Do Canker Pathogens Limit the Supply of CO2 for Photosynthesis? Estimation of Stomatal and Internal Limitations in Maple Leaves in Response to Phytophthora cactorum Inoculations

Phytophthora diseases are a major limiting factor to crop, forest, and amenity plant growth worldwide. Canker pathogens can affect net CO₂ assimilation (A_n) though phloem and xylem infections. While prior studies have shown that Phytophthora canker infections limit A_n, reports regarding the underlying mechanisms are variable. One possible pathway is through lowering CO₂ supply by reduced stomatal (g_s) and mesophyll conductance (g_m). Our objective of this study was to quantify CO₂ supply limitations in response to P. cactorum inoculations in red maple (Acer rubrum). We hypothesized that the inoculated plants will reduce both g_s and g_m, compared with the non-inoculated controls. We tested this hypothesis using the two methods to determine g_s and g_m: 1) a method combining leaf gas exchange, chlorophyll fluorescence, and the biochemical model of photosynthesis; and 2) a method utilizing the photosynthesis model with leaf gas-exchange measurements made at the two O₂ concentrations (2% and 21%). Our results show that g_s and g_m in the maple leaves decreased due to P. cactorum inoculations, leading to the limitation of CO₂ supply to the site of carboxylation. The two different g_m estimation methods produced comparable g_m values and similar response patterns.