Asian soybean rust (Phakopsora pachyrhizi-ASR) is one of the major diseases that occur in soybean and causes great damage to commercial crops. Therefore, the goal of this work was to investigate the relationship between biochemical and photosyntetic parameters in soybean with ASR. Two experiments were performed in a randomized complete block with three treatments (water, Tween 20, and methyl jasmonate). The evaluated traits were: severity, chlorophyll pools, concentration of phenolic compounds content, enzyme activity, and photosyntetic parameters. Based on Pearson correlation, correlation network and path analysis it was verified that the severity had high correlation with almost all traits evaluated meanwhile photosynthesis is weakly related with almost all traits. Therefore, the occurrence of ASR affects directly the traits related to enzymatic activity and phenolic compounds content in soybean plants inoculated with ASR. The management of ASR is important to keep in normal levels the rates of photosynthesis carried out by the plant, and thus not affect the yield. Besides that, understanding the biochemical mechanisms and ecophysiological responses that occur during the soybean-P. pachyrhizi interaction has a great importance for breeding programs, as it will help for obtaining resistant cultivars or efficient methods in fungus control.

Asian rust affects the physiology of soybean plants and causes losses in yield. Repeatability coefficients may help breeders to know how many measurements are needed to obtain a suitable reliability for a target trait. Therefore, the objectives of this study were to determine the repeatability coefficients of 14 traits in soybean plants inoculated with Phakopsora pachyrhizi and to establish the minimum number of measurements needed to predict the breeding value with high accuracy. Experiments were performed in a 3x2 factorial arrangement with three treatments and two inoculations in a random block design. Repeatability coefficients, coefficients of determination and number of measurements needed to obtain a certain reliability were estimated using ANOVA, principal component analysis based on the covariance matrix and the correlation matrix, structural analysis and mixed model. It was observed that the principal component analysis based on the covariance matrix out-performed other methods for almost all traits. Significant differences were observed for all traits except internal CO2 concentration for the treatment effects. For the measurement effects, all traits were significantly different. In addition, significant differences were found for all Treatment x Measurement interaction traits except coumestrol, chitinase and chlorophyll content. Six measurements were suitable to obtain a coefficient of determination higher than 0.7 for all traits based on principal component analysis. The information obtained from this research will help breeders and physiologists determine exactly how many measurements are needed to evaluate each trait in soybean plants infected by P. pachyrhizi with a desirable reliability.

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Considering blast, caused by Pyricularia oryzae, as a critical threat to wheat yield and the importance of photosynthesis as the source of energy used by plants to activate their mechanisms of defence to counteract pathogen infections, this study aimed to investigate whether wheat plants compromised in their photosynthetic process by using an inhibitor of photosynthesis could become more susceptible to blast. Plants from cultivars BRS 220 (moderately resistant) and BRS Guamirim (susceptible) were non‐sprayed (‐DCMU plants) or sprayed (+DCMU plants) with a solution (10 μM) of 3‐([3,4‐dichlorophenyl]‐1,1‐dimethylurea) (DCMU) at 24 hr before inoculation with P. oryzae. Blast progressed much faster on the leaves of +DCMU plants from cultivars BRS 220 and BRS Guamirim in comparison with ‐DCMU plants of these cultivars. Increased susceptibility of plants from the two cultivars to blast caused by DCMU was associated with its negative effect on the photosynthetic apparatus, decreasing phenylalanine ammonia‐lyase activity and reducing the concentration of total soluble phenolics. Chitinase activity for the infected plants from the two cultivars was lower compared with non‐inoculated ‐DCMU plants but was greater for inoculated +DCMU plants. Distinctly, β‐1,3‐glucanases activity was kept stable for inoculated +DCMU plants. In conclusion, manipulation of the photosynthetic process of wheat plants by using DCMU allowed to understand their response to P. oryzae infection at the physiological level and reinforced the concept that photosynthesis was important for wheat resistance to blast.

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