Lloyd (1908) discovered that the amount of starch in guard cells increases during night and decreases during day time. The reverse is true for sugar content of the guard cells. According to him the starch sugar interconversion is responsible for the opening and closing of the stomata. He believed that the change in the OP causes the interconversion of starch – sugar. Lottfield (1921) also discovered the same phenomenon in the guard cells, and he also reported that for the surrounding epidermal cells, the reverse (i.e. increased sugar and decreased starch during night) is true.

Sayre (1926) observed that stomata remain open when the medium is either alkaline or neutral even in dark and are closed in acidic pH even during day time. This means to say that the pH of the medium decides the interconversion of starch and sugar. According to Sayre (1926) utilization of CO, during photosynthesis causes the increase in pH resulting in the conversion of starch to sugar. Scarth (1932) Small et al (1942) supported the idea of Sayre.

Scarth (1932) proposed that during day time CO2 liberated by respiration is utilized by mesophyll cells in photosynthesis, which results in lowering of H ion concentration. Whenever the pH is high the enzymatic conversion of starch to sugar is favoured. In darkness, CO2 accumulates in the intercellular spaces and increases the H ion concentration and decreases the pH promoting con­version of sugar into starch.

The discovery of the enzyme phosphorylase in guard cells by Hanes (1940) greatly supported the starch sugar interconversion theory. Subsequently Yung and Tun (1948), Yung and Willist (1954) have confirmed the participation A & B, conjoint, collateral and open C & D, conjoint, Collateral and closed; E and F, conjoint, bicollateral and open; G and H, concentric and amphicribral; I and J, concentric and amphivasal; of phosphorylase enzyme in the interconversion of starch and sugar in the guard cells. This can be represented as follows.

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Steward (1964) proposed a scheme which is slightly modified. According to him Glucose 1. phosphate should have its phosphate removed to be ulti­mately able to increase the concentration of the cell sap. The process re­quires the participation of biological energy.