Which of the following sets of conditions favour the opening of stomata?

The opening of stomata is an active process that depends on the turgidity of the guard cells. According to the active potassium ion transport theory, during the day, starch is converted into malic acid, which dissociates into malate and $\text{H}^+$ ions.

The $\text{H}^+$ ions are actively pumped out of the guard cells, and $\text{K}^+$ ions are taken in from the surrounding subsidiary cells. $\text{Cl}^-$ ions are also taken up to maintain electrical neutrality. The $\text{K}^+$ ions combine with malate to form potassium malate.

The accumulation of potassium malate and the conversion of starch to sugar increase the osmotic concentration inside the guard cells. This causes water to move into the guard cells from the surrounding cells (endosmosis). As a result, the guard cells become turgid, their outer thin walls bulge outward, and the inner thick walls are pulled apart, leading to the opening of the stomatal pore.

Answer: Conversion of startch to sugar in guard cells, Formation of Potassium malate in the guard cells, Endosmosis in the guard cells, Uptake of $\text{K}^+$ and $\text{Cl}^-$ ions by guard cells.