Elevated [CO₂] and increased N supply reduce leaf disease and related photosynthetic impacts on Solidago rigida

To evaluate whether leaf spot disease and related effects on photosynthesis are influenced by increased nitrogen (N) input and elevated atmospheric CO ₂ concentration ([CO₂]), we examined disease incidence and photosynthetic rate of Solidago rigida grown in monoculture under ambient or elevated (560 μmol mol^-1) [CO₂] and ambient or elevated (+4 g N m^-2 year^-1) N conditions in a field experiment in Minnesota, USA. Disease incidence was lower in plots with either elevated [CO₂] or enriched N (-57 and -37%, respectively) than in plots with ambient conditions. Elevated [CO₂] had no significant effect on total plant biomass, or on photosynthetic rate, but reduced tissue%N by 13%. In contrast, N fertilization increased both biomass and total plant N by 70%, and as a consequence tissue%N was unaffected and photosynthetic rate was lower on N fertilized plants than on unfertilized plants. Regardless of treatment, photosynthetic rate was reduced on leaves with disease symptoms. On average across all treatments, asymptomatic leaf tissue on diseased leaves had 53% lower photosynthetic rate than non-diseased leaves, indicating that the negative effect from the disease extended beyond the visual lesion area. Our results show that, in this instance, indirect effects from elevated [CO₂], i.e., lower disease incidence, had a stronger effect on realized photosynthetic rate than the direct effect of higher [CO₂].