The stomata are tiny pores present in the epidermal surface of leaves, young stems and in certain fruits (banana, cucumber, etc.). They are also found in almost all the young aerial parts of the plant body. The different types of stomata are anomocytic type, anisocytic type, paracytic type, diacytic type and actinocytic type. Stomata plays a very important role in transpiration, conservation of water, gaseous exchange, absorption of water, ascent of gap and absorption of mineral salts by regulating the transpiration.
Stomata usually consist of two guard cells, which can be kidney shaped or dumb cell shape, surrounding a pore. Stomatal pore is generally elliptical in surface view and allow H2O, CO2 and O2 molecules to pass through it. The guard cells are usually much smaller in size as compared to other epidermal cells and generally have thick walls towards pore and thin walls on opposite side. The guard cell walls have special elastic properties. The adjoining cell walls of two guard cells around pore are free and are not attached with each other, which help them to stretch laterally during stomatal opening. Plasmodesmata is absent between the guard cells and accessory cells. A variety of factors affect the opening and closing of stomata by altering the size of stomatal pore such as light and dark, CO2 concentration, water supply, pH of the cell sap etc. Opening of the stomata takes place when the guard cells become turgid and due to this their thin walls get extended and thick walls become slightly concave. On the other hand, on losing water the guard cell become flaccid and their thick walls revert back to their original position thus closing the stomatal pore.
Factors affecting the stomatal movement
Carbon dioxide concentration Higher concentration of CO2 in leaves causes closing of stomata and low concentration of CO2 in intercellular spaces of leaves cause opening of stomata.
Light: The maximum light intensity required for opening of stomata is about 1/1000 to 1/30 of full sunlight. On exposure to light stomata opens and in the dark closing of stomata takes place except in succulent plants. Blue and red lights are most effective wavelength of light responsible for opening of stomata.
Temperature: At temperature 0oC or below 0oC stomata remain close. Rise in temperature upto 30oC causes stomatal opening but temperature above 30oC causes closing of stomata.
Mechanism of Stomatal Movement
Following are the hypothesis explaining mechanism of stomatal movement.
Starch – Sugar Hypothesis The starch sugar hypothesis was formulated in 1923 by J.D. Sayre and extensively studied by Scarth 1932. According to this hypothesis, the mechanism of stomatal movement in light and dark is as follows:
In light: In bright light, the process of photosynthesis occurs in green leaves and this process consumes carbon dioxide from the surrounding medium. This leads to decrease in CO2 concentration in substomatal cavity and an increase in the pH of guard cells. This increased pH of guard cell favours the activity of enzyme phosphorylase, which hydrolyses the starch into glucose-1-phosphate. A glucose-1-phosphate molecule dissolves in the cell sap of guard cells and results in increase in the concentration of cell sap and osmotic potential of cell. This increase in osmotic pressure results an increase in its diffusion pressure deficit in comparison to surrounding cells. This leads to movement of water from surrounding cells to guard cell and guard cells become turgid and opening of stomata takes place.
In dark: As in dark the process of photosynthesis does not occur in plants. The carbon dioxide accumulates in the intercellular spaces and the level of carbon dioxide increases in the substomatal cavity which results an increase in the acidity of guard cells by increasing hydrogen ion concentration. Due to increase in the H+ concentration the pH of cell sap is decreased. This low pH favours the conversion of glucose-1-phosphate back to starch. The osmotic pressure of the cell sap decreases which results in the decrease of its diffusion pressure deficit gradient. The guard cells become flaccid and closing of stomatal pore takes place.
Steward’s hypothesis: This hypothesis was given by Steward in 1964. This hypothesis is explained below
In light: Due to photosynthesis, there is consumption of respiratory carbon dioxide present in inter-cellular spaces in light. This lowers the hydrogen ion concentration of cell sap and pH of guard cells is increased. This high pH favours the activity of enzyme phosphorylase, which converts starch into glucose-1-phosphate, which is further converted to glucose-6-phosphate by enzyme phospho-glucomutase. The enzyme phosphatase converts glucose-6-phosphate to glucose and phosphate. The glucose and phosphate increases the concentration of cell sap, osmotic pressure of guard cells and diffusion pressure deficit. This results in the movement of water into the guard cells from surrounding cells. The guard cells become turgid and swell and opening of stomatal pore takes place.
In dark: As photosynthesis does not occur, the level of respiratory CO2 in substomatal cavity is increased. This results in the decrease in the pH of guard cells. The glucose molecules are then converted to glucose-1-phosphate making use of respiratory ATP in presence of enzyme hexokinase. The glucose-1-phosphate molecules are converted to starch in presence of enzyme phosphorylase. The osmotic pressure of cell sap decreases due to synthesis of starch. This also leads to decrease in diffusion pressure deficit in plant cells. The guard cells lose water to the surrounding cells and become flaccid and closing of stomata takes place
Potassium ion transport and hormonal regulation theory: This theory was proposed by Levitt in 1974 and extensively studied by Raschke 1975 and Bowling 1976. A brief of this theory is given below
In light: Exposure of blue light leads to breaking of stored starch in plant into three carbon compound- phosphoenol pyruvic acid in the guard cells. Phosphoenol pyruvic acid so formed then combines with carbon dioxide to produce a four-carbon compound- oxaloacetic acid with the help of enzyme phosphoenol pyruvate carboxylase (PEP carboxylase), which becomes active at high pH. Oxaloacetic acid is converted to malic acid, which further dissociates into malate anion and hydrogen ion in the guard cells. Ion exchange takes place after this in which protons of guard cells are transported to surrounding epidermal cells; potassium ions are taken into the guard cells down an electrical gradient. This process is an active process and requires ATP. Increased concentration of potassium ions and malate ions in the guard cells causes sufficient decrease in the osmotic potential that causes reduction in water potential. This leads to entry of water from surrounding cells into guard‟s cells and opening of stomata takes place.
In dark: Lowering of pH takes place as carbon dioxide concentration is increased in the sub-stomatal cavity. Low pH and water stress in leaves results in the activation of an inhibitor hormone abscisic acid. Abscisic acid (ABA) acts on the guard cell membrane and opens calcium ion channels. It results in the entry of calcium ions into the guard cells and stoppage of ATP-dependent proton pump takes place. It is hypothesized that loss of water from the guard cells reduces turgor, which results in the closure of stomata.