Short Notes on the Glucose Interconversion Theory


With the discovery of presence of phosphorylase enzyme in guard cells by Yin and Tung (1948), Steward (1964) proposed the theory of starch-glucose interconversion. According to then the following sequence of reactions occur at high PH. in day time.

Formation of gluscoe in the guard cells increases the turgor of guard cells. Thus stomata get opened in day time.

In night or dark condition the reverse reacting occurs and glucose is first converted to glucose – 1- PO, with the help of ATP.


Steward’s scheme of metabolic reactions in guard cells involved in opening and closing of stomata:

However, there are some objections to this theory for which it is not universally accepted. The objections are as follows:

(a) Sugar has never been observed to occur in the guard cells when starch disappears at the time of opening of stomata. On the other hand starch gets converted to organic acids.

(b) In monocotyledons guard cells do not synthesise starch. closing of stomata


(c) Stomatal closure occurs even at midday although there is light and there is no change in starch contents.

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