Introgression of the low browning trait from the wild Mexican species Solanum hjertingii into cultivated potato (S-tuberosum L.)

Internal discoloration of tubers resulting from impact damage (blackspot bruise) is a serious quality problem in potato production and utilization, reducing profits to growers and increasing costs for processors. Resistance to blackspot bruise has been identified in the wild species Solanum hjertingii and is therefore a potential germplasm resource for genetic resistance to this problem. A bridging cross between S. hjertingii and a cultivated diploid clone was used to produce a triploid hybrid population that exhibited very low tuber browning potential, indicating a dominant pattern of inheritance for this trait. The triploid progeny were subjected to in vitro chromosome doubling and the resulting hexaploid clones were screened for browning potential. A hexaploid clone selected for low browning was reciprocally crossed with cultivated S. tuberosum cultivars exhibiting high susceptibility to blackspot bruise. Tubers obtained from the seed progeny of these 4x-6x crosses (hereafter referred to as the BC1 populations) were evaluated for browning potential and polyphenol oxidase (PPO) activity. Tubers from the BC1 populations displayed a very low potential for melanin production, while PPO activity was quite variable. The low Pearson correlation coefficient (r(2) = 0.45), between browning potential and PPO activity suggests that the mechanism of blackspot bruise resistance derived from S. hjertingii cannot be explained simply as a reduction in the initial PPO activity. The expression of substantial resistance to browning and dominant expression pattern in these BC1 progeny indicate that utilizing genetic elements derived from S. hjertingii provides a robust approach for developing blackspot bruise resistant potato varieties.