Unit 2 GCE Biology: Plant Question

By negative pressure im guessing you mean hydrostatic pressure? If so ill explain:
Water is drawn up the xylem under tension in the process of transpiration pull.As water is lost by evaporation from the aerial parts of a plant it is replaced.Because of the cohesion between water molecules that form a long continuous column it causes the water to be drawn up under tension to replace this lost water.It is due to the presence of lignin lining the walls of the xylem that allows the xylem to withstand this tension.Therefore we call this the "cohesion tension theory".

I understand that hydrostatic pressure is part of a separate theory: Another method of water being transported up the xylem is root pressure.Due to the active transport of nitrate ions from the surrounding cells into the xylem this lowers the water potential of the xylem.Therefore water moves down the water potential gradient and into the xylem by osmosis.This creates a high hydrostatic pressure and therefore forces water up the xylem.

I hope ive helped