The transpiration stream of water literally "pulls" water out of the soil as water moves from the xylem elements into the tree canopy— driven by the energy available to evaporate water. As water flows, it encounters resistances from the soil, moving through the root cortex and membranes, passing along and through the xylem vessels, changing phases to a gas in the stomatal cavity, and passing through the stomata into the air. The flow of water from the soil to the root surface is generally not limiting until either the soil dries and shrinks, causing gaps between the root system and the soil, or the water films around soil particles become so thin that hydraulic conductivity to the root is reduced. Water movement through the external root cortex is not limiting, but when water moves through the endodermal cell membrane to pass through the Casparian strip and then moves through a second membrane to transfer into the xylem, there is considerable resistance due to the cell membrane. Water flow in the xy-lem is generally not limiting unless extensive cavitation or blockage has occurred due to water stress or biotic stress such as disease or insect damage. The stomatal cavity and opening are the final resistance in the flow of water, and the plant can control the size of the stomatal opening and, hence, the rate of water vapor transport from the leaf. The size of the stomatal opening, or stomatal conductance, is related to environmental and biotic factors. Stomatal aperture generally reaches a maximum at 25 to 50 percent of full sunlight, and the tree will maintain high conductance unless other factors cause stomatal closure. Stomates tend to open as the leaf temperature rises and close as the relative humidity decreases. Water stress will cause stomates to close. Water stress develops when the energy to evaporate water exceeds the transport of water to be evaporated. The resulting deficit of water results in tension developing in the xylem column of water. Stomatal closure reduces the transport of water from the leaf and allows time for water transport from the soil to reduce the tension. When the tension exceeds a threshold, the column of water begins to break within the xylem vessels. The cavitation of the xylem vessels further reduces the transport rate and supply of water to the canopy and results in plant wilting, stomatal closure, and the reduction of photosynthesis.
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