Equilibrium between soil water potential and predawn water potential of two pre-saharan shrub species of Tunisia

An experimental study of the relationship between predawn water potential and soil water potential was performed under controlled conditions on two shrub species, Seriphidium incultum (Delile) Ling (syn. Artemisia herba-alba Asso) and Anthyllis henoniana Batt. The results show that predawn water potential for both species remained strongly negative even when soil water potential was close to zero. The difference between observed values and values expected from water transfer models was greater in Seriphidium (−2 MPa) than in Anthyllis (−1 MPa). The results of resaturation experiments on whole plants and cut shoots indicate two strong resistances to rehydration, one at the soil-root xylem level and the other at the xylem-leaf cells level. A hypothesis is put forward concerning the ecological significance of non-equilibrium between plant and soil.