Transpiration is a physiological phenomenon in which water is lost from certain surfaces of the plant body in the form of water vapors. Based on the structures involved, transpiration has been classified into cuticular, lenticular and stomatal transpiration.
Plants lose most of the water by transpiration process that too through stomata. However the total amount of water lost, including all kinds of processes, from a single plant, varies from species to species, which inturn depends upon the structural features of the plant organs involved in transpiration and environmental factors. Xerophytic plants with their reduced leaf surfaces, thick cuticle, multiple epidermal layers, shrunken stomata, have structurally adopted so as to prevent loss of water. But the majority of plants which belong to mesophytic community are so designed, they loose maximum amount of water, in fact they are the plants which deplete most of the soil water.
A single cotton plant looses about one liter of water a day, which is equivalent to 1 cm of rain water per day per acre. One the other hand, corn plants growing in one acre loose water which is equivalent to 3-4 mm of rain water per day. A single maple tree with a large number of foliage is found to loose about 100-125 liters of water per hour. An acre of deciduous forests in tropical regions loose about 7,000 to 20,000 liters of water per day. The above examples indicate that most of the annual rain water obsored by plants is transpired this way.
This process is often called epidermal transpiration for some water is lost through the cuticle found at the outer surface of the epidermal layer of cells. Cuticle is a layer which is made up of complex waxy substances called Cutin. In fact, the cutin is synthesized and deposited by epidermal cells on its outer surface. It is a mixture of free hydroxy fatty acids and soaps. The cuticle layer thus deposited may be thin or thick, but it covers the entire surfaces of the mesophytic plant body including dorsal and ventral epidermal surfaces of leaves. But aquatic plants are lacking in the cuticle.
The purpose of secreting cuticle layers is to prevent the loss of water from the surface cells. But the cuticle layers are not continuous, because mechanical stress, brittleness or injury may cause the cuticle to break open here and there as small fissures. It is through these openings, water is lost to atmosphere from the under lying epidermal cells. Still the total amount of water lost is not that significant, in terms of the total amount of water transpired by stomatal mechanism.
The green leaves, with their vivid shapes and colors, perform many vital functions, such as synthesis of food material by photosynthesis, transport of photosynthate and they also loose water through special structures they possess. The epidermal structures that loose water are called stomatal apparatus. These structures greatly felicitate the exchange of gases, but they are also mainly responsible for loosing more than 90% of the water that is lost from the plant body.
Stoma is a pore and the term stomata refer to pores. As they are the epidermal openings, they are found in both dorsal and ventral epidermal layers, but their distribution pattern varies from species to species. Though the distinct pattern of distribution of stomata is determined by its genome, environmental factors like shade, sunlight, and habitat have a profound influence on the distribution pattern of stomata. Nonetheless, based on the relative distribution pattern between the dorsal and ventral surfaces of the leaves, five groups of plants have been identified. In the recent years, the stomatal distribution pattern has also been used as one of the parameters for identifying and placing the species in a particular group.
1. Apple / Mulberry Type: In this category of plants, stomata are present only on the ventral surface and the stomata are almost absent on the dorsal surface. Ex. Quercus, Acer, Apple, Mulberry, etc.
2. Potato Type: Plants belonging to this category contain stomata distributed on both the surfaces but greater number is found on the ventral surfaces than the dorsal surface. Ex., Tomato, Potato, Cabbage, Brassica, Helianthus, Pea, Beans etc.
3. Oat Type: Most of the plants belonging to grass members have isobilateral leaves, because of their equal disposition to light and shadow. They contain equal number of stomata on both the surfaces. Such distribution is not just restricted to only monocot grass members, but also found in many groups of dicot species; which suggests a common gene pool for expressing their character; ex. Oat, Rice, Wheat, Eucalyptus, etc.
4. Nymphea Type: Certain aquatic plants like Nymphea, Nelumbium produce leaves which float on the surface of water where the dorsal surface is exposed to light and the ventral surface is in contact with water. The leaves belonging to such category contain greater number of stomata on the dorsal side than the ventral surface. Ex. Vioctoria regia, Nymphea, Nelumbium etc.
5. Potomegation type: Potomegaton is an aquatic plant. The leaves of these plants contain stomata which are non functional. No environmental factor can induce them to be functional as the other stomata. Hence, such ineffective stomata are called vestigial stomata. Ex., Potomegaton, Blixa, Vallisnaria, etc.
The number of stomata found per unit areas is called stomatal frequency. But if the total number of stomata present is compared to the total number of epidermal cells in a given unit area, one finds a remarkable correlation. This correlation can be expressed as stomata index.
