According to Stewart's starch hydrolysis theory, which one of the following is the principal reason for the opening of stomata during the daytime?

Question

Conversion of sugar into starch in guard cells.

Influx of $K^+$ ions into guard cells under the influence of the ABA hormone.

Photosynthetic utilization of $CO_2$ in guard cells.

Efflux of $K^+$ ions from guard cells under the influence of the ABA hormone.

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