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Y.-K. Chen, J.P. Palta, and J.B. Bamberg

Wild potato species provide a valuable source of genetic variability for the improvement of freezing tolerance in cultivated potato, Solanum tuberosum (tbr). However, breeding for freezing tolerance by using wild genetic resources has been hampered by contradictory results regarding the genetic control of this trait. Both dominance and recessiveness for this trait have been reported. To explore the genetic control of freezing tolerance, the expression of freezing tolerance was investigated in various interspecific F1 and somatic hybrids between hardy and sensitive species. In addition to 2 years of field evaluation, freezing tolerance before and after acclimation was characterized separately under controlled environments to dissect the two independent genetic components of freezing tolerance, namely nonacclimated freezing tolerance (NA) and acclimation capacity (ACC). The expression of freezing tolerance, including NA and ACC, was closer to that of hardy parent, sensitive parent, or approximate parental mean, depending on species combination. However, the expression of freezing tolerance tended to be greater when the hybrids contained more sets of chromosomes from the hardy parent than from the sensitive parent. The significance of hardy: sensitive genomic ratio was further supported by using sexual and somatic hybrids between tbr and S. commersonii (cmm) to achieve different genomic ratios without the confounding effect of species. Therefore, we propose that the hardy: sensitive genomic ratio is an important determinant for the expression level of freezing tolerance before and after cold acclimation.