Which of the following statements are correct? P. Water potential is a measure of the free energy of water per unit volume.

Q. Water potential is influenced by concentration, pressure and gravity.

R. An open beaker full of pure water at 20°C has 0 MPa water potential value. S. Water in plants moves from higher to lower water potentials.

All four statements about water potential are correct. P correctly defines water potential as free energy per unit volume. Q correctly states the three components: concentration (osmotic), pressure, and gravity. R is correct - pure water at atmospheric pressure and standard temperature is defined as 0 MPa. S is correct - water moves from higher to lower water potential. Therefore D (all statements are correct) is the right answer.