The effect of ploidy, parental chloroplast type and parental nuclear genome dosage on net photosynthesis, Rubisco activity and chloroplast ultrastructure was studied among somatic hybrids of diploid S. brevidens and dihaploid S. tuberosum. An increase in nuclear ploidy resulted in an increase in net photosynthesis and specific leaf weight. There were no significant differences in net photosynthesis or Rubisco activity between the hybrids having different parental chloroplast type. Examination of the hexaploid hybrids indicated that Rubisco activity was affected by nuclear-organelle genome incompatibility, the most affected combination being tuberosum chloroplast type with brevidens nuclear genome. Examination of chloroplast ultrastructure revealed wide variation in the size of chloroplasts, starch granules, plastoglobuli and in grana stacking among the hybrids and between fusion parents.