Biochar Amendment Alters the Effects of Phytophthora Cankers on Leaf Physiology of Acer rubrum

Previously we have shown that biochar amendment in potting mix significantly reduced the size of necrotic lesions caused by Phytophthora spp. on Acer rubrum and Quercus rubra seedlings. In addition, we have shown that biochar amendment mitigated the negative impacts of inoculation on biomass in A. rubrum and on plant water status in Q. rubra.  In this study, we present the findings of a subsequent two-year experiment conducted with Acer rubrum with an emphasis on leaf physiological responses.  Specifically, we examined maximum CO₂ assimilation rates (A_max), maximum photochemical efficiency of PSII (Fv/Fm), stem water potential, leaf phenolic content, leaf starch content, and leaf chlorophyll content (SPAD measures) in order to gain an understanding of the underlying physiological mechanisms associated with the increased pathogen resistance conferred by biochar amendment. When data from both years were combined, multiple measures indicated that leaf physiological performance was improved in inoculated plants that had been amended with 5% biochar compared to the inoculated plants with no biochar amendment.  Biochar amended plants showed reduced canker expansion, higher A_max, higher stomatal conductance to water vapor, higher Fv/Fm, and higher chlorophyll content over the course of the experiments. These differences were significant in some weeks (P < 0.05), and the trend was consistent throughout the experiments.  Foliar starch content, stem water potential, and systemic phenolic content data were inconclusive. While the exact physiological mechanisms remain unclear, it is likely that the lower CO₂ assimilation rates observed in inoculated plants were due to a reduced stomatal conductance resulting from a pathogen derived signal and/or hormonal response to wounding, leading to limitations in the supply of CO₂ at the site of carboxylation, rather than a reduced demand for CO₂ resulting from feedback inhibition.  Reduction of stem lesions observed in biochar treatment compared with the inoculated control is likely a result of systemic resistance associated with higher photosynthetic productivity. Further studies will be needed in order to determine the exact mechanisms underlying the induced resistance against lesion development in biochar amended seedlings.