Root pressure is maximum when

Root pressure is maximum when Transpiration is low and absorption high.
The transpiration process in plants creates a suction pressure that pulls up water from the xylem of the roots to the stem and then to the leaves.
Root pressure is the transverse osmotic pressure within the cells of a root system that causes sap to rise through a plant stem to the leaves.
When root pressure is maximum, the rate of absorption is higher whereas the loss of water through transpiration is less.