SOIL-MOISTURE IN DEEP LEVELS IS THE MAIN SOURCE OF MID-DAY WATER POTENTIAL VARIATIONS IN OAK (QUERCUS SP)

There are daily variations in xylem water potential (psi(f)) in oak: p si(f) is close to zero at the end of the day and before dawn (predawn potential), it then drastically decreases to a mid-day minimum. predaw n and mid-day potentials vary from one day to another. Soil moisture i n surface levels (50 cm) accounted for a large part of the changes in psi(f) at the beginning and at the end of the day. Mid-day water poten tial changes were mostly related to soil moisture in deep levels (100 cm) whereas air water potential was the most important source of psi(f ) variations in the morning and in the afternoon. During the day-time, oaks react to an increase in water deficit by diversifying their wate r supply; whereas superficial roots are considered to be the only orga ns involved in water uptake under conditions of low water demand, deep roots may markedly contribute to water supply when water deficit is h igh.