Consortium for Educational Communication

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1 Consortium for Educational Communication Module on Mechanism of opening and closing of stomata By A. M. Chalkoo Department of Botany Govt. Degree College Boys Sopore Cell No:

Text Water movement between the guard cell and surrounding tissue is driven by changes in guard cell osmotic potential.

The fundamental role of osmotic potential was demonstrated in the mid- nineteenth century by von Mohl (1856), who showed that open stomata could be made to close by placing them in solutions of

It is surrounded by a pair of specialized epidermal cells called guard cells, which act as a turgor-driven valve that open and close the pores in response to given environmental conditions.

The concavo-convex curvature of two guard cells is variable and causes stomatal pore to open and close.

2 Text Water movement between the guard cell and surrounding tissue is driven by changes in guard cell osmotic potential. The resulting changes in turgor are translated, via a specialized cell wall structure into changes in the size of the stomatal pore. The fundamental role of osmotic potential was demonstrated in the mid- nineteenth century by von Mohl (1856), who showed that open stomata could be made to close by placing them in solutions of increasing osmotic strength. A stoma (pl. stomata) is a microscopic pore on the surface (epidermis) of land plants. It is surrounded by a pair of specialized epidermal cells called guard cells, which act as a turgor-driven valve that open and close the pores in response to given environmental conditions. The guard cells are connected with the adjacent epidermal cells through plasmodesmata. Typical stomates of dicots consist of two kidney shaped guard cells. They are joined at their ends. The concavo-convex curvature of two guard cells is variable and causes stomatal pore to open and close. In cereals, members of cyperaceae and some palms, the guard cells are dumb-bell shaped in outline. Their expanded ends are thin walled while middle portions are highly thick walled. Guard cells contain a few chloroplasts, whereas their neighbouring epidermal cells seldom do. The presence of countless numbers of stomata is critical for plant function. Typically, the plant epidermis is tightly sealed by wax-coated, interlocking epidermal pavement cells, which protect the plant body from the dry atmosphere and UV-rays. At the same time plants must be able to exchange carbon dioxide and oxygen, for photosynthesis and respiration. Stomata act as a gateway for efficient gas exchange and water movement from the roots through the vasculature to the atmosphere.transpiration via stomata supplies water and minerals to the entire plant system The unique structure of the stomata makes the plant more efficient, and better able to cope during different environmental conditions. Less advanced plants, such as the liverworts, do not have stomata. These plants have continuously open pores, and

3 therefore, can only survive in extremely wet environments. The use of stomata, however, allows plants to survive in many different types of climates. Stomata are present in the sporophyte generation of all land plant groups except liverworts. Dicotyledons usually have more stomata on the lower epidermis than the upper epidermis. Monocotyledons, on the other hand, usually have the same number of stomata on the two epidermises. In plants with floating leaves, stomata may be found only on the upper epidermis; submerged leaves may lack stomata entirely. In oleander or pine, the stomates occur in a substomatal crypt. Such sunken stomates are apparently an adaption to reduce transpiration. Stomata appeared with the first vascular plants in the lower Devonian. The properties of stomatal cell walls play a role in the mechanics and the physiology of stomatal movements. Stomates function the way they do because of special features in the submicroscopic anatomy of their cell walls. The cellulose microfibrils,or micelles, that make up the plant cell walls are arranged around the circumference of the elongated guard cells as though they were radiating from a region at the centre of the stomata.the result of this arrangement of microfibrils, called radial micellation,is that when a guard cell expands by taking up water, it cannot increase much in diameter,because the microfibrils do not stretch much along their length; therefore,because two guard cells are attached to each other at both ends, they bend outward when they swell, which opens the stomata. During opening movements, the plasma lemma is convoluted and probably related to ion influx. Microtubules have been shown to be involved in wall building and probably in stomatal functioning. Guard cell mitochondrial membranes undergo configurational changes during stomatal movements and those of open stomata have a larger vacuolar volume with one or few large vacuoles, whereas in closed stomata, there are numerous small vacuoles. In some species, ribosomes are free in the cytoplasm of guard cells of open stomata, whereas in those of closed stomata, they are associated in polysomes. Also, in guard cells of open stomata, nuclei can possess a reticular condensed chromatin and in those of closed stomata, a homogeneous chromatin. These ultra structural

4 features are associated with the higher synthetic protein and RNA activities of the guard cells when stomata are closed. Mechanism of sto matal opening: Stomata function as turgor-operated valves because their opening and closing movement is governed by turgor changes of the guard cells. Due to endosmosis, an increase in turgor of guard cells takes place which finally results in stretching and bulging out of their outer thin walls. This results in the pulling apart of the opposite inner thicker walls creating an opening or pore in guard cells of stomata. When the turgor pressure of guard cells decreases, inner walls sag, leading to closure of space between them. This is due to the los of water (exosmosis) from guard cells, resulting in thicker walls to move closer and finally shut the opening. Several theories have been put forth to explain the opening and closing of stomata. 1) Turgor Pressure Theory: At the end of the 19th century, plant physiologists believed that, as guard cells contain chloroplasts, they are capable of photosynthesis. Consequently H2O present in guard cells is used up in the said process; at the same time glucose is synthesized. Thus both the steps cause an increase in the diffusion pressure deficit (DPD) of cells and as a result, water from neighbouring cells enters into guard cells creating an increased turgor pressure which leads to opening of stomata. At the outset, this presumption is quite attractive, but experimental facts are different. Amount of water consumed during photosynthesis is so low that it cannot have any significant effect on DPD of guard cells, while glucose synthesized during photosynthesis is a fact, but qualitative analysis shows that

