Blackman's principle of limiting factors - class-XI

Potassium ions are involved in turgor movement of guard cells. Formation of organic acids like malic acid in guard cells promotes intake of potassium into the guard cells, which causes a decrease in water potential leading to intake of water by guard cells.

Potassium
(K) and Magnesium (Mg) are essential macronutrients for plants.

Osmosis is the process which regulates the opening and closing of stomata by guard cells. When potassium ions accumulate in the cells, the water potential drops and endosmosis occurs making the cell turgid. This causes the cells to take a shape so that the stomatal pore is opened up. When water moves out of the cells, the cells become flaccid and the pore closes.

Modern theory of stomatal movement is proposed by Levitt. According to it, entry of K+ ions leads to the opening of stomata during day time and exist of k+ results the closure of stomata during night time. Simultaneously, carbon dioxide assimilation occur during day time in the guard cells.

So, the correct option is ‘CO₂ assimilation
by guard cells only’.

Stomatal movement are influenced by many environmental factors like temperature, light humidity, water availability (moist soil), CO$ _2$ concentration, growth hormones, minerals and pH.

It is generally well accepted that stomata opens, when guard cells become turgid due to entry of water. The water enters the guard cells from the surrounding cells, when the guard cells accumulate ions, which causes a decrease in water potential. It has been established that the ion, which accumulates in guard cells is the potassium ion. When plants are exposed to light, the guard cells accumulate large amounts of $K^+$. Light appears to stimulate active movement of $K^+$ ions from surrounding cells into the vacuoles of the guard cells. In guard cells, osmotic and hence, water potentials become very negative due to the presence of KCI, potassium malate or dipotassium malate or both. A water potential gradient is established from the accessory water movement, increased turgidity and stomatal opening.

• K+ ion theory was proposed by Levitt in 1974. K ions are balanced by malate ions present in guard cells and also by taking in some Cl ions. 
• This ion exchange occurs through the expenditure of ATP energy.
• Plant hormone ABA acts on guard cells, which interfere with the exchange of K+ H+ ions in guard cells, results in a reverse of the reaction of the opening of stomata, hence stomata closed.
  
•  Hence, Active K+ ion theory is the latest explanation for the closure of stomata.
• So, the correct answer is  'Active K+ ion theory'.

There are three main theories about the mechanism of stomatal movement.

1. 'Guard Cell Photosynthesis' (proposed by Schwendener, 1881).
2. 'Starch Hydrolysis' and Starch sugar interconversion theory (proposed by Sayre, 1923 and 1926 and other contributions were made by Lloyd and Steward)
3. Malate or potassium ion pump hypothesis (proposed by Levitt, 1974). 

Starch hydrolysis theory is the classical theory for stomatal opening and closing. It was originally proposed by Sayre, 1923. According to this theory, guard cells contain starch, which is hydrolysed to form glucose under high pH caused due to reduced carbon dioxide concentration. The carbon dioxide concentration is reduced due to the process of photosynthesis Glucose increases the osmotic concentration and hence, DPD of guard cells. The later absorb water from epidermal cells, swell up and open the stomatal pore. Major contributors to this theory were Lloyd, 1908, who proposed starch sugar interconversions; Sayre, proposed that starch hydrolysis occurs with rise of pH and Steward, suggested that closing movement requires ATP.

Malate or potassium ion pump hypothesis was first proposed by Levitt, 1974. According to this theory, there is a decrease of carbon dioxide in guard cells. During light exposure there is hydrolysis of starch to form phospho enol pyruvate (PEP). PEP combines with carbon dioxide to form organic acids, like oxalic acid. The organic acids get ionised and the proton passes out of guard cells and potassium ion is actively absorbed and accumulated. 

Wilting of leaves occur in response to plasmolysis, when plant cells lose lot of water due to exosmosis. During day time, when temperature is high, there is high transpiration if plants are not able to absorb enough water the leaves will wilt. After sunset the temperature and consequently rate of transpiration drop. Hence, there is less water loss from plants and plants generally overcome the wilt.

Most widely accepted theory for opening and closing of guard cells is the malate or potassium ion pump theory, first proposed by Levitt, 1974. 

When stomata open, K+ ions are actively pumped into guard cells, and organic acids (like malic acid) are produced, which dissociate into H+ and malate ions. The removal of H+ ions from the guard cells causes the pH to increase.

CAM (Crassulacean Acid Metabolism) plants open their stomata at night to take in CO2 to minimize water loss due to transpiration in hot, dry environments.

