Co-localizations among QTLs may be expected when significant genetic correlations are observed among quantitative traits. In such a case, identification of co-localizing QTLs may be explained by a QTL having a pleiotropic effect on different traits. Furthermore, the sign of the substitution effect of co-localizing QTLs should be in this case in agreement with that of the correlation coefficient. In our experiment growth parameters were generally and negatively correlated with D. In five cases QTLs for D and growth-related traits coincided on the same linkage group. Opposite signs of the substitution effect were observed on the LG1F, LG6F, LG7F (the three QTLs for D and height2000) and LG8F, whereas concordant signs were observed on LG3M and LG7F (considering the QTL for diameter2002). Overall, this result agrees with the negative and weak phenotypic correlation between D and growth suggesting a genetic correlation between the two traits. Recently, Lauteri et al. (personal communication) found negative and strong genetic correlations between D and growth traits in chestnut seedlings, highlighting the interesting adaptive value of this kind of relation. However, because of the complex determinism of D, the sign of the relationship between plant growth and D itself is likely to be dependent on genotype–environment interactions and difficult to predict (Farquhar et al. 1989). For instance, Brendel et al. (2002) found a positive phenotypic correlation between d13C and mean ring width in Pinus pinaster (which corresponds to a negative correlation with D). A negative correlation between D and tree height was also found in Picea mariana on a dry site, but the relation was not confirmed in a site characterized by higher water availability (Flanagan & Johnsen 1995). Brendel et al. (2002) did not obtain significant genetic correlation between d13C and mean ring width. No co-localization of QTLs for the two traits was found in their case. In their study on P. pinaster a rather moderate genetic control was reported for both ring growth and d13C of cellulose. Furthermore these authors hypothesize a dependence of d13C, but not of growth, from photosynthetic assimilation, supporting the lack of co-localization of QTLs for these traits in P. pinaster. In the present study, the chestnut F1 progeny was obtained from a female parent originating from a drought-adapted population, which is expected to present genetic adaptation to Mediterranean-type. Lauteri et al. (1997) showed that this drought-adapted ecotype was well adapted to the Italian site where the full-sib family was grown, showing higher photosynthetic capacity, and higher stomata and mesophyll conductance when compared with the wet-adapted population which the male parent came from. In such a situation we can suppose that allele combinations for or of adaptation to drought conditions were detected by our QTL analysis in the female map. The negative correlation between D and growth and the opposite sign of four co-localized QTLs found in the female map are in agreement with a whole-plant structure adaptation to drought conditions proposed by Lauteri et al. (1997). Following this hypothesis, the relatively higher photosynthetic performances of the drought-adapted ecotype may be linked to an increased carbon allocation to roots, beneficial to take up deep and reliable soil water sources, and to a lower above-ground juvenile growth. Indeed, the opposite trends of D and growth, taking into account the four co-localized QTLs in the female parent, are consistent with this adaptive outline, confirming a whole-plant evolutionary strategy in this chestnut population.