Theories Of Stomatal Movement
Various theories have been put forward to explain the change in osmotic potential of the guard cells.
Photosynthesis in Guard Cells
Guard cells contain chloroplasts, they synthesize sugar during day time that causes increase in osmotic potential of the guard cells and a subsequent increase in turgor pressure. At night, the sugar is converted into starch and osmotic potential of the guard cell is reduced and stomata are closed. But it has been observed that guard cells protoplast lacks enzymes requires for reduction of carbon dioxide leading to the formation of sugars. Also, the stomata in CAM plants open, during night and photosynthesis is absent in them.
Starch-Sugar Interconversion Hypothesis of Stomata movements
Several workers observed that starch contents of guard cells are high in dark and low in light. Also, it was observed that starch contents decrease when the stomata are open and increase when closed. Therefore, it was suggested that in the presence of light the starch is converted into sugar and increases osmotic potential of the guard cells. The sugars used in synthesis of starch in guard cells in believed to be transported from the adjacent cells.
Similarly, stomata are sensitive to pH changes. The low pH favors closure of stomata and high pH favors opening of stomata. It was also observed that in the presence of light pH increases and starch is degraded to sugar. The reverse occurs in dark. Diastase was believed to be the enzyme catalyzing the starch-sugar interconversion. The pH changes considered are to be brought by carbon dioxide. In the presence of light, the carbon dioxide is consumed during photosynthesis resulting in increase in pH and in darkness carbon dioxide and other organic acids accumulate causing decrease in pH. These observations led to conclusion that pH changes favor starch-sugar interconversion.
Presence of Glucose 1-Phosphate and Enzyme Phosphorylase
The evidence of presence of glucose 1-phosphate and enzyme phosphorylase in guard cells led to the belief that starch-sugar interconversion is brought by phosphorylase since it is more sensitive to p11 changes. At about pH 7 the starch is converted to glucose 1-phosphate and about p11 5 starch is synthesized. The glucose 1-phosphate is converted into glucose 6-phospahte and then split into glucose and phosphate. The glucose dissolves and increases the osmotic potential of the guard cell. In dark, reverse occurs and resynthesis of glucose 1-phosphate resulting in decrease of osmotic potential of guard cells and closure of stomata photophosphorylation in guard cells provide ATP for phosphorylation.
Active K+ Ion Transport and Hormonal Regulation Theory
The accumulation of K+ ions is favored by light and carbon dioxide free air, and the ions move out of guard cells when leaves are shifted to dark. The K+ ion concentration increases many times when the stomata are open (400 – 800 mM) and decreases when closed (100 mM). Similarly, changes in starch concentration were observed with increase in K+ ion concentration. The starch contents were found to decrease in the presence of light when the stomata are open and increase in dark when the stomata are closed. It is considered that light stimulates guard cells to accumulate potassium and becomes turgid. This response is triggered by the illumination of a blue-light receptor in a guard cell, perhaps built into the plasma membrane. Activation of these blue-light receptors stimulates the activation of ATP powered proton pumps in the plasma membrane of the guard cells, which in turn, promotes the uptake of K+.
In the presence of light, the pH increases due to carbon dioxide which is used in photosynthesis and the starch is converted to a 3-C compound, phosphenolpyruvic acid (PEP). The PEP combines with carbon dioxide to produce oxaloacetate 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 combine to form potassium malate that increases the solute potential and lowers the water potential of guard cells. The water moves into the guard cells from the adjacent cells to make them turgid and the stomata open. The K+ ions move out of guard cells to balance the 10- ions entering the cell. In dark, the carbon dioxide produced during respiration accumulate and pH falls resulting in conversion of malic to starch. The K+ ions and water moves out of the guard cells lowering the osmotic potential of the cells. Loss of turgor results in the closure of stomata. Also, ABA (abscisic acid) is produced under water stress conditions and plays a role in the closure of stomata.