The equation for determining the stomatal index (1) is, 1=S/E+Sx100
Where S=the number of stomata, E=total number of epidermal cells in a unit area of the leaf. A study on stomatal index of leaves at different positions and different areas in the same leaf and plant provides very interesting information about the structure, frequency, distribution and ultimately their function.
Leaves having such a large number of stomata, virtually acts as wet surfaces which are comparable to porous clay pots filled with water. Through the pores water escapes as vapors in all directions, as a result water vapor shells develop over the pores. When the amount of water lost from a single large surface area is compared with the amount of water lost through very minute pores found in clay pot; it shows that the latter looses water more efficiently than the former. This is because a perimeter of all small pores in a given area is greater than the total surface area of large opening. The diffusion shells overlap with each other, because of close proximity, still stomata loose water very efficiently. This is because the sum total of all stomatal perimeters is greater than the circumference of the whole leaf. This happens despite the total area of all stomata put together occupies just 1 to 2 % of the total surface are of the leaves. In clay pot, the loss of water from its surface has cooling effect on water, that is why people store water in clay pots in summers.
The pore that is present in the epidermal layers is called stoma; the plural form is stomata. Such pores are not just openings, but each pore is surrounded by a set of epidermal cells which are highly specialized in their structure and function. Such a structure is called stomatal Apparatus.
In fact, each stomatal apparatus develops from a single epidermal cell; after mitotic division the middle lamella between the daughter cells gets digested and disappears. This is a highly programmed process for no other cells are affected. However, with the disappearance of the middle wall, an opening is formed between the daughter cells. The two daughter cells present on either side of the pore are called guard cells, which further undergo differentiation and specialization. The cell walls of the guard cells facing pores become thicker and some of the deposition of additional cell wall material radiates from the inner wall towards the opposite wall, but the cell wall found at the other side of the guard cells, remains thin. The epidermal cells found around the guard cells are called subsidiary cells.
In most of the dicot members, guard cells appear as kidney shaped structures, but in monocots they look like dumbbell shaped structures, where the elongated ends are thin walled and flexible and the middle region consists of thick wall. The pore size of the stomata varies from species to species.
Unlike other epidermal cells guard cells are specialized. They contain functional chloroplasts, where as other epidermal cells are totally lacking is functional chloroplasts; guard cells possess well defined nucleus, mitochondria and other cell organelles typical of any other plant cells. Both the ends of guard cells are tightly sealed. Furthermore, subsidiary cells and underlying spongy parenchymatous cells are in contact with guard cells by plasmodesmata, which felicitate the movement of cytoplasmic components from one cell to the other. The presence of starch granules in guard cells during night and disappearance of the same at day times is another important feature of guard cells.
Water found in xylem elements diffuse into and fill the intercellular spaces and apparent free spaces of mesophyll cells. Then water from AFS i.e. apparent free space and inter cellular spaces escape as water vapors into spaces found in the spongy and other parenchymatous tissue, provided the RH of the atmospheric air present in the spaces is very low.
If the stomata are open, the water vapors immediately escape into the atmosphere, other wise the water vapors remain within the leaves. So it is the movement of guard cells that ultimately controls the loss of water through the stomata. But the activity and turgour movements of guard cells are further regulated by various factors like light, temperature, concentration of CO2 internal content of water, relative humidity of the atmosphere, etc.
Opening and closing of stomata exhibits diurnal rhythm, in the sense stomata open during day time and close at nights. So light has an important role in this process. The analysis of action spectrum reveals that blue light and red bands together are very effective in the opening of stomata. Incidentally the same action spectrum is also active in photosynthesis. Finer analysis shows that blue light is more effective in opening of the stomata than red light.
As temperature provides energy for the movement of cellular components within the protoplasm; its decrease or increase has a profound effect on the cellular metabolism. Guard cells are no exception to this rule. At low temperatures such as 00 C degree to 8 0C, stomata remain closed, but with the increase in temperature from 8 to 30 0C or so, stomata open and further show a temperature coefficient equal to 2. However, high temperature has contrary effects, and induces the closing of stomata.
Correlative studies between the concentration of CO2 and opening closing of stomata reveal that higher concentration of CO2 induces the closing of the stomata even during day conditions. On the other hand, if the conc. Of CO2 is very low, it induces the stomata opening even in the absence of light. Changes in the levels of CO2 content in the environs of plants are very well known. Due to active photosynthesis operating in the leaves, the concentration of CO2 decreases rapidly, but at nights, the CO2 concentration builds up because of respiration.
