According to the proton transport theory of stomatal mechanism,...

According to the active potassium ion transport theory (proton transport theory) proposed by Levitt, stomatal opening during the daytime is driven by the accumulation of potassium ions in the guard cells.

During the daytime, starch in the guard cells is converted into malic acid. Malic acid then dissociates into malate anions and protons (H^+$).<br><br>These protons are actively pumped out of the guard cells into the surrounding subsidiary cells, and in exchange, potassium ions (K^+$) are transported into the guard cells. The potassium ions combine with malate to form potassium malate, which increases the osmotic concentration of the guard cells, leading to endosmosis and stomatal opening.

Thus, protons are formed in the guard cells when malic acid dissociates to form malate during the daytime.

Answer: Malic acid dissociates to form malate during day time