Temperature and water availability are major abiotic factors for plants. Stomata need to be closed to conserve water but opened to cool the plant by the evaporation of water.The opening and closing of stomata in response to abiotic stress is controlled largely by ABA. Leaf cells release ABA when under stress and when levels of soil water fall ABA is produced in the roots. ABA is transported to the leaves and binds to plasma membrane receptors on guard cells where it changes the ionic concentrations of the cell and reduces the turgor pressure so guard cells close the stomata to preserve water.