Stomata opening is primarily regulated by light, CO2 concentration, water status, and K+ ions. Nitrogen gas (N2) does not directly affect the opening or closing mechanism of stomata.

According to the most widely accepted view, stomata open or close due to turgor movement in guard cells. When guard cells gain water they swell and become turgid and stomata open. When guard cells lose water they become flaccid and stomata close. 

Potassium is an important macronutrient for plants. Potassium ions cause turgor movement of guard cells. 

Transpiration occurs through stomata. The opening and closing of stomatal pore depends on the turgidity of guard cells. When guard cells become turgid the stomata open and when guard cells become flaccid the stomata close. 

During stomatal opening starch gets converted to organic acids, like malic acid. The dissociation of organic acid increases proton concentration. The protons move out of guard cells and to maintain electrical neutrality potassium ions move into the guard cells. The inflow of potassium ions results in a drop in water potential in guard cells, due to which water moves into guard cells making them turgid and stomata open.

According to the modern K+ malate theory, the accumulation of potassium malate in the guard cells lowers the water potential, causing water to enter and the stomata to open.

According to the most widely accepted theory of stomatal opening and closing, potassium ions are involved in turgor movement of guard cells. Inflow of potassium ions decreases water potential in guard cells, which leads to inflow of water. Consequently guard cells swell and stomatal pore opens. When potassium ions move out, these events are reversed and stomatal pore closes.

Transpiration is the process of loss of water, in vapour form, through stomata. The process depends on the turgidity of guard cell. An increase in turgidity of guard cells opens stomatal pore, whereas a decrease in turgidity of guard cells closes stomatal pore. The change in turgidity of guard cells is brought about by reversible influx and efflux of potassium ions.

Starch is osmotically inactive and does not contribute to decrease in water potential. Conversion of starch to organic acids like malic acid promotes uptake of ions like potassium. This decreases water potential and helps in uptake of water. Thus making guard cells turgid and opening of stomata.

Changes in turgidity of guard cells, regulate the opening and closing of stomata. When guard cells are turgid, stomatal pore opens and when guard cells are flaccid stomatal pore closes.

Potassium ions movement into and out of guard cells is related to the turgidity and consequent opening and closing of stomatal pore.

Guard cells have chloroplast, on exposure to light, there is synthesis of sugar in guard cells. The increase in sugar concentration causes water to move in from surrounding cells, thus making guard cells turgid. Increase in turgidity of guard cells leads to stomatal opening.

Amophila are xerophytic grasses, which show rolling of leaf as a xerophytic adaptation to conserve water. Nerium leaves show multilayered epidermis, thick layer of cuticle and sunken stomata, but do not show rolling of leaves. Melia is not a xerophyte.

The starch-sugar interconversion theory, which suggests that starch is converted to sugar (glucose) in light to increase osmotic pressure, was proposed by Steward.

Stomata open and closes due to water and ion movement. When the light falls on the parts of the plant, chloroplast carries out photosynthesis. ATP from photosynthesis powers sodium and potassium pump. Then the K$^+$ ions are pumped (influx of K$^+$) into the guard cells, by creating a concentration gradient and also water enter into the guard cell through osmosis. As a result of this, guard cells swells and stomata open. The reverse process occurs in the closing of stomata, i.e, loss of K$^+$ ions (efflux of K$^+$) by guard cells and water also leave the guard cells by the process osmosis. As a result, guard cells become flaccid and stomata close. 

Botanist Sayre observed that stomatal opening is connected to change in ph of guard cells due to carbon dioxide. 

According to Lloyd the stomatal movement is regulated by interconversion of starch and organic acids. When starch is converted into organic acid in the presence of enzymes and light, guard cells get turgidity and eventually causes the opening of stomata.

The opening and closing of stomata are controlled by many factors such as light, pH, amount of water and solute and the osmotic pressure.

(A) Hydrolysis is the breaking of the bond due to reaction with water. Starch in plants is converted into PEP through the breaking of bonds as it reacts with water.

Stomata are the structure present in the leaf which meant for exchange of gases and loss of water. Opening and closing of stomata are regulated by guard cells. Lloyd found that in daytime starch gets converted into sugar which accumulates in guard cells.

(A) Hexokinase is the enzyme catalysing the production of fructose from glucose.

Steward's theory posits that the photosynthetic utilization of CO2 in guard cells during the day increases the pH, which activates phosphorylase to convert starch into sugar, thereby increasing osmotic pressure.

