Biochemical prerequisites for resistance to biotic stress in apple (Malus domestica Borkh.)

The current study aimed to investigate the effect of an exclusion net and aphid infestation on some biochemical defense parameters of several apple varieties. The experiment was set up in 2021-2022 in the Agricultural University – Plovdiv, Bulgaria. During the first experimental year, the influence of exclusion net on phenolic content, DPPH radical scavenging, and salicylic acid in Super Chief, Fujion, Pinova, and Enterprise varieties were investigated. During the second experimental year, the activity of the enzymes guaiacol peroxidase, syringaldazine peroxidase, phenylalanine ammonia-lyase, and the content of salicylic acid in infested and uninfested leaves of Gemini and Modi varieties outside the exclusion net was compared. The results of the screening of Super Chief, Fujion, Pinova, and Enterprise varieties for the content of phenolic compounds and salicylic acid showed that all the varieties grown in the open field demonstrated higher content of gallic acid and salicylic acid, which clearly shows the response of plants to stressful conditions. Significantly increased values of phenylalanine ammonia-lyase, guaiacol peroxidase, and syringaldazine peroxidase in pest-infested leaves of Modi cultivar compared to non-infested leaves of the same cultivar were observed. In contrast, an increase in the activity only of guaiacol peroxidase, but not the other two enzymes, was observed in the Gemini variety. The increased activity of the peroxidase enzymes correlates with the lower content of gallic acid and quercetin (one of the substrates for the action of the enzymes) in the infested leaves. Keywords: aphid, apple, phenols, peroxidase, phenylalanine ammonia-lyase, salicylic acid