5 glucose does not contribute anything significant towards the DPD increase in guard cells. Thus, the theory fails to explain the true mechanism. 2) Starch Hydrolysis theory: Sayre (1926) and Searly (1932) proposed their theories, which are even though different from one another but show some similarities. The observations such as, a) Presence of starch in guard cells at night and its disappearance during day time, b) opening of stomata under high ph condition and closing at low ph,and C) decrease of CO2 level during day time and increase at night, are very well correlated to each other. During the day time green plants start synthesizing glucose by fixing CO2. Thus the concentration of CO2 decreases within the leaves, as well as outside the leaves. This decrease in CO2 concentration raises or increases the ph of the cell sap of guard cells. Due to this change in ph, certain enzymes found in guard cells get activated and start hydrolyzing the starch (osmotically inactive) which results in release of glucose (osmotically active). This enzymatic conversion causes an increase in the DPD of guard cells. Automatically a DPD gradient is created between the guard cells and neighbouring cells which makes the water to diffuse into guard cells. As a result, turgor pressure increases and guard cells bulge and open. The entire process is reversed during night (darkness) by converting osmotically active Glucose to osmotically inactive starch. This is due to the increased concentration of CO2 because of continued respiration. This theory has got a good support from the discovery of starch phosphorylase enzyme in guard cells by Yung and Tan (1948). PH 7

6 Starch + Pi --> Glucose 1 P PH 5 Though this theory has some support from the experimental evidences, it is not known whether the same enzyme is involved in the dark conversion of Glucose 1-P to Starch in closing of the stomata, or not. Thus, the theory has remained incomplete. 3.Steward s Theory: Steward (1964) based on previous observations, modified the theory of Starch hydrolysis. He concurred with the earlier view up to the formation of G-1-P but he opined that G-1-P is not osmotically active, and to produce osmotically active molecules G-1-P has to be converted to glucose, via phosphoglucomutase and phosphotase action. It is increase of glucose concentration that causes the DPD gradient which results in the increase in turgor pressure(tp) of the guard cells and opening of stomata. Further he viewed that closing of the stomata is an active process, because glucose is converted to starch through hexokinase which requires ATP. The conversion of glucose to starch leads to the fall of DPD level causing the guard cells to become flaccid and therefore the stomata close. This theory became popular and was widely accepted. But soon people realized that Steward s hypothesis was contradicted by certain observations like, a) closure of stomata in the mid-day b) some guard cells are lacking in starch, c) effect of glycollate on opening of stomata. Thus, alternate explanations were sought.

7 4. Malate or K + ion pump Theory. The main features of the theory were put forward by Levitt(1974). Levitt in 1974 combined the points in Scarth s and Steward s hypothesis and gave a modified version of the mechanism of stomatal movement which was called the proton - potassium pump hypothesis. There are two mechanisms for the active transport of potassium ion which are supposed to take place in quick succession. These are - 1. Light induced transport of protons (H + ) from the cytoplasm into the chloroplasts (during photosynthesis) creates a negative potential which eventu ally stops continued movement of proton. As a result of this K + ions from the surrounding cells move into the cytoplasm of the guard cells. This increases the positive potential and the movement of proton into the chloroplasts continues. Consequently the ph of cytoplasm (of guard cells) is raised to 8-9, while that of the chloroplast is reduced to 5. At a higher ph carbon dioxide, changes to carbonic acid (HC03) (in the cytoplasm). 2. Due to photosynthetic activity in the chloroplast, starch is produced and it is converted into phosphoenol pyruvic acid (PEP) during glycolysis in the chloroplast. PEP diffuses into cytoplasm (of the guard cells) where in the presence of the enzyme PEP carboxylase it(pep) reacts with carbonic acid (HCO3), to form organic acids.phosphoenol pyruvate can also be formed by pyruvic acid of respiratory pathway. With the help of PEP carboxylase, it combines with available CO2 to produce oxalic acid which gets changed into malic acid.malic acid dissociates into H + and malate. H + ions pass out of the guard cells. This in turn favours the influx of K + ions( accompanied by Cl - ions) into the cyto plasm of guard cells increasing their osmotic pressure (OP).Guard cells maintain their electroneutrality by balancing K + with malate and Cl -.In the combined state they pass into the small vacuoles and increase the osmotic concentration of the guard cells. As a result guard cells absorb water from the nearby epidermal cells through

