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Evolved low-mass stars of a wide range of metallicity bear signatures of anon-standard mixing event in their surface abundances of Li, C, and N, and intheir 12C/13C ratio. A Na overabundance has also been reported in some giantsof open clusters but remains controversial. The cause of the extra-mixing hasbeen attributed to thermohaline convection that should take place after the RGBbump for low-mass stars and on the early-AGB for more massive objects. To trackthe occurrence of this process over a wide mass range, we derive in ahomogeneous way the abundances of C, N, O, and Na, as well as the 12C/13C ratioin a sample of 31 giants of 10 open clusters with turn-off masses from 1.7 to3.1 Msun. A group of first ascent red giants with M/Msun \leq 2.5 exhibitslower [N/C] ratios than those measured in clump giants of the same mass range,suggesting an additional increase in the [N/C] ratio after the first dredge-up.The sodium abundances corrected from NLTE are found to be about solar. [Na/Fe]shows a slight increase of 0.10 dex as a function of stellar mass in the 1.8 to3.2 Msun range covered by our sample, in agreement with standard firstdredge-up predictions. Our results do not support previous claims of sodiumoverabundances as high as +0.60 dex. An anti-correlation between 12C/13C andturn-off mass is identified and interpreted as being caused by a post-bumpthermohaline mixing. Moreover, we find low 12C/13C ratios in a fewintermediate-mass early-AGB stars, confirming that an extra-mixing process alsooperates in stars that do not experienced the RGB bump. In this case, theextra-mixing possibly acts on the early-AGB, in agreement with theoreticalexpectations for thermohaline mixing. [abridged]

CNONa and 12C/13C in giants of 10 open clusters