Question

why water potential is the sum of the pressure and solute potential

Answer 1

Water potential is the overall potential of the system. Adding of solute in pure water will lowers the water potential. It is because solute particles bind with the water molecules and water potential becomes negative (water potential of pure water is zero) .
when water which is entered inside plant cell moves, it creates pressure in outward direction on plant cell wall .This pressure will increase the water potential. So water potential is the sum of solute potential and pressure potential.