Chlorophyll a fluorescence imaging of soya bean leaflets infected by Corynespora cassiicola

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