Plants contain a continuous water column from the roots, where water is absorbed, to the leaves, where water is lost through evaporation via the stomata. When a plant’s cells require water, opening stomata makes water potential in the xylem strongly negative and water is pulled from the soil into the roots and into the xylem strongly and quickly. However, the water potential gradient can be too steep, causing cavitation (bubbles) in the xylem, which slows down water transport. The optimum transpiration rate occurs when water potential and cavitation are balanced in the right way. According to research published recently in New Phytologist, plants are able to maintain a transpiration rate very close to the maximum theoretical transpiration potential, allowing partial cavitation but not letting it limit hydraulic conductivity.
Here, Manzoni et al. from Amilcare Porporato’s group at Duke University, compared the theoretical optimum transpiration rate with actual transpiration ates in a number of tree species (grouped into boreal, temperate, Mediterranean, tropical dry, and tropical moist species). Their parameters for calculating the theoretical optimum were extensive, including soil water potential, xylem hydraulic conductivity, and canopy height.
The actual maximum transpiration rate of these species was then collected from published papers, and sorted according to climate and the conditions under which the analysis was done. Only the data from well-irrigated systems was used. The average observed peak transpiration rate was close to the theoretical maximum transpiration rate, and both were fairly conserved among plant types of a similar size in a particular climate. (more…)
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