Describe how negative pressure potentials (Tension) are generated at cell wall air-interfaces.

In the middle of the afternoon, the osmotic potential in the roots drops significantly to -2.0 due to active changes in the solute concentration in the cells. What do we call this process? Where do these solutes originate from?

At noon the leaf total water potential of the symplast has decreased to -0.8 MPa when compared to sunrise(-0.4 MPa). At noon, the osmotic potential of the symplast is -1.6 MPa (pressure potential is 0.8 MPa). What accounts for the difference in osmotic and pressure potentials at noon with respect to sunrise?