4.5.1.2.1: Adaptations to Reduce Transpiration

Learning Objective

Describe the various adaptations that help plants reduce transpiration rate.

Plants have evolved over time to adapt to their local environment and reduce transpiration. Leaves are covered by a waxy cuticle on the outer surface that prevents the loss of water. Plants that grow in dry environments and plants that grow on other plants (epiphytes) have a much thicker waxy cuticle than those growing in more moderate, well-watered environments (Figure \(\PageIndex{1}\)). Additionally, they often have a thick covering of trichomes or of stomata that are sunken below the leaf’s surface (Figure \(\PageIndex{2}\)). These adaptations impede air flow across the stomatal pore and reduce transpiration. Multiple epidermal layers are also commonly found in these types of plants .

Figure \(\PageIndex{1}\): Plants are suited to their local environment. Plants adapted to dry conditions such as the prickly pear cactus (Opuntia sp., left) and epiphytes like this tropical Aeschynanthus perrottetii (right) have adapted to very limited water resources. The leaves of a prickly pear are modified into spines, which lowers the surface area-to-volume ratio and reduces water loss. Photosynthesis takes place in the stem, which also stores water. A. perottetii leaves have a waxy cuticle that prevents water loss. The left image is a modification of work by Jon Sullivan, and the right image is a modification of work by L. Shyamal/Wikimedia Commons.

Figure \(\PageIndex{2}\): Cross section of an oleander (Nerium sp.) leaf. Oleander is adapted to dry conditions. Stomata are sunken into stomatal crypts and bordered by trichomes, reducing transpiration. Magnified at 400x. Image modified from Berkshire Community College Bioscience Image Library (public domain).

The size and shape of photosynthetic structures also influences transpiration rate. Succulent plants, common in deserts, have thick, fleshy leaves or stems (Figure \(\PageIndex{1}\), left). Other plants, such as the evergreen shrubs of the chaparral, have small, thick, tough leaves (Figure \(\PageIndex{3}\)). Compared to thin, broad leaves, these shapes reduce surface area-to-volume ratio and decreases the opportunity for water loss. Plants with thin, broad leaves that live in climates with hot, dry seasons (such as chaparral or tropical forests that have a wet and dry season) may be deciduous, losing their leaves during these seasons to limit transpiration (Figure \(\PageIndex{4}\)).

Figure \(\PageIndex{3}\): (left) The thick, tough leaves of chamise (Adenostoma fasciculatum) limit transpiration during dry, hot summers. Image by Tom Hilton (CC-BY). (right) Sal tree (Shorea robusta) is a tropical deciduous tree, losing its leaves during the dry season. Image by J.M.Garg (CC-BY-SA).

As discussed in 13.7: Photorespiration and Photosynthetic Pathways, CAM plants close their stomata during the day when light and high temperatures would otherwise increase transpiration rate. C₄ plants reduce the need to frequently open stomata by creating a high carbon dioxide concentration in the bundle sheath cells, which conduct the Calvin cycle. Regardless of photosynthetic pathway, plants can open and close stomata to regulate transpiration rate based on environmental conditions.

Attribution

• Curated and authored by Melissa Ha using 30.5 Transport of Water and Solutes in Plants from Biology 2e by OpenStax (licensed CC-BY). Access for free at openstax.org.