Since the late 1960s, researchers have observed that starch in the chloroplasts of the guard cells breaks down during the day and accumulates in the dark. Based on this, carbohydrates have historically been regarded as the primary osmotica modulating stomatal opening. However, the discovery of an important role for potassium uptake has led to the replacement of that starch-sugar hypothesis. Current research now focuses mainly on how $K^+$ is transported in and out of cells when the stomata open or close. However, questions remain concerning photoreceptors, and the functioning of guard cell chloroplasts is still disputed. Coincidentally, some recent study results have again suggested that sucrose may playa major role in guard cell osmoregulation, thus supporting the original theory of starch-sugar involvement.