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Uploaded byTahira Rai
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Mechanism of stomatal regulation

Stomata are tiny pores on plant leaves that open and close to regulate gas exchange. They are bordered by a pair of guard cells that swell and contract to control the opening width. During the day, photosynthesis in guard cells produces sugar, increasing their turgor pressure and causing stomata to open. At night, the lack of photosynthesis allows starch to accumulate, decreasing turgor pressure and prompting stomatal closure. Potassium ion transport is also important - influx of K+ into guard cells during the day increases their solute content and turgor, while efflux at night decreases turgor. Together, these mechanisms allow plants to control gas exchange through stomatal openings in response to light

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Plant Physiology•
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MECHANISM OF STOMATAL REGULATION Presented by: TAHIRA ALI RAI
Introduction:  Plant breathe through tiny openings in leaves are called stomata.  The stomata are very minute holes.  Stomata open and close to allow the intake of carbon dioxide and the release of oxygen.  found on the epidermis of the leaves.  Each stoma is surrounded by two kidney-shaped epidermal cells, known as guard cells.  The stomata found in all the aerial parts of the plant.  They are never found on roots.  The epidermal cells surrounding the guard cells of the stoma are known as accessory or subsidiary cells.
Stomata on leave surface
 The guard cells contain chloroplasts.  These cells contain much amount of protoplasm  The number of stomata may range from thousands to lacs per square centimeter on the surface of the leaf.  The stomata may be found on both the surfaces of the leaf, but their number is always greater on the lower surface.  The upper surface of the leaves of several xerophytes also lacks the stomata.
 The free floating leaves of the water plants bear stomata only on their upper surface. Frequency of stomata on the upper and lower surface of leave
Basics Mechanism Of Opening And Closing Of Stomata:  In general it is considered that stomata open at day and closed at night. Stomatal movements are regulated by the change of turgor pressure in guard cells.  When water enters the guard cell, it swells and its unevenly thickened walls stretch up resulting in the opening of stomata.  This is due to concave non-elastic nature of inner wall pulled away from each other and stretching of the convex elastic natured outer wall of guard cell.
Theories: Different theories have been proposed regarding opening and closing of stomata. The important theories of stomatal movement are as follows, 1. Theory of Photosynthesis in guard cells 2. Starch – Sugar interconversion theory 3. Active transport potassium ion concept
Theory of Photosynthesis in guard cells: Von Mohl (1856) observed that stomata open in light and close in the night. According to him,  The mechanism of the closing and opening of the stomata depends upon the presence of sugar and starch in the guard cells.  During day time or in the presence of light, the guard cells of the stomata contain sugar synthesized by their chloroplasts.  The sugar is soluble and increases the concentration of the sap of guard cells.
 Due to higher concentration of the cytoplasm of guard cells, the water comes to them from the neighboring cells by osmosis and they become turgid. With the result the stomata remain open.  In the night or in the absence of light the sugar (glucose) present in guard cells converts into the starch. The starch is insoluble, and this way the cell sap of the guard cells remains lower concentration than those of neighboring cells, and the neighboring cells take out the water from the guard cells by osmosis making them loose and the stomata closed.
Demerits:  Chloroplast of guard cells is poorly developed and incapable of performing photosynthesis.
Starch – Sugar Interconversion theory:  According to Lloyd (1908), turgidity of guard cell depends on interconversion, of starch and sugar.  It was supported by Loftfield (1921) as he found guard cells containing sugar during the daytime when they are open and starch during the night when they are closed.  Sayre (1920) observed that the opening and closing of stomata depends upon change in pH of guard cells.
 According to him stomata open at high pH during day time and become closed at low pH at night. Utilization of CO2 by photosynthesis during light period causes an increase in pH resulting in the conversion of starch to sugar  Sugar increase in cell favours endosmosis and increases the turgor pressure which leads to opening of stomata. Likewise, accumulation of CO2 in cells during night decrease the pH level resulting in the conversion of sugar to starch. Starch decreases the turgor pressure of guard cell and stomata close.
 The discovery of enzyme phosphorylase in guard cells by Hanes (1940) greatly supports the starch-sugar interconversion theory. The enzyme phosphorylase hydrolyses starch into sugar and high pH followed by endosmosis and the opening of stomata during light. The vice versa take place during the night.
 Steward (1964) proposed a slightly modified scheme of starch-sugar interconversion theory. According to him, Glucose-1-phosphate is osmotically inactive. Removal of phosphate from Glucose- 1-phosphate converts to Glucose which is osmotically active and increases the concentration of guard cell leading to opening of stomata.
Objections to Starch-sugar interconversion theory:  In monocots, guard cell does not have starch.  There is no evidence to show the presence of sugar at a time when starch disappears and stomata open.  It fails to explain the drastic change in pH from 5 to 7 by change of CO2.  The conversion of starch into sugar is not fast process as compared to the opening and closing of stomata.
Active Potassium (K+) ion Theory:  Role of potassium K+ in stomatal opening is now universally accepted. This was observed for the first time by Fujino (1967) that opening of stomata occurs due to the influx of K+ ions into the guard cells.  The sources of K+ ions are subsidiary and epidermal cells, thereby increasing the concentration from 50 mM to 300 mM in guard cells.  ATP helps in entry of K+ ions into the guard cells.  In the presence of light the pH increase due to absence of CO2 which is used in photosynthesis and the starch is converted to a 3 C compound phosphenolpyruvic acid .
 The PEP combines with CO2 to produce oxaloacetic acid which is converted to malic acid.  The malic acid being a weak acid dissociates into H+ ions and malate ions.  K+ and malate ions combines to form potassium malate that increase the solute potential and lowers the water potential of the guard cells.  The water move into the guard cells to make them turigd and the stomata open.
 In dark, CO2 produced during respiration accumulates and pH falls resulting in conversion of malic acid to starch.  The k+ ion and water move out of the guard cells lowering the osmotic potential of the cells.  Loss of turgor result in closure of stomata.  Also ABA ( abcissic acid) produced under water stress conditions and plays a role in closure of the stomata.
Opening and closing of stomata
References:  https://www.biologydiscussion.com/plants/stomatal- mechanism/stomatal-mechanism-in-plant-cells-botany  http://www.brainkart.com/article/Mechanism-of-Stomatal- Movement--_33071/