OSMOTIC ADJUSTMENT IN ASH AND WATER-STRES S SITUATION

This paper shows the existence of an osmotic adjustment in leaves of a sh in a water stress situation. During a summer drought the predawn wa ter potential fell to values ns low as -4.2 MPa. Pressure-volume curve s demonstrated that osmotic potential at full turgor decreased from - 0.48 to -0.94 MPa (tree 1) and from -0.84 MPa to -1.57 MPa (tree 2) du ring water stress. This gives an average degree of osmotic adjustment of 0.59 MPa. On the other hand, the forest alluvial ash did not show a lly osmotic adjustment since the water stress was compensated by the g round water. In leaves, the percentage of soluble carbon (relative to total carbon) increased when drought started, but the soluble nitrogen level did not change significantly. Together these data suggest the i nvolvement of an osmotic adjustment in which nan-nitrogenous organic c ompounds could be concerned.