This study assessed the leaf pigments and the activity of peroxidase, polyphenol oxidase, and phenylalanine ammonia-lyase enzymes in soybean local cultivars, i.e., Genetic-1, Tomaris, Baraka, Nafis, and Sochilmas, under the influence of the phytopathogenic fungus Alternaria alternata. In local soybean (Glycine max L.) cultivars during the budding and flowering periods, the A. alternata micromycetes influenced and decreased the chlorophyll a and b index compared with the control, validating the relationship of soybean leaf chlorophyll with the phytopathogenic micromycetes. Compared with the control variant, carotenoid content in the leaves of some soybean cultivars increased to varying degrees in variants with phytopathogenic micromycetes while decreasing in other genotypes to varying degrees. An outcome of the study revealed that the amount of leaf pigments in local soybean varieties infected with the phytopathogenic micromycete A. alternata is higher during the plant’s flowering than during its budding period. Compared with other soybean cultivars, the Baraka and Nafis appeared with enhanced peroxidase enzyme activity by the artificial exposure to the A. alternata fungus. The results further revealed that polyphenol oxidase enzyme activity also increased by 117.7% and 152.0% in the cultivars Tomaris and Baraka, respectively, under the influence of A. alternata micromycetes compared with the control. The activity of phenylalanine ammonia-lyase enzyme was higher in the soybean cultivars Tomaris and Nafis under the influence of A. alternata than in other cultivars and the control. The activity of the peroxidase, polyphenol oxidase enzyme, and phenylalanine ammonia-lyase heightened in all the soybean cultivars under the influence of the phytopathogenic micromycete A. alternata compared with the control treatment.

Keywords: Soybean (G. max L.), Alternaria alternata, budding, flowering, chlorophyll, carotenoid, peroxidase, polyphenol oxidase, phenylalanine ammonia-lyase, control, physiological and biochemical traits

Key findings: In the physio-biochemical defense system of the soybean (G. max L.), it was evident that a sharp increase of the protective enzymes occurred in resistant genotypes by showing considerable tolerance to the fungus A. alternata. Therefore, the local soybean cultivars Tomaris and Nafis were the choice resistant cultivars to A. alternata that can serve as base materials in future breeding programs to develop the soybean-resistant cultivars to alternariosis.