Stomata are the structure present in the leaf which meant for exchange of gases and loss of water. Opening and closing of stomata are regulated by guard cells. Lloyd found that in daytime starch gets converted into sugar which accumulates in guard cells.

• Stomata are the structure present in the leaf which meant for exchange of gases and loss of water. 
• Opening and closing of stomata are regulated by guard cells. 
  
• Levitt found that in daytime starch gets converted into sugar which accumulates in guard cells.
  
• Fujino found that in the presence of light there is the influx of potassium ion in guard cells which causes the opening of stomata.
  
• High pH favours the closing of stomata.
  

The opening and closing of stomata are controlled by the accumulation of potassium ion. Potassium ion is found in the guard cell of the plant. The increase in the solute potential and a decrease in water potential is affected by potassium ion in the plant.

The influx of $K^+$ into guard cells during stomatal opening is active.

Accumulation of maleic acid in stomata causes its closure. 

When there is the rise in pH it causes hydrolysis of starch into sugar which accumulates in guard cells which results in the opening of cells. For this mechanism, cytokinins, malic acid, and cAMP are required.

Guard cells get swollen up or turgid due to absorption of water so the stomatal pore is opened up.

Starch hydrolysis theory is the classical theory for stomatal opening and closing. It was originally proposed by Sayre, 1923. According to this theory, guard cells contain starch, which is hydrolysed to form glucose under high pH caused due to reduced carbon dioxide concentration. The carbon dioxide concentration is reduced due to the process of photosynthesis. Glucose increases the osmotic concentration and hence, DPD of guard cells. The latter absorb water from epidermal cells, swell up and open the stomatal pore.

During day, light photosynthesis occurs in guard cells as they contain chloroplast. The
soluble sugars formed in this process may contribute in increasing the osmotic potential of
guard cells and hence resulting in stomatal opening. However, very small amounts of soluble
sugars (osmotically active) have been extracted from the guard cells which are insufficient to
affect water potential.
As a result of photosynthesis CO$ _2$ concentration in guard cells decreases which leads
to increased pH up of organic acids, chiefly malic acid during this period in guard cells. The
formation of malic acid would produce proton that could operate in an ATP-driven proton K+
exchange pump moving protons into the adjacent epidermal cells and K ions into guard cells
and thus may contribute in increasing the osmotic pressure of the guard cells and leading to
stomatal opening.
Reverse process would occur in darkness.

Plants have photoreceptors, which are proteins that are specially designed to perceive light and signal certain biological effects in the plant. There are two types of photoreceptor proteins in plants - Phytochromes and Blue light receptors. Phytochromes absorb red light and far red light and help the plants to sense seasonal changes in night length, or photoperiod, thereby controlling the opening and closing of stomata.

When the leaf is exposed to light, the process of photosynthesis begins. As the photosynthetic reactions proceed in the guard cells, the residual carbon dioxide is converted to carbohydrates. The disappearance of carbon dioxide from the cytosol of the guard cell results in an increase in the cellular pH. 
As the pH rises, the activity of the enzymes, that convert starch and sugars to organic acids increases. The higher concentration of organic acids results in a higher concentration of hydrogen ions. The hydrogen ions of the guard cells are then exchanged for potassium ions in the subsidiary cells.
This increased concentration of potassium, combined with the higher levels of organic acids, lowers the osmotic potential of the guard cells, and, since water moves from regions of high osmotic potential to regions of lower osmotic potential, water will move from the subsidiary cells into the guard cells. This movement of water increases the turgor pressure (inner pressure) of the guard cells and causes them to swell. Thus, the stomata open.

The stomatal opening follows when solutes from near epidermal and mesophyll cell enter the guard cell lowering its osmotic potential and water potential. Also rising in K ions in guard cell allows movement of water into the guard cell from neighbouring cells and makes the guard cell swollen due to water accumulation which results in the opening of stomata. 

Stomata opens and closes due to water and ion movement. When the light falls on the parts of plant, chloroplast carries out photosynthesis. ATP from photosynthesis powers sodium and potassium pump. Then the K+ ions are pumped into the guard cells, by creating a concentration gradient. For this uptake of K+ ions it requires protons (H+) ions then, K+ ions are exchange for the protons H+.  This is an active ionic exchange and requires ATP energy and cytokinin (a plant hormone). In this way, the concentration of K+ ions increases in guard cells and at the same time, the concentration of H+ ions decreases in guard cells.  As a result of this, guard cells swells and stomata opens.So, the major solute taken in by the guard cells is K+, which is responsible for the opening of guard cells.