Potassium is the element responsible for the opening and closing of stomata.

Explanation:
The opening and closing of stomata, the tiny pores on the surface of plant leaves, is primarily regulated by the movement of potassium ions (K+) in the guard cells. The process is governed by the potassium pump theory.

Basically, the active transport of potassium ions into and out of the guard cells leads to changes in the osmotic pressure and turgor pressure within the cells. This change results in the opening and closing of the stomata.