The American chestnut (Castanea dentata) dominated eastern forests in the United States prior to the introduction of the fungal pathogen Cryphonectria parasitica in the early 1900s. While the chestnuts were largely extirpated from forests, sustained efforts by The American Chestnut Foundation to reintroduce the chestnuts have produced BC3F3 hybrids by crossing American and Chinese chestnuts. These hybrids have shown moderate levels of blight resistance while maintaining their American morphology, but it is unknown if other physiological traits were changed through the breeding program. Specifically, changes in the hybrid’s drought response is important as pure Americans chestnuts were known for their drought tolerance and preference of drier, ridgetop sites. Determining if they hybrid chestnuts have comparable drought tolerance and can maintain their blight resistance under drought will be important for the hybrid’s successful reintroduction.
Here a factorial treatment design of drought and pathogen infection was applied to American, Chinese, and BC3F3 hybrid seedlings grown in a common garden. Gas exchange, pre-dawn leaf water potential, canker size, estimated turgor loss point, and estimated solute potential were all measured and compared between species and treatments over the course of one growing season.
Results/Conclusions
Drought treatments showed lower rates of CO2 assimilation and stomatal conductance compared to the control (p-value < 0.01). American chestnuts had higher rates of assimilation and stomatal conductance compared to the Chinese under both treatments (p-value < 0.01). Hybrids showed intermediate rates in the control treatment but showed the lowest rates of stomatal conductance under drought treatment, significantly lower than the Americans (p-value <0.01). We found that all chestnuts responded to decreasing leaf water potentials by osmotic adjustment (p-value <0.01), and found the well-watered controls showed Americans having lower estimated turgor loss points compared to the Chinese (p-value = 0.02) with hybrids being intermediate between Chinese and American. Americans showed larger canker sizes than both Chinese and hybrids (p-value < 0.01) while hybrids showed intermediate canker sizes between American’s and Chinese. Cankers were on average larger in the drought treatment but results proved nonsignificant for all species.
Our findings indicate that hybrids may have inherited more Chinese rather than American chestnut drought response characteristics from the breeding program. Such a change in drought response would suggest more mesic sites to be more appropriate for hybrid reintroduction rather than the classical dry ridgetop sites pure Americans favored.