Solanum commersonii cytoplasm does not improve freezing tolerance in substitution backcross hybrids with frost-sensitive potato species

Solanum commersonii Dun. (cmm) is the most frost hardy wild potato species known, being able to tolerate an acute freezing episode to about -5 C and further acclimate to tolerate -10 C after being exposed to chilling temperatures for several days. Breeding with this species to incorporate its frost-hardiness traits can be accomplished by standard sexual hybridization or protoplast fusion. These methods can result in hybrids that vary in contribution of the cmm plastome. To test the effect of cmm cytoplasm, cytoplasmic substitution backcross hybrids were made with three very frost-sensitive species, S. brachistotrichum, S. cardiophyllum, and S. pinnatisectum, by using S. commersonii as the female to make an F, then performing repeated backcross (BC) using the sensitive species as males. Relative freezing tolerance (RFT) of all genotypes was assessed by measurement of ion leakage of excised terminal leaflets subjected to a controlled ice nucleation and simulated freeze-thaw stress. Even against the background of these very sensitive species' genomes, the cmm cytoplasm of substitution hybrids promoted insignificant improvement in frost hardiness or ability to acclimate. We conclude that either (1) cmm cytoplasm does not contribute to frost hardiness, or (2) if cmm cytoplasmic frost hardiness genes do exist, they must be epistatic to (depend on the presence of) nuclear hardiness genes for expression.