Physiological responses of pepper (Capsicum annum) to combined ozone and pathogen stress

Tropospheric ozone [O₃] is a secondary air pollutant formed from the photochemical oxidation of volatile organic compounds in the presence of nitrogen oxides, and it is one of the most damaging air pollutants to crops. O₃ entry into the plant generates reactive oxygen species leading to cellular damage and oxidative stress, leading to decreased primary production and yield. Increased O₃ exposure has also been shown to have secondary impacts on plants by altering the incidence and response to plant pathogens. We used the Capsicum annum (pepper)-Xanthomonas perforans pathosystem to investigate the impact of elevated O₃ (eO₃) on plants with and without exposure to Xanthomonas, using a disease-susceptible and disease-resistant pepper cultivar. Gas exchange measurements revealed decreases in diurnal photosynthetic rate (A') and stomatal conductance ( $g_s^{\text{'}}$ ), and maximum rate of electron transport (J_max) in the disease-resistant cultivar, but no decrease in the disease-susceptible cultivar in eO₃, regardless of Xanthomonas presence. Maximum rates of carboxylation (V_c,max), midday A and g_s rates at the middle canopy, and decreases in aboveground biomass were negatively affected by eO₃ in both cultivars. We also observed a decrease in stomatal sluggishness as measured through the Ball-Berry-Woodrow model in all treatments in the disease-resistant cultivar. We hypothesize that the mechanism conferring disease resistance to Xanthomonas in pepper also renders the plant less tolerant to eO₃ stress through changes in stomatal responsiveness. Findings from this study help expand our understanding of the trade-off of disease resistance with abiotic stresses imposed by future climate change.