Physiological adjustments to enhance tolerance or avoidance of drought were studied in three desert
plants growing in Wadi Natash (Eastern Desert, Egypt). Studied plants (Zilla spinosa (L. ) Prantl, Citrullus
colocynthis (L. ) Schrad and Morettia philaeana (Dalile) DC. ) were collected from four stands. Cell sap
osmotic potential, some organic (soluble sugars, total free amino acids, and soluble proteins) and
inorganic (Ca2+, Mg2+, Na+, K+, Cl- and SO4
2-) solute concentration were determined. In addition, cover
degree of investigated species (according to the cover-abundance scale of Braun-Blanquet) was also
estimated. Substantial osmotic adjustment (up to -1 .71 MPa) was observed in Z. spinosa collected from
stand 4. Z. spinosa was dependent on soluble sugars, soluble proteins, free amino acids, SO4
2-, K+, Cland
Mg2+ to readjust their internal osmotic pressure and to improve its water status. I t preferred Mg2+
more than the two other species. C. colocynthis accumulated inorganic solutes more than Z. spinosa
and less free amino acids. The results suggest that osmotic adjustment was the main water relationship
adaptation to cope with drought. Accumulation of soluble sugars, soluble proteins, K+, Cl- and SO4
2- at
higher concentration often assist in turgor maintenance and helped to enhance drought tolerance.
Key words: Adjustment; Chlorophyll; Organic solute; Osmotic potential; Soluble sugars