Osmotic diffusion of water from xylem to inter cellular spaces through mesophyll cells:
In side the leaf mesophyll cells are in contract with xylem and on the other hand with inter cellular spaces above the stomata. When water saturates the cell wal protoplasm and vacuoles of mesophyll cells by the water supplied by wxlem of leaf, then the cells become turgid. Their diffusion pressure deficit and osmotic pressure decrease with the result that they release water in form of vapours with the result that they release water in form of vapours into the inter cellular spaces close to stomata by osmotic diffusion. In turn the O.P. and D.P.D of mesophyll cells become higher and hence they draw water from xylem by osmotic diffusion.
ii. Opening and closing of stomata:
When the water from mesophyll cells and reach the mercellular spaces above stomata in form of vapour then stomatal movement or closing and opening of stomata is necessary for transpiration. The chief mechanism involved in stomatal transpiration is the mechanism of stomatal movement.
Mechanism of Stomatal Movement:
Stomatal movements are brought about by the changes in the volume and shape of fuard cells. This contraction and expansion of the fuard cells is due to turgidity and flacidity respectively. When guard cells absorb water from the surrounding cells and become turgid a pressure is created that pushes the outer thinner wall to bulga outward. Outward bulging pulls apart the inner thick wall of the fuard cells that caused the opening of the stomata.
Stomatal movements caused due to five different factors and are five different types.
i. Photoactive movement :
Movement that controlled by light due to increase in pH on reduction of Co2 or due to hydrolysis of starch into glucose.
ii. Scoto - active movements :
This is found in succulent plants in which stomata opens at night due to organic acid metabolism.
iii. Hydro -active movement: Loss of water from epidermal cells cause opening and their turgidity cause closing of stomata.
iv. Autonomous movement :
Movement of stomata by rhythemic pulsatory activity due to Co2 concentration or change in cell sap concentration.
v. Passive and active movements :
Stomata open in active state and open in passive state due to change in turgidity.
There are several hypothesis has been proposed to explain stomatal movement.
i. Mohl's hypothesis :
Von mohal (1856) gave the hypothesis that, the chloroplast present in guard cells manufactures substances which increase the osmotic pressure of guard cells. As a result of which endosmosis takes place and that increases the turgidity in guard cells, consequently cause opening of stomata.
In high concretions of Co2 around stomata would cause opening of stomatal pore, but the pore closes. On the other hand guard cells have feeble role in photosythesis in compared to mesophyll. Hence the phythesis was rejected.
ii. Starch-sugar hypothesis :
This hypothesis was postulated by Lloyd (1908), loft fields (1921) and sayre (1926). These workers noted that, starch content of guard cells is high during night and low during day time. Suyere further observed that, stomata xloses at a pH lower or higher that pH 4.2-4.4.
This hypothesis postulates that:
i. During day time Co2 which released in respiration is utilized in photosynthesis of mesophyll cells. Therefore concentration of Co2 around guard cells and neighbouring cells reduced with rise in pH.
ii. High pH favours conversion of starch into osmotically active teducing sufars which get soluble in cell sap.
iii. In dark Co2 is accumulated in guard cells as photosynthesis stopped. It cause fall in pH of guard cells. At low pH conversion of sugar into starch takes place. Guard cells become flacid and stomata closed. The enzyme phosphorylases present in the chloroplast catalyses this reaction in presence of inoganic phospate, as follow.
iv. Steward's Hypothesis :
Steward (1964) criticized this above swtarch sugar hypothesis proposed by lloyed and other and pointed out that ,unless glucose 1 - phosphate is furtherer broken down to glucose and inorganic phosphate, no appreciable change occur in the osmotic pressure steward proposed his own scheme, According to which.
i. At high pH the opening of stomata is caused by conversion of starch into glouse.
ii. The closing of stomata requires metabolic energy (ATP), O2 and the enzyme hexokinase which help in conversion of sutars into starch.
Starch-sugar hypothesis is also subhected to criticism in following ground.
i. It fails to explain rise of pH on basis of Co2 concentration.
j. Sugar never noticed in cell sap of fuard cells during opening of stomata.
k. Starch sugar introversion is very slow which does not effect quick stomatal movement.
K+ ion transport mechanism of stomatal movement:
Fujino (1959, 1967) proposed that opening and closing of stomata is directly related to k+ ion conventration of the guard cells. In other words, these phenomena are governed by active transport of k+ ions into the guard cells and out of them.
Rasnchke (1975) sumerised the process as follow:
i. Disappearance of starch from guard cells.
j. Production of organic acids (Malic acid)
k. Migration of H+ ion from fuard cells.
l. Up take of K+ ions into the vacuoles of guard cells.
m. Up take of cl ions into the vaxuoles. Thus stomata opens.
n. Stomatal closure is brought about by outward movementof k+ ion and CL ion from the fuard cells to sorround cells.
Proton transport concept:
Levitt (1974) proposed that the photoactive stomatal opening and closure mechanism and called it as proton transport concept. It is similar to L+ ion transport mechanism.
Transpiration as a Necessary Evil:
i. Transpiration has paramount importance as transpiration pull help in ascent of sap and influence the rate of water absorption by plants.
j. Transpiration cause cooling thus controls the internal temperature of plant body.
k. It helps in gaseous exchange, besides the above importance transpiration cause.
l. Water deficit in plant resulting wilting permanent wilting or death of plants.
m. It causes shedding of leaves.
n. Unnecessary wastage of water from soil and plant body.
o. Structural adoptions required with expense of tissue in xerophyes.
Inspite of above diadvtages plant cannot avoid transpiration, for which curtis (1926) called transpiration as a necessary evil.
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