In Brazil, eucalyptus have been planted under different climatic and edaphic conditions. As regard to climate, the plantations have been located in a wide range of conditions, from regions with no restrictions to regions with strong water deficit. Similarly, soils are quite variable in terms of fertility and physical characteristics. The combination of soil and water restrictions can be overcome by the selection of genetic material adapted to such conditions. Soil low fertility can easily be resolved by mineral fertilization, whereas climatic and soil physical restrictions are of more difficult solution. Selecting genotypes adapted to such restrictions has been one of the alternatives adopted in the Brazilian silviculture. Early selection attributes, as biomarkers of such adaptations, including morphological, anatomical and physiological characteristics, are of great practical interest. The objective of this study was to evaluate characteristics and morpho-anatomic adaptations of eucalypt clones, differing in tolerance to water deficit and relate these adaptations to isotopic carbon discrimination (13CO2 ()). The study was carried out in commercial eucalypt stands located in two different climate conditions (mean of 900 mm and 1,500 mm per year) on an soil Yellow Ultisol, in the Northeast of Bahia State, Brazil. In both sites, three clones (1277, a hybrid of Eucalyptus urophylla x E. camaldulensis, 1404, E. urophylla, and 1407, a hybrid of E. urophylla x E. grandis) respectively considered of high, medium and low tolerance to water deficit, were planted at 3 x 3 m spacing and the stands were approximately 5 year old at the assessment time. In each site were selected and felled four trees of average size (diameter and height) per clone. At the stem base, a disc of 5 cm width was removed for 13 CO2 () determination. Wood samples were collected from each disc in radial sections, dried in the laboratory and submitted to a mass spectrometer. Ten completely expanded and active leaves of the third or fourth worl of branches located at the top, half and base of the crown were also collected from each sampled tree for morphometric analyses and dimension determination (area, width, length and perimeter). An additional leaf was also collected for micrimorphometric analyses (cuticle, epidermis and mesophyll width and intercellular space). In the region of lower precipitation, clone 1277 presented higher Δ13C, lower leaf area at the top of the crown, lower water diffusion time of leaves at the half and bottom of the crown and less mesophyll intercellular space than the other two clones. All these characteristics are indicators of adaptation to xeromorphic conditions. Therefore, quantitative and qualitative characteristics were associated with the tolerance to water restrictions. Xeric characteristics were more evident on clone 1277, which showed higher tolerance and adaptation to conditions where water restrictions predominate.
