AP BIOLOGY

Detailed explanation-1: -Force developed in cortex of root which pushes water into xylem of root is root pressure. As various minerals ions from the soil are actively transported into the vascular tissue of the roots, water follows and increase the pressure inside the xylem. This positive pressure is called root pressure.

Detailed explanation-2: -The effect of root pressure in the transport of water is more important at night as: The stomata remain closed during the night time. In all higher plants, the movement of water chiefly occurs due to root pressure and transpiration pull.

Detailed explanation-3: -As ions accumulate in the root xylem, the osmotic potential of the xylem solution falls causing the passive uptake of water from the soil by osmosis into the xylem. As pressure builds up within the xylem due to osmotic water uptake, the xylem solution is forced upward to the leaves by mass flow.

Detailed explanation-4: -The active spread of mineral nutrient ions to root xylem is responsible for root pressure. They collect in the root xylem and decrease the water potential without transpiration to move the ions up the stem. Then, because of osmosis, waters spread from soil to root xylem.