In recent years plant physiologists have demonstrated how the concentration of CO2 affects the cytoplasmic pH. Decrease in the concentration of CO2, the cellular pH increases, i.e. becomes more basic; the same is reversed if the concentration of CO2 is increased in the environs of leaves. The change(s) in the cellular pH has been found to effect or affect the activity of certain enzymes. In plants, increase in pH has been found to activate starch phosphorylase enzyme which activates the break down osmotically inactive starch into osmotically active glucose-P; while low pH can inactivate the starch phosphorylase but activates starch synthesizing enzymes. As a result, the concentrations of osmotically active molecules of guard cells change. This in turn brings about turgour movements of guard cells.
INTRA AND EXTRA CELLULAR MOISTURE CONTENT.
The moisture content of atmosphere is referred to as relative humidity. The RH of the atmosphere influences the rate of transpiration to a greater extent. But under extreme stress conditions like very low RH, in spite of favorable conditions like actinic light, low CO2 levels, etc. stomata close, this is because, the low RH acts as an unfavorable stress factor on guard cells, which immediately respond to such factors and induce closing. It has been found that under water stress conditions, whether it is due to a change in RH of the atmosphere or the non-availability of water, cells release abscissins that is a plant growth inhabiting hormone. Abscissin inturn brings about changes in the permeability of guard cells and induces the closing of stomatal opening similarly the non-availability of water in soil also induces closing of the stomata by the above said hormonal mechanism
The most important single factor that is ultimately responsible for inducing the turgour movement is the change in the osmotic concentration or osmotic potential of guard cell. Once the osmotic pressure builds up in the guard cell, water from the neighboring cells moves into guard cells by diffusion thus turgour pressure builds up within the cells. As the ends of the guard cells are tightly sealed, the cells can expand only in the lateral direction. As the cell wall of the guard cells towards subsidiary cells s thin, they are stretched laterally under increased turgour pressure. As a result, the anterior thick wall of the guard cell also buckles under tension; thus the stoma opens. The closing of stomata is almost reverse of the opening process, where the guard cell loose turgidity and come back to the normal position.
Most of the theories proposed from time to time have tried to explain the mechanism by which guard cells build up osmotic pressure which leads to turgour pressure so guard cells can open and close the pores. Still it is difficult to find any one single theory which explains all the operative forces leading to opening and closing of the stomata. Each one of the mechanisms simultaneously operates in inducing the opening and closing has been explained as a comprehensive scheme of events.
With the onset of day, sunlight triggers a series of reactions which have compounding and confluencing effects, which results in the turgour movement of guard cells to open the stoma (pore)
ACTIVATION OF STARCH HYDROLYSIS
The process of photosynthesis, which is triggered by the sunlight, with time fixes considerable amount of CO2 into carbohydrates. This causes a rapid fall in the concentration of CO2 in the immediate environs of the leaves including the inner spaces found within the mesophyll tissue. The decrease in the concentration of CO2 results in the increase of pH in the cellular cytoplasm. In guard cells, unlike other cells increase in the cellular pH activates enzyme similar to that of starch phosphorylase. The specific hydrolysing enzymes have an optimal pH; in this case it is 7.2 to 7.5. Such activated enzymes hydrolyze osmotically inactive polymers like amylose and amylopectins into glucose or glucose phosphates. The glucose phosphate is immediately dephosphorylated to glucose. It does not make much difference because both glucose and glucose phosphates are equally osmotically active. However, the increase in glucose concentration within the guard cell provides the motive force for the movement of water from the other cells into guard cells. As a result of the entry of water turgour pressure builds up and guard cells bulge to open the pore. The presence and the activity of such starch hydrolysing enzymes in guard cells have been reported by Heath Zelith, Steward and others.
Light besides inducing photosynthesis, activates cytokinin in guard cells which inturn activates ATPase dependent K/H pumps found in the plasma membranes of guard cells located towards subsidiary cells. The activated pumps transport potassium ions, in massive quantities, from the subsidiary cells into the guard cells. In exchange, H+ ions are effluxed. There by the
osmotic concentration of K ions increases and that sets the chain of events like movement of water, increase in TP, and finally opening of stomata. In this process, subsidiary cells play a significant role in providing K+ ions for the guard cells. The energy required for K influx is supplied by noncyclic photophosphorylation activity of chloroplasts found within the guard cells.
