Tag: photosynthesis in higher plants

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.