Through modern back-cross breeding, The American Chestnut Foundation has worked to develop hybrid chestnuts through crosses of pure American chestnuts (Castanea dentata) and Chinese chestnuts (Castanea molissima). The goal of the breeding program is to select for disease resistance against chestnut blight. By back-crossing with American chestnuts to retain American characteristics, all traits other than disease resistance in chestnut hybrids should be more similar to American trees than to Chinese. However, Cryphonectria parasistica attacks the hydraulic architecture of infect trees, so it is unknown if the disease resistance in the hybrid chestnuts has also altered stem hydraulics.
To test this, four genotypes of BC3F3 hybrid chestnuts with known resistance were grown alongside pure American chestnuts, and Chinese chestnuts. Saplings were infected with a highly virulent strain of Cryphonectric parasitica commonly used for disease resistance screening. An artificial drought treatment was used to see if hydraulic behavior was different in hybrids when not infected. A combined drought and pathogen infection treatment was also used to determine how the stressors interact. Specific hydraulic conductivity (Ks) was measured on stems at the end of summer. Only visibly infected stems were selected from the pathogen and drought-pathogen treatments.
Results/Conclusions
Chinese chestnuts were found to have a statistically higher Ks than both pure American (p-value = 0.005) and hybrid chestnuts (p-value = 0.024) with no differences being found between American’s and hybrids. The pathogen and drought-pathogen treatments showed the most significant reductions in Ks across all species (p-value < 0.05). Mortality, determined by zero flow through the stem, was most prevalent in combined drought-pathogen group with statistically higher mortality than both the drought and pathogen treatments alone (p-value < 0.05). When comparing percent changes in average Ks between species and across treatments, the behavior of the hybrids is closer to that of pure American chestnuts than to that of Chinese. hybrids and American chestnuts showed a greater loss of conductivity in the pathogen treatment compared to the Chinese chestnuts but showed less of a loss in the drought-pathogen treatment. While the hybrids show less of an effect of co-occurring drought and pathogen infection, they also show a greater effect of pathogen infection potentially implying they are not as resistant as Chinese chestnuts even after breeding. These findings imply the hybrid chestnuts perform more similar to American chestnuts in terms of their hydraulics under drought and chestnut blight infection.