La Tunisie a de vastes etendues marginales, arides, semi-arides et desertiques (environ trois quarts de la superficie du pays). Le reboisement figure parmi les moyens de reconstitution de ces zones degradees. Casuarina glauca Sieber, largement utilisee en dehors de son aire de distribution d'origine, a ete testee dans la presente etude quant a son degre d'acclimatation et d'endurcissement a la secheresse. Trois regimes hydriques, simulant les conditions des sites de reboisement, ont ete appliques a des plants de C. glauca. Ces regimes englobent un traitement temoin (T : potentiel hydrique de base [[PSI].sub.b] = -0,2 MPa), un stress modere (M : [[PSI].sub.b] = -0,5 MPa) et un stress severe (S : [[PSI].sub.b] = -1,0 MPa). Les parametres hydriques generes a partir des courbes pression-volume ainsi que la masse seche accumulee ont ete mesures au demarrage de l'application des traitements et apres deux, quatre et six mois. La masse seche totale des plants soumis aux regies hydriques (M et S) a ete significativement affectee par la restriction hydrique. Cependant, l'allocation de masse seche a ete nettement orientee en faveur des racines. Les regimes hydriques appliques ont eu un effet significatif sur le rapport masse aerienne a saturation / masse seche correspondante (TM/DM). L'evolution des parametres hydriques sous les regimes M ou S revele une capacite adaptative de C. glauca, caracterisee par une baisse significative des potentiels osmotiques a pleine turgescence ([[PSI].sub.[pi].sup.100]) et a turgescence nulle ([[PSI].sub.[pi].sup.0]), un ajustement osmotique (AO) accru atteignant 0,31 et 0,56 MPa, et une augmentation significative du module d'elasticite volumique ([[epsilon].sub.max]) atteignant 26 % et 68 % du temoin, respectivement, a la fin des traitements. La diminution du rapport TM/DM combinee a un accroissement de [[epsilon].sub.max] au cours des traitements indique la presence d'un ajustement de l'elasticite de la paroi cellulaire. Ces traits ont ete d'autant plus marques en presence du stress severe. Les resultats obtenus suggerent que l'application d'un preconditionnement a la secheresse en pepiniere pendant quatre mois avant la mise en terre en site de plantation, d'intensite M ou S selon le site hote, pourrait conferer aux plants des adaptations morphologiques et physiologiques permettant d'ameliorer leur taux de survie et leur croissance dans les reboisements effectues dans les zones arides et semi-arides. Mots-cles : reboisement, Casuarina glauca, secheresse, allocation de biomasse, relations hydriques. Tunisia has vast land tracts that are marginal, arid, semiarid, or desert (about three quarters of the surface of the country). Reforestation of these regions is one of the methods being used to rehabilitate degraded areas. Casuarina glauca Sieber, a fast-growing tree species that is widely used outside of its native range, was tested in the present study with respect to its degree of acclimatization and hardening in response to drought. Three water regimes, simulating the conditions of the reforestation sites, were applied to C. glauca seedlings. These regimes included a control treatment (T : predawn water potential [[PSI].sub.b] = -0.2 MPa), a moderate stress treatment (M : [[PSI].sub.b] = -0.5 MPa), and a severe stress treatment (S : [[PSI].sub.b] = -1.0 MPa). Water relations parameters, estimated by pressure-volume curves through a repeat pressurization method, as well as accumulated dry mass were measured at the beginning of the experiment and 2, 4, and 6 months following treatment imposition. The total dry mass of M and S plants was significantly affected by the treatments. However, water stress favoured dry mass allocation to roots and decreased the shoot turgid mass / dry mass ratio (TM/DM). The evolution of water relations parameters under M or S regimes revealed an adaptive capacity of C. glauca seedlings, as expressed by a significant decrease of the osmotic potentials at full turgor ([[PSI].sub.[pi].sup.100]) and at the turgor loss point ([[PSI].sub.[pi].sup.0]), an osmotic adjustment in water-stressed plants reaching 0.31 and 0.56 MPa, and a significant increase in [[epsilon].sub.max] reaching 26 % and 68 % of that of the control plants, respectively, at the end of the drought period. A reduction in shoot TM/DM combined with increases in [[epsilon].sub.max] during water stress suggests the presence of a cell wall adjustment. These effects were more pronounced under the severe stress. Our results suggest that preconditioning seedlings to drought under nursery conditions by applying a moderate or severe stress, depending on the conditions of the planting site, for 4 months prior to outplanting, possibly will allow the seedlings to adapt morphologically and physiologically and improve their chance of survival in arid and semiarid zones. Key words: reforestation, Casuarina glauca, drought, biomass allocation, water relations., Introduction D'une superficie limitee (1226 x [10.sup.3] ha), les formations forestieres naturelles et artificielles tunisiennes constituent 7,5 % de la superficie du pays (FAO FRA 2005) et assurent une production [...]