Physiological Comparison of Pure American and American-Chinese Chestnut Hybrids

American chestnut (Castanea dentata), once a dominant canopy species in eastern North America, succumbed to invasive pathogens in the early 20th century. The American Chestnut Foundation attempts to breed pathogen resistance using backcrossed hybrids with Chinese chestnut (Castanea mollissima ). We compared steady-state photosynthetic characteristics among backcrossed chestnut hybrids (BC1F1 with 3/4 American genotype and BC3F3 with 15/16 American genotype) and pure American chestnut. Photosynthetic capacity was compared among these three chestnut classes with steady-state light response curves. Dark respiration rates, quantum yield, and maximum photosynthetic rates differed among these chestnut types, but light compensation point, a measure of shade tolerance, did not. In July and August, we measured plant water potential (at predawn, morning and afternoon) and leaf gas exchange (morning and afternoon) under ambient conditions between the 15/16 and 3/4 AC hybrids. Trees in the pure American chestnut population suffered blight-induced dieback resulting in shrubby plants that differed in size and light environment, so these were not used in diurnal comparisons. Despite similarities in soil moisture and pre-dawn water potential, there were significant differences in daily gas exchange and water potential. Trees in the 15/16 American chestnut population generally had higher gas exchange rates in the afternoon, while also experiencing lower plant water potential and a greater diurnal change in water potential. These results suggest that these trees may be better able to perform under moderate atmospherically-induced water stress than trees in the 3/4 American chestnut population. Quantifying relationships between water relations, gas exchange, and environmental factors allow evaluation of long-term performance and may better inform site selection and management practices.