34.15: Adaptations that Reduce Water Loss
Though evaporation from plant leaves drives transpiration, it also results in loss of water. Because water is critical for photosynthetic reactions and other cellular processes, evolutionary pressures on plants in different environments have driven the acquisition of adaptations that reduce water loss.
In land plants, the uppermost cell layer of a plant leaf, called the epidermis, is coated with a waxy substance called the cuticle. This hydrophobic layer is composed of the polymer cutin and other plant-derived waxes that are synthesized by epidermal cells. These substances prevent unwanted water loss and the entry of unneeded solutes. The specific composition and thickness of the cuticle vary according to plant species and environment. Other leaf adaptations can also minimize evaporation, primarily by reducing surface area. For example, some grasses have a folded structure that reduces water loss. Alternatively, other grass species undergo a rolling of the blade to protect against evaporation. Some desert-dwelling plants have leaves coated in microscopic hairs that trap water vapor, therefore reducing evaporation.
Water primarily evaporates through tiny holes in plant leaves called stomata. The stomata of some plants are located exclusively on the lower leaf surface, protecting them from excessive heat-associated evaporation. Other plants trap water vapor near stomata that are located in pits on their leaves, reducing evaporative water loss, as the guard cells that flank the stomatal opening can sense relative humidity. Some desert plants open their stomata only at night when evaporation is less likely to occur. This strategy is called Crassulacean Acid Metabolism (CAM), and plants that use it capture and fix carbon dioxide at night, and run light-dependent photosynthetic reactions during the day. Some scientists have proposed bioengineering plants to decouple carbon fixation from photosynthesis by utilizing CAM as a mitigation effort for evaporation associated with warming global temperatures.