Photosynthetic activity in green leaves have two important effects; first it decreases the concentration of CO2 and the second it increases the intracellular concentration of oxygen; the latter triggers the glycollate metabolism. In the presence of higher concentration of oxygen, RUBP carboxylase found in chloroplasts acts as RUBP oxygenease. This enzyme oxidizes the RUBP to phosphoglycerate and glycollate phosphate. Synthesis and accumulation of glycollate along with non cyclic ATP formation in guard cells has been observed. Zelith, and Walker have further shown that glycollate is very effective in the opening of stomata. To top it, the chloroplasts found in guard cells have been found to synthesize malate by PEP carboxylase activity. The glycollate and malate metabolism not only provides energy and hydrogen ions for ATPase pumps, but they also contribute in building up of osmotic concentration, which initiates the events like entry of water, increase in TP and finally the opening of the stomata.
In all the above said reactions light triggers various events where each pathway leads to the accumulation of one or other chemical components thus cause a DPD gradient between guard cells and subsidiary cells. Even if one of the above mentioned three pathways is not operating, stomata can still open with other mechanisms. With all the pathways operating the total effect is confluencing and rapid. As subsidiary cells provide K+ ions and water in large quantities for the guard cells, they play an important role in opening and closing of the stomata.
With the sun setting, the process of photosynthesis stops; as a result three set of reactions which are initiated during the opening o stomata also stop. However, respiratory process continues, with the result the concentration of CO2 gradually increases and oxygen level decreases. With the increase in the levels of CO2, the pH of the guard cells decreases. Hence, starch hydrolyzing enzymes become inactive, but the same pH activates the enzymes responsible for the synthesis of osmotically inactive starch by utilizing the glucoses available in the guard cells. Hence, the osmotic pressure of guard cells decreases and water starts moving out into subsidiary cells. While subsidiary cells are enlarging guard cells loose their cellular TP and regain the normal shape thus the stomata close. Similarly, increase in carbon dioxide and decrease in oxygen inhibits glycollate/malate metabolism; which further affects the osmoticum of guard cells. Finally it leads to closing of the stomata.
Furthermore, the change in the PH of guard cells due to increase in CO2 during night also induces the release of abscisic acid from the chloroplasts (ABA is synthesized in plastids). Abscissin has a reverse effect on the ATPase pump, where the influx of K ions is stopped. Instead the efflux of K ions is induced. Thus the guard cells loose osmotic concentration, which inturn induces the closing of the stomata. In recent years the effect of ABA on turgour movement of cells has gained credibility, because of many experimental evidences. The abscisic acid is now certainly known to induce the closing of stomata. In fact the mid day closure of stomata due to water stress, is attributed to ABA is effect, because under water stress condition, the synthesis and release of ABA increases significantly. All in all the above mentioned pathways have a compounding effect on the DPD of the guard cells, which ultimately leads to the closure of stomata.
Whatever factors that regulate the movement of guard cells also control the rate of transpiration. Among them light, temperature, RH of the atmosphere, soil water, air currents, stomatal index, etc. are the major factors that control the rate of transpiration.
Sunlight is an important source of energy for all living organisms. During photosynthesis plants capture, convert and conserve the solar energy in the form of chemical energy. In fact, light also triggers a series of multipronged reactions within the guard cells, which ultimately leads to the opening of the stomata, thus transpiration is favored in day time, but not during nights.
Most of the plants, both in tropical and temperate regions grow luxuriantly at 25 0C to 30 0C. Because of this 25-30 0C is the most favored temperature for maximum enzymatic activities. So the same temperature is also effective in opening of the stomata, hence the loss of water by transpiration. Low temperature, as prevailed in certain temperate regions, plants do not loose any water because stomata remains closed. But increased temperature, though it felicitates the opening of stomata, has a profound effect on the relative humidity of the atmosphere, which may have a compounding effect.
The relative moisture content of the atmosphere is called relative humidity which greatly varies, because of various factors like, season of the year, altitude, location, temperature, etc. For example, in summer season, because of high temperature, the RH of the atmosphere is low and the rate of transpiration is always high. The reverse is the case on a rainy day during rainy season. Plants growing either in temperate climate or tropical climates, at very high altitudes, because of extreme low temperatures stomata remain closed and they don’t loose any water. So the on the contrary, if the temperature is high the RH will below so the effect of temperature on relative humidity is very significant. So also it affects the opening and closing of the stomata.
Another significant effect of RH on transpiration is the rate of transpiration and the magnitude of water loss. At 50% RH, the DPD of the atmosphere is about 1000 bars, while the DPD of the leaf cells is just about 40 bars or less than that. A situation like this creates such a steep gradient, water is virtually pulled out of the plants into the atmosphere hence the transpiration is greatly fecilitated. The rapidity with which plants loose water is more or less controlled by the relative humidity of the atmosphere, provided that the other factors are favourable for the stomata to remain open or closed.
