EQUILIBRIUM BETWEEN SOIL-WATER POTENTIAL AND PREDAWN WATER POTENTIAL OF 2 PRE-SAHARAN SHRUB SPECIES OF TUNISIA

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