Gas exchange between a leaf and the atmosphere occurs through nonselective, adjustable stomata in the epidermis. A stoma is defined by the surrounding pair of guard cells, which are kidney-shaped epidermal cells that face each other. Certain conditions stimulate the guard cells to accumulate potassium salts.
Stomal closure occurs when the guard-cell pair loses solutes.
Guard cells have evolved special attributes to fulfill their critical role. They are small and have enhanced capacity for ion flux. Their chloroplasts are specialized for starch storage and mobilization. Almost uniquely, they lack protoplasmic connections with adjacent cells, and their vacuole is a dynamic structure. The implications of guard-cell function to the utilization of finite resources such as water and soil and to agricultural productivity justify our intense efforts to understand them better.