2019 ASHS Annual Conference

Redbay (Persea borbonia), a member of the Lauraceae family, is a tree native to forests of the southeastern United States. It is planted as landscape tree and used as a medicinal and ceremonial plant by the Seminole tribes. Most members of the Lauraceae family, including redbay, are susceptible to laurel wilt, a devastating vascular disease caused by the fungus Raffaelea lauricola, which was introduced into the United States in 2002. Trees respond to laurel wilt by rapidly developing gums and tyloses (outgrowths of xylem vessel cells) impeding water and nutrient transport. Infected trees wilt and eventually die within weeks to months. We used stable isotopes of carbon (C) and leaf gas exchange measurements to determine if inhibition of photosynthesis in laurel wilt-infected trees is due to mainly to stomatal factors in response to water stress. Two-year-old redbay trees were inoculated with R. lauricola or deionized water (control). Disease symptoms (visual rating of 1-10 with 1 indicating no symptoms), net photosynthesis (Pn), transpiration (Tr), stomatal conductance (gs), substomatal CO₂ concentration (Ci), leaf nitrogen (N) content, and the ratio of ¹³C/¹²C (δ¹³C) were determined prior to inoculation and when inoculated trees exhibited disease symptoms. At the end of the experiment stem segments of all plants were plated on a semi-selective medium for recovery of the pathogen. R. lauricola was recovered from all infected trees and none of the controls. Infected trees had lower Pn, Tr, gs, and higher Ci than control trees. There was a positive linear correlation between Pn and gs in inoculated and control trees. Leaf N content was linearly correlated with δ¹³C in control trees. Therefore, leaf N content was used as a covariate to assess the effects of the disease on δ¹³C. Redbay trees infected with laurel wilt had higher (less negative) δ¹³C values than the controls. The decrease in δ¹³C, Tr, and gs that we observed in inoculated trees is similar to what has been observed in drought stressed trees of other species. However, we also observed an increase in Ci in inoculated trees, which is opposite of what is expected if inhibition of photosynthesis is due to solely to stomatal factors as in the case of drought stress. Thus, reduced Pn in redbay trees with laurel wilt appears to be due to both stomatal and non-stomatal (biochemical) factors.