蒸散

Transpiration is the evaporation of water from the aerial parts of a plant, primarily through pores called stomata on leaves. Plants must open their stomata to absorb carbon dioxide for photosynthesis and release oxygen, but this inevitably causes water loss. Although it seems inefficient, transpiration is unavoidable because it maintains the transportation of water and nutrients throughout the plant while enabling gas exchange with the atmosphere.

Water enters plant roots by osmosis and travels upward through a vascular tissue called xylem in a channel known as the transpiration stream. When water evaporates from stomata, water molecules below are pulled upward to replace it. This upward movement occurs because water molecules stick to each other and to xylem walls through cohesion and adhesion, creating continuous flow from roots to leaves.

Guard cells are specialized cells that border each stomatal pore and control whether the pore opens or closes. When guard cells expand, they open the pore to allow carbon dioxide uptake and oxygen release. When they contract, they close the pore to reduce water loss. This regulation allows plants to balance the need for gas exchange with the need to conserve water.

Desert plants have evolved multiple adaptations to minimize transpiration. They typically have small leaf surface areas, reducing the area available for water evaporation. Desert plants also have fewer stomata per unit area on their leaves compared to rainforest plants, limiting water loss. In contrast, rainforest plants develop large leaves with high stomatal density to maximize photosynthesis and support rapid growth in water-rich environments.

Water potential is the measure of the free energy of water, and water molecules move from areas of higher water potential to areas of lower water potential. When evaporation occurs in leaves, it creates areas with lower water potential, causing water from roots and stems to be driven toward the leaves. This water potential gradient, combined with cohesion and adhesion properties of water molecules, enables continuous upward water transport throughout the plant.

Temperature is the primary environmental factor driving transpiration rates; higher temperatures increase water loss through faster evaporation. Water availability, wind, and sunlight also influence transpiration. Plants in hot, arid environments face selective pressure to conserve water, while plants in tropical rainforests experience different pressures favoring rapid water transport to outcompete neighbors or grow taller to avoid herbivores.

Scientists can measure transpiration rates indirectly using a potometer, a device that measures the rate of water uptake in a leafy plant. The assumption is that transpiration causes water uptake, which can be quantified. Additionally, scientists can determine relative transpiration rates by observing leaf structures, such as the size and number of stomata per unit area, which reflect a plant's adaptation to its environment.