Importance
Root pressure can transport water and dissolved mineral nutrients from roots through the xylem to the tops of relatively short plants when transpiration is low or zero. The maximum root pressure measured is about 0.6 megapascals but some species never generate any root pressure. The main contributor to the movement of water and mineral nutrients upward in vascular plants is considered to be the transpirational pull. However, sunflower plants grown in 100% relative humidity grew normally and accumulated the same amount of mineral nutrients as plants in normal humidity, which had a transpiration rate 10 to 15 times the plants in 100% humidity. Thus, transpiration may not be as important in upward mineral nutrient transport in relatively short plants as often assumed.
Xylem vessels sometimes empty over winter. Root pressure may be important in refilling the xylem vessels. However, in some species vessels refill without root pressure.
Root pressure is often high in some deciduous trees before they leaf out. Transpiration is minimal without leaves, and organic solutes are being mobilized so decrease the xylem water potential. Sugar maple accumulates high concentrations of sugars in its xylem early in the spring, which is the source of maple sugar. Some trees "bleed" xylem sap profusely when their stems are pruned in late winter or early spring, e.g. maple and elm. Such bleeding is similar to root pressure only sugars, rather than ions, may lower the xylem water potential. In the unique case of maple trees, sap bleeding is caused by changes in stem pressure and not root pressure.
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