8 endosmosis, swell up and create a pore in between them. The influx of K + ions (into cytoplasm) and efflux of protons is energized by H +, K + ATPase pump. For every molecule of ATP hydrolyzed to ADP (near the plasma membrane) one K + ion will enter into the cytoplasm in exchange for one H + ion. Mechanism of stomatal closure: During darkness H + ions start diffusing out of chloroplast into the cytoplasm. This continues till (COO - ), ions are available to combine with H + to form (COOH)2 molecules. These organic acid molecules will cause a reverse influx of K + ions. They move out of the guard cells into the subsidiary cells. This will decrease the osmotic pressure of guard cell cytoplasm. As a result, exosmosis takes place and guard cells shrink, bringing about the closure of the stomata. Blue-light wavelengths of daylight, detected by zeaxanthin (a carotenoid) activate proton pumps in the guard cell membranes, which proceed to extrude protons from the cytoplasm of the cell; this creates a proton motive force (an electrochemical gradient across the membrane) which opens voltage operated channels in the membrane, allowing positive K ions to flow passively into the cell, from the surrounding tissues. Chloride ions also enter the cell, with their movement coupled to the re-entry of some of the extruded protons (Cl/H symport) to act as counter-ions to the potassium. Water passively follows these ions into the guard cells, and as their turgidity increases so the stomatal pore opens, in the morning. As the day progresses the osmotic role of potassium is supplemented by that of sucrose, which can be generated by several means, including starch hydrolysis and photosynthesis. At the end of the day (by which time the potassium accumulation has dissipated) presumingly it is the fall in the concentration of sucrose that initiates the loss of water and reduced turgor pressure, which causes closure of the stomatal pore. ABA is the messenger that causes stomates to close under slowly developing water stress. There are two feedback loops that control stomatal opening and closing. When CO 2 decreases in the intercellular spaces and thus in the guard cells,k + moves into guard cells and stomates open,allowing CO 2 to diffuse in, and thus completing the first loop. This meets the first need of

9 photosynthesis, (in nonsucculents it also leads to transpiration). If water stress develops, ABA appears in the water that moves to the guard cells, so the stomates close, completing the second loop. The two loops interact: The degree of stomatal response to ABA depends upon CO 2 concentration in the guard cells, and response to CO 2 depends upon ABA.One feedback loop provides CO 2 for photosynthesis; the other protects against excessive water loss.stomates,as Raschke (1976) has said, have been delegated the task of providing food while preventing thirst. Factors influencing stomatal movement 1.Water Guard cells can loose water into three different directions: outwards, into the neighbouring subsidiary cell, and into the respiratory cavity that is a part of the intercellular system lying beneath the guard cells. An equilibrium between the water vapour of the atmosphere and the respiratory cavity results when the stomata are opened. Plants form an intermediate distributor, since a large difference in water potential between the moist soil and the normally dry atmosphere is very common. Plants profit from the concentration gradient (that is a gain of energy for them), while the closing movements of the stomata exert a decisive, regulating influence. They close when too much water is lost, or when not enough supply exists. The osmotic pressure of the stomata is far larger in the guard cells than in the subsidiary cells. This ratio shifts in favour of the subsidiary cells when the stomata are closed. 2.Light and Carbon Dioxide. The stomata of most plant species are closed in darkness. The light intensity required for stomatal opening is quite low (250 ft candles in tobacco). Even moon light is sufficient in some cases. Light, however, stimulates opening. The action spectrum is similar to that of photosynthesis. Blue light is especially effective in causing phosphorylation and activation of the plasma membrane H + -ATPase that creates this potential. The stomata of CAM-plants, like Crassulaceans, are opened during the night. They depend on the accumulation of carbon dioxide during the night. These

10 plants store the carbon dioxide as malate or aspartate and feed it into the CALVIN cycle during daytime. Opened stomata would cause intolerable transpiration losses in the areas that CAM-plants live in. A low concentration of carbon dioxide (in the respiratory cavity) causes the stomata to open; a high concentration leads to their closing. Photosynthesis starts with the first light of the day, because enough carbon dioxide has been accumulated. Photosynthesis takes place in guard cells, too, since they contain chloroplasts in contrast to the subsidiary cells. This activity again is related to the rise of the osmotic value and thus also to the opening of the stomata. Thus, opening and closing of stomata is regulated by two independent controlling cycles (that of water and that of carbon dioxide). Regulation via water potential is an effective mechanism. On one hand, the amount of water in the direct vicinity of the guard cells is calculated, and on the other hand, the water potential of far-away parts of the tissue is computed via the effect of ABA. The water and the carbon dioxide cycle may compete in case of closed stomata, since carbon dioxide is usually a limiting factor in photosynthetically active tissues. The stomata remain nevertheless closed at simultaneous lack of water. The rate of photosynthesis decreases to a low level, though it does not stopped at all due to the carbon dioxide that is repeatedly produced a new in considerable amounts by the respiratory processes within the plant.

Chapter 4-2. Transpiration diffusion of water vapor

Chapter 4-2. Transpiration diffusion of water vapor Chapter 4-2 Transpiration diffusion of water vapor Transpiration diffusion of water vapor Diffusion is primary means of any further movement of the water out of the leaf. That is water movement is controlled