Increased humidity in the air surrounding the leaf decreases the water potential gradient between the saturated air in the leaf air spaces and the air surrounding the leaf, resulting in a decreased rate of leaf water loss. However, when the humidity of the air surrounding the leaf is very low, the water potential of the air is low. Therefore, the water potential gradient between the air spaces of the leaf and the surrounding air is high, and the rate of leaf water loss increases. Conditions that cause a decreased rate of water loss from leaves result in a decreased water potential gradient from stem to leaf and therefore in a decreased rate of water movement up the stem to the leaves. Conditions that cause an increased rate of water loss from leaves result in an increase in the water potential gradient from stem to leaf and therefore in an increase in the rate of water movement up the stem to the leaves.
Closed stomata prevent excessive water loss.
Lack of water in the air spaces of the leaf is a limiting factor in the light dependent reactions of photosynthesis. Closed stomata prevent CO 2 from entering and O 2 limits the light independent reactions of photosynthesis. A decrease in the CO 2 coupled with an increase in O 2 in the air spaces of the leaf can result in the cells converting to photorespiration. Adaptations to reduce leaf water loss include a reduced number of stomates, an increase in the thickness of the leaf cuticle, a decrease in leaf surface area, and adaptations that decrease air movements around stomates, such as dense hairs and sunken stomates. Because leaves are all different in size, reporting the water loss without considering a unit area would provide non-comparable data.