Considering the importance of northern leaf blight, caused by Exserohilum turcicum, to decrease maize yield, this study evaluated electrolyte leakage, content of H2O2, malondialdehyde (MDA), chlorophyll (Chl) a, Chl b, and carotenoids (CAR), and activities of superoxide dismutase, catalase, ascorbate peroxidase, peroxidase, glutathione reductase, glutathione-S-transferase, and glutathione peroxidase, as well as expressions of superoxide dismutase (sod), catalase (cat 1), ascorbate peroxidase (apx 2), peroxidase (pox), glutathione reductase (gr 1), and glutathione-S-transferase (gst 23) genes, and leaf gas exchange and Chl a fluorescence parameters in maize leaves infected with E. turcicum. The content of H2O2 and MDA was high at 15 and 20 d after inoculation (dai) and electrolyte leakage was high from 10 dai onward in infected leaves. Net assimilation rate, stomatal conductance to water vapour, transpiration rate, internal CO2 concentration, maximum quantum yield of photosystem (PS) II, and effective quantum yield of PS II significantly decreased as the disease developed. Quantum yield of regulated energy dissipation significantly increased at 10 and 15 dai but decreased at 20 dai whereas the quantum yield of non-regulated energy dissipation significantly increased at 15 and 20 dai. The content of Chl a, Chl b, and CAR decreased in infected leaves. The activities of the enzymes increased, and the relative expressions of the genes sod, cat 1, apx 2, pox, gr 1, and gst 23 were higher in infected leaves compared to non-infected ones mainly from 10 dai onward. In conclusion, photosynthesis in infected leaves was dramatically affected and a late involvement of the antioxidant metabolism was not sufficiently helpful to counteract the deleterious effects of fungal infection.

Considering the importance of target spot, caused by Corynespora cassiicola, to impact soya bean yield, this study investigated the C. cassiicola‐induced perturbations in the photosynthesis of soya bean plants from cultivars Fundacep 59 (moderately resistant) and TMG 132 (susceptible) by examining the chlorophyll (Chl) a fluorescence parameters and imaging and the photosynthetic pigment pools. The area under disease progress curve decreased by 38% for plants from cultivar Fundacep 59 in comparison with plants from cultivar TMG 132. The concentration of total Chla+b significantly increased by 16% and the Chla+b/carotenoids ratio significantly decreased by 11% for plants from cultivar Fundacep 59 in comparison with plants from cultivar TMG 132. Significant decreases of 31% and 8%, respectively, for the concentration of Chla+b and the Chla+b/carotenoids ratio occurred for inoculated plants in comparison with noninoculated ones regardless of the cultivar. As target spot developed, the values of maximal photosystem II quantum yield (Fv/Fm), effective photosystem II quantum yield [Y(II)], quantum yield of regulated energy dissipation [Y(NPQ)] and quantum yield of nonregulated energy dissipation [Y(NO)] increased, especially for plants from cultivar TMG 132. There were positive correlations of area under disease progress curve (AUDPC) with both Y(NPQ) and Y(NO) regardless of the cultivar. For cultivar TMG 132, the correlation between AUDPC and the electron transport rate was negative. Based on the analysis of the Chl a fluorescence, it was possible to elucidate the enormous potential of target spot to impair the functionality on the photosynthetic apparatus that may help the plants to invest in metabolic pathways involved in host defence responses.