Frogeye leaf spot (FLS), caused by the fungus Cercospora sojina, is one of the most important soybean diseases and can cause great yield losses. Several studies have demonstrated that silicon (Si) enhances the plant antioxidant system, especially when they are subjected to stresses. Thus, this study was designed to evaluate the effect of Si on soybean resistance to FLS, on the antioxidant system, on the concentration of reactive oxygen species and on cellular damage during the infection process of C. sojina. Plants from cultivars Bossier and Conquista, susceptible and resistant to FLS, respectively, were supplied with either 0 (−Si) or 2 mm (+Si) and non‐inoculated or inoculated with C. sojina. FLS severity was greater for Bossier than for Conquista, regardless of the Si supply, and it was increased by Si for both cultivars. The activities of the most antioxidant enzymes were lower in the +Si plants than in the −Si plants when they were not inoculated. Inoculated plants usually showed an increased enzyme activities and higher concentrations of ascorbate and reduced glutathione than did the non‐inoculated plants, regardless of Si supply. At advanced stages of fungal infection, the +Si‐inoculated plants from Bossier had higher activity of most antioxidant enzymes and higher concentrations of superoxide and malondialdehyde compared to the non‐inoculated plants as a result of an increased oxidative stress. The results from this study provide the first evidence that Si reduces the basal activity of antioxidant enzymes in soybean leaves leading to an increase in host susceptibility to FLS.

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Considering the potential of white mold, caused by the fungus Sclerotinia sclerotiorum, to reduce tomato production, this study aimed to determinate the effect of manganese (Mn) phosphite on the resistance of tomato plants to this disease by assessing the photosynthetic performance (gas exchange and chlorophyll a fluorescence), the activities of defence enzymes and those related to the antioxidant metabolism as well as the concentrations of photosynthetic pigments, malondialdehyde (MDA), hydrogen peroxide (H2O2) and superoxide anion (O2−). The in vitro assays showed that S. sclerotiorum mycelial growth was inhibited by Mn phosphite in a dose‐response manner. The spray of Mn phosphite reduced white mold severity on the leaves of tomato plants. Additionally, there was a higher foliar Mn concentration for plants sprayed with Mn phosphite. The negative effects of S. sclerotiorum infection in the photosynthetic process were mitigated by Mn phosphite application as noticed by the net carbon assimilation rate, stomatal conductance to water vapour, transpiration rate, maximal photosystem II quantum yield values and concentration of photosynthetic pigments. The concentrations of MDA, H2O2 and O2‐ on inoculated leaves were lower upon Mn phosphite spray. In general, the activities of defence enzymes and those related to the antioxidant metabolism were higher for water‐sprayed plants inoculated with S. sclerotiorum in comparison to those inoculated and sprayed with Mn phosphite. Based on the present study results, the application of Mn phosphite may represent a feasible alternative for white mold management in tomato plants.

ResumoConsiderando o efeito positivo do silício (Si) na melhoria do desempenho fisiológico de várias espécies de plantas quando infectadas por patógenos, este trabalho teve como meta investigar o efeito desse elemento na capacidade fotossintética e no metabolismo antioxidativo de plantas dos cultivares de algodoeiro NuOpal e BRS Buriti, supridas ou não com Si, durante o processo infeccioso de Ramularia areola. A infecção por R. areola afetou negativamente a taxa líquida de fixação do CO 2 (A), a condutância estomática (g s ), a transpiração e a razão entre concentração interna e ambiente de CO 2 nas plantas dos cultivares NuOpal e BRS Buriti não supridas com Si em relação às plantas supridas com esse elemento. Na presença do Si, houve aumento em A e g s além das concentrações de clorofila a, clorofila b e carotenóides nas plantas dos dois cultivares. A concentração foliar de Si aumentou, o que contribuiu para decrescer em 38% a área abaixo da curva do progresso da mancha de ramulária nas plantas supridas com Si, em relação às plantas não supridas com esse elemento. O fornecimento de Si às plantas de algodoeiro dos dois cultivares aumentou a atividade da dismutase do superóxido (SOD), da peroxidase do ascorbato (APX) e da lipoxigenase (LOX), mas houve redução na atividade da catalase (CAT) e na concentração de peróxido de hidrogênio (H 2 O 2 ) e de aldeído malônico (MDA), houve ainda uma maior perda de eletrólitos. A atividade da glutationa redutase (GR) foi maior nas plantas do cultivar BRS Buriti supridas com Si, enquanto que nas plantas do cultivar NuOpal houve aumento da atividade da glutationa redutase apenas aos 21 dias após inoculação (dai). Conclui-se que a capacidade fotossintética e o sistema antioxidativo das plantas supridas com Si sofreram menor impacto durante o processo infeccioso de R. areola.Palavras-chave: fotossíntese, mancha foliar, nutrição mineral.Leaf gas exchange and oxidative stress on cotton plants supplied with silicon and infected by Ramularia areola AbstractConsidering the positive effect of silicon (Si) in improving the physiological performance of several plant species when infected by pathogens, this study investigated the effect of this element in the photosynthetic capacity and on the antioxidant metabolism of cotton plants from cultivars BRS NuOpal and Buriti, supplied or not with Si, during the infectious process of Ramularia areola. The infection by R. areola negatively affected the net carbon assimilation rate (A), stomatal conductance to water vapor (g s ), internalto-ambient CO 2 concentration ratio and transpiration rate on plants of the two cultivars non-supplied with Si in comparison to plants supplied with this element. In the presence of Si, there was an increase in A and g s as well as on the concentrations of chlorophyll a, chlorophyll b and carotenoids in plants of both cultivars. The Si supply increased the foliar Si concentration, which contributed to decrease the area under ramularia leaf spot progress curve by 38% in comparison to plants non-supplied with this e...