Water found in the soil is absorbed by the roots and the same is transported through the stem to the aerial regions of the plant body. And finally most of the water is lost by the way of transpiration. Thus the water is continuously depleted from the soil to atmosphere through the plant structures. If water is available in the soil and conditions are favourable for transpiration, plants go on depleting the water from soil eternally. But the availability of water in the soil, at times, acts as a deterrent factor in transpiration. Adequate supply of water to transpiring plant is not found, the stomata close, because of water stress. This state continues till the root system is replenished with sufficient water. Thus the soil water content plays an important role in the loss of water.
The velocity of wind over the surface of leaves contributes a greater impact on the water potential gradient between the surrounding atmosphere and the plant surfaces. In still air, depending upon the DPD gradient, plants may transpire water rapidly or slowly. But with high winds velocity water is displaced from the surface of leaves rapidly, which results is greater loss of water, however under such conditions of water stress, stomata close and inhibit transpiration.
Leaves are the most important structures responsible for transpiration. The number of stomata and distribution of stomata provide the potential surface areas for transpiration. Xerophytes contain reduced or modified leaves, where transpiration is almost nil. On the contrary, a mesophyte has a greater number of stomata, which results in the greater loss of water per unit area per a unit of time. While plants like Potomegaton, though living and leading a submerged habitat in water, they don’t loose any water, for they have non functional stomata. But under normal conditions, if the surface area is provided for transpiration, plants loose greater amount of water and vice versa. The structural adaptation of the transpiration is mostly dependent on the genetic potentiality as well as the plasticity with which they adapt to the environment.
The process of transpiration has been damned as an unavoidable evil. It is more or less true; because the vagaries of rain, which determines the supply of water to plants. Whether water is available or not, plants go on loosing water till they wilt temporarily or permanently; probably it may lead to death if the conditions are hostile. Insufficient supply of water, due to the rapid depletion of water in the soil causes a greater stress on the metabolism of plant cells and on its growth and development, then it prevents the absorption and transportation of minerals to different regions of the plant. This also has a greater bearing on the food production of the world.
The above said observation have necessitated the research workers to find out some substances which prevent transpiration which are fashionably called anti transpirants. Substances like phenyl mercuric acetate, hydroxydeconol, etc., on spraying, they are found to form very thin films over the surfaces of leaves and prevent the loss of water. But some of these substances have also been found to be deterrents, in the sense; they prevent the entry of CO2 into leaves, which has deteriorating effect on photosynthetic yield. Search for genetic variants where the most important crop, plants like cereals, pulses, which provide the food for the entire human population on this planet, could or should possess no stomata at all or have non functional vestigial stomata, thus farmers can grow their crops and harvest with one or two rains without endangering them to drought.
In spite of calling transpiration devil of unavoidable evil, the beneficial effects to plants are many.
1. Cooling Effect: Plant leaf surfaces virtually act as porous wet surfaces. With exposure to atmosphere, plants loose water constantly till the stomata close or empty all the water found in the soil. The loss of water is just like the loss of water from the porous clay pot. With the loss of water from the porous clay pot the temperature of water goes down in the pot. The same effect has been noticed in plants also. Despite, plants being heated up by sun rays and also by internal burning of metabolic fuel, plants maintain their normal temperature. Plants though do not have any homeostatic mechanisms as in the case of higher animals, they manage their body temperature amazingly and effectively. If they couldn’t, they would have burnt up long ago.
2. Ascent of Sap: Another significant effect of transpiration is on the movement of water upwards. Plants, small or large, dwarf or tall, have to transport the water from the roots to all other regions. In some plants like Australian Acacia (350ft). Sequoia semipervians (400ft), which have grown up to a height of 350-400 feet, the water is transported to such great heights. This is made possible by only transpiration. Due to loss of water from transpiring surfaces, all put together exerts a tremendous suction force or transpiration pull on the water column found in the linearly oriented xylem elements through which water is pulled upwards to heights. With ease and facility. This process also helps in absorption of water and minerals effectively. Thus transpiration is an indispensable process for absorption and transportation of water and minerals in plants.
Rapid transpiration brings in an overall effect on recirculation of water within the cells. This also leads to the development of mechanical tissue in the neighborhood of translocation structure, probably to give strength to such structures. The rapid transpiration depletes the water from the soil, which probably provides a motive force for the extensive growth of roots in
search of water.