What primarily causes root pressure in plants?

Root pressure in plants is primarily caused by the active transport of salts into the xylem. When salts, such as potassium and sodium ions, are actively transported from the soil into the root cells, it leads to a higher concentration of solutes within the xylem. This increase in solute concentration reduces the water potential inside the xylem vessels, causing water to flow into the xylem from the surrounding root cells and ultimately from the soil through osmosis.

As water moves into the xylem, it creates positive pressure within the xylem vessels, which can push water and nutrients upward through the plant. This process not only supports the transport of water but also plays a significant role in keeping the plant hydrated and maintaining turgor pressure in cells.

In contrast, decreased temperature in the soil can affect plant processes, but it does not directly lead to root pressure. Osmosis of water out of the roots would result in a decrease in pressure, not an increase. Photosynthesis in the leaves primarily impacts water movement through transpiration, leading to negative pressure in the xylem rather than root pressure. Thus, the active transport of salts is the primary mechanism that builds root pressure in plants.