Leaf stomata open when special cells on either side of the pore become full of water. One reason for poor plant growth during a water shortage is that the stomata do not open as often.
Stomata are small openings in the epidermis of plants, especially on the leaves, that open to allow for gas and water exchange. Surrounding the stomata are guard cells; guard cells open and close stomata through the uptake of water, ions and sugars. The uptake of ions and sugars gives guard cells a higher concentration of solutes than the surrounding cells, which allows water to enter the guard cells via osmosis. As guard cells fill with water and the pressure inside them increases, they bend and cause stomata to open. The stomata close again when water leaves the guard cells, causing cellular pressure to decrease.
The hormone abscisic acid is important in reducing cellular growth in plants. In most plants, it allows for dormancy in seeds and buds, preventing damage in these structures when growing conditions are not ideal. When abscisic acid enters guard cells, they shrink and close stomata. This function of abscisic acid is prevalent in drought-tolerant plants as a means of preventing excessive water loss.