E6 Leaf - Atmosphere Interface

Topic

The leaf-atmosphere interface is the site of exchanges of carbon dioxide, water vapor, and heat between the leaf and the surrounding air; each of these three exchanges is regulated—albeit in different ways—by two elements of this interface: the stomata and the boundary layer of still air overlying the leaf.

Stomata simultaneously regulate the uptake of carbon dioxide (needed for photosynthesis) and the loss of water vapor (which constitutes transpiration): when stomata open wider, carbon dioxide uptake and photosynthesis increase, but water vapor loss and transpiration also rise; conversely, when stomata close, both carbon dioxide uptake and water vapor loss decrease. This simultaneous dual regulation—whereby a leaf cannot increase photosynthesis without also increasing transpiration—represents the central trade-off in plant ecophysiology.

In addition to the stomata, the leaf-atmosphere interface includes a boundary layer of still air that forms directly over the leaf surface; this layer adds resistance to the exchange of carbon dioxide, water vapor, and heat beyond the resistance already imposed by the stomata themselves. The thicker this boundary layer, the greater the additional resistance to exchange, making it more difficult for the leaf to exchange carbon dioxide, water vapor, and heat with the surrounding air, even when the stomata are fully open.

The thickness of this boundary layer depends on leaf size and wind speed: the larger the leaf, the thicker the boundary layer of still air that tends to form on its surface, because the air travels a greater distance before being renewed; And the higher the wind speed, the thinner the boundary layer of stagnant air becomes, because the wind more rapidly replenishes the air in direct contact with the leaf.

Ultimately, the leaf-atmosphere interface regulates the exchange of carbon dioxide, water vapor, and heat between the leaf and the air through two interacting resistances: the resistance imposed by the stomata—governed by the fundamental trade-off between photosynthesis and transpiration—and the additional resistance to exchange imposed by the boundary layer of stagnant air, which is determined by leaf size and wind speed. Together, these two resistances determine the actual amounts of carbon dioxide, water vapor, and heat passing through the leaf-atmosphere interface at any given moment.

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