Which process is responsible for opening …

Stomata are small pores or opening present in the epidermal cells of leaves. They open during the day and close during night. They take carbon dioxide required during photosynthesis during the day. Also, they release excess water released in the process of respiration during the night along with carbon dioxide. This opening and closing of stomata is controlled by concentration of solutes in the guard cell.

During daytime, the pH of the guard cells is alkaline. This results in the hydrolysis of water. The solutes from neighbouring epidermal and mesophyll cells enter the guard cells and lowers the osmotic potential and water potential of guard cells. This lowered water potential and osmotic potential allow the movement of water into guard cells from neighbouring cells. Due to this, guard cells become turgid. This results in the opening of the guard cells. During night time, carbon dioxide accumulates in the guard cells. Hence, it becomes acidic. The acidic pH favors the formation of starch in the guard cells. Hence, the solute concentration decreases. This makes the water from the guard cells to move away into neighbouring cells. The guard cells then become flaccid with no water. They collapse against each other and result in the closing of stomata.