Tag: blackman's principle of limiting factors

• 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.