Q.What role does root pressure play in water movements in plants?

Root pressure, generated by active ion uptake in roots, pushes water a short way up the xylem and causes guttation, but by itself cannot explain water transport in tall trees — that job belongs mainly to transpiration pull.

Water and minerals enter root cells and eventually reach the xylem. As living root cells actively (using metabolic energy) accumulate mineral ions from the soil solution into the xylem, the water potential inside the root xylem becomes lower (more negative) than that of the surrounding soil water. Water therefore moves into the xylem by osmosis, continuously, building up a positive pressure inside the root xylem — this is root pressure.

Role in water movement:

1. It pushes the column of water and dissolved solutes upward into the stem, but this force is generally sufficient to move water only a short distance up the plant.
2. It is directly demonstrated by guttation — the appearance of drops of water (containing some dissolved substances) at the tips or edges of leaves in the early morning, especially in small herbaceous plants, when root pressure forces xylem sap out through special pores (hydathodes) because transpiration is minimal (high humidity, closed stomata).
3. Root pressure is strongest at night and early morning (when transpiration is low) and becomes negligible during the day when transpiration is high. …