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.

Water is critical for plants to photosynthesize, metabolize, and maintain cellular structure. Hence, excessive loss of water is problematic for plants. What adaptations have evolved to allow plants to endure and resist water loss?

Most plants have a waxy cuticle that covers the leaf surface, preventing the evaporation of water. The specific composition of the cuticle influences its water 

Tiny openings on the surface, called stomata, facilitate gas exchange and transpiration. To minimize water loss, plants adjust the density and location of stomata on developing leaves in response to water and light availability. 

In most deciduous trees, for instance, stomata are located on the undersides of leaves. Additionally, the density of stomata is higher on leaves near the center of the tree and lower on leaves at its periphery.

Conservation of water is especially critical for desert-dwelling plants. The desert shrub brittlebush traps water around its leaves with tiny leaf hairs. These hairs, called trichomes, deflect the sun and diminish the drying effect of wind.

Other desert plants, such as Opuntia, store water in their fleshy stems to protect against drought. In addition, cacti have modified leaves, called spines, that reduce evaporation and dissipate heat.

Plants in arid environments can also reduce evaporation by only taking in carbon dioxide at night. During the day, the stomata remain closed. This process is called crassulacean acid metabolism, or CAM.

Specific leaf architectures may also help reduce water loss. Small or fine leaves reduce evaporation. Grasses acquired rolled or folded leaf structures that likewise reduce surface area and, therefore, evaporation.