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Transpiration as Flux

Synthesis from Sources

Attributions to Come

 


Introduction [1]

Transpiration is the process of water movement through a plant and its evaporation from aerial parts, such as from leaves but also from stems andflowers. Leaf surfaces are dotted with pores which are called stomata, and in most plants they are more numerous on the undersides of the foliage. The stomata are bordered by guard cells and their stomatal accessory cells (together known as stomatal complex) that open and close the pore.[1]Transpiration occurs through the stomatal apertures, and can be thought of as a necessary "cost" associated with the opening of the stomata to allow the diffusion of carbon dioxide gas from the air for photosynthesis. Transpiration also cools plants, changes osmotic pressure of cells, and enables mass flow of mineral nutrients and water from roots to shoots.

Mass flow of liquid water from the roots to the leaves is driven in part by capillary action, but primarily driven by water potential differences. In taller plants and trees, the force of gravity can only be overcome by the decrease in hydrostatic (water) pressure in the upper parts of the plants due to the diffusion of water out of stomata into the atmosphere. Water is absorbed at the roots by osmosis, and any dissolved mineral nutrients travel with it through the xylem.

Regulation

Plants regulate the rate of transpiration by the degree of stomatal opening. The rate of transpiration is also influenced by the evaporative demand of the atmosphere surrounding the leaf such as humidity, temperature, wind and incident sunlight. Soil water supply and soil temperature can influence stomatal opening, and thus transpiration rate. The amount of water lost by a plant also depends on its size and the amount of water absorbed at the roots. Transpiration accounts for most of the water loss by a plant, but some direct evaporation also takes place through the cuticle[citation needed] of the leaves and young stems. Transpiration serves to evaporatively cool plants as the escaping water vapor carries away heat energy.

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Overview of transpiration

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  1. Water is passively transported into the roots and then into the xylem.
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  1. The forces of cohesion and adhesion cause the water molecules to form a column in the xylem.
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  1. Water moves from the xylem into the mesophyll cells, evaporates from their surfaces and leaves the plant by diffusion through the stomata.

 

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Stoma in a tomato leaf shown via colorized scanning electron microscope