Root pressure

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Root pressure is the osmotic pressure inside the root xylem that pushes water up the stem toward the leaves. It mainly happens when soil is moist and transpiration is low, such as at night or on cloudy days. When transpiration is high, the xylem sap is pulled upward rather than pushed, so it’s under tension rather than pressure.

Root pressure results from the active uptake of mineral ions into the root xylem. These ions lower the water potential, so water moves from the soil into the root by osmosis. The water accumulates in the xylem and creates pressure that can push water up the plant. You can observe it by cutting a shoot near the soil; xylem sap will exude from the cut stem for hours, and you can measure the pressure with a gauge.

But root pressure alone cannot explain how water reaches the tops of the tallest trees. The maximum pressure observed is about 0.6 megapascals, which could lift water only a few meters. For tall trees, transpiration-driven pull is the main force moving water upward.

An important part of the root’s anatomy is the endodermis, a single cell layer between the cortex and the pericycle. The endodermis has the Casparian strip, a waxy barrier that stops ions from moving passively through cell walls. Water and ions travel through cell walls via the apoplast pathway, but ions must be actively transported across endodermal membranes to enter or leave the endodermis. Once inside the endodermis, ions move through the living cytoplasm (the symplast) and can then enter the xylem. In the xylem itself, water moves upward through vessels and tracheids that lack membranes, so the Casparian strip prevents unwanted diffusion of ions back out.

Root pressure can push water and minerals up the xylem in relatively short plants when transpiration is low, and it can help refill xylem vessels after winter. But not all species rely on it; some refill without root pressure. Root pressure is often high in deciduous trees just before they leaf out, when leaves are absent and solutes are mobilized to lower the xylem water potential. For maples, early spring sap flow (maple sugar) is related to changes in stem pressure, and not solely to root pressure. It is common in many grasses to have some root pressure, and in bamboos, the level of root pressure is linked to the maximum height of a clone.