Phytotoxic substances released by invasive plants have been reported to have anti-pathogen, anti-herbivore, and allelopathic activity. The aim of this study was to determine the allelopathic influence of the Ambrosia trifida L. on oxidative stress parameters (the lipid peroxidation process; reduced glutathione (GSH) content; and activity of antioxidant enzymes catalase (CAT), superoxide dismutase (SOD), and peroxidase (PX)) and phenolic compounds (total phenolic and tannin content) in maize (Zea mays L.), soybean (Glycine max L.), and sunflower (Helianthus annuus L.) crops to explore the effect of released allelochemicals through A. trifida root on crops. An analysis by HPLC confirmed the presence of protocatechuic acid, p-hydroxybenzoic acid, vanillic acid, and syringic acid as major components in the A. trifida. Based on the obtained results for oxidative stress parameters, it can be concluded that the sunflower was the most sensitive species to A. trifida allelochemicals among the tested crops. The other two crops tested showed a different sensitivity to A. trifida. The soybean did not show sensitivity, while the maize showed sensitivity only 10 days after the sowing.

The aim of this study was to determine the effect of the hydrolates obtained as the by-products of the Thymus vulgaris essential oil steam distillation process. The bioassays, which were undertaken in order to determine the effect on germination and initial growth of seedlings of some cultivated and weed species, were performed under controlled conditions with different concentrations of the hydrolates. Seeds of Glycine max, Helianthus annuus, Zea mays, Triticum aestivum, Daucus carota subsp. sativus, Allium cepa, Medicago sativa, and Trifolium repens, and six weed species—Amaranthus retroflexus, Chenopodium album, Portulaca oleracea, Echinochloa crus-galli, Sorghum halepense, and Solanum nigrum—were treated with 10, 20, 50, and 100% T. vulgaris hydrolate solution. The obtained results showed that the T. vulgaris hydrolate had the least negative effect on the germination of cultivated species, such as soybean, sunflower and maize, whereas clover and alfalfa were the most sensitive. By comparison, all the tested weed species expressed high susceptibility. It can be concluded that the T. vulgaris hydrolate has an herbicidal effect, in addition to its potential as a biopesticide in terms of integrated weed management.

Oxidative stress in soybean plants infected with Diaporthe isolates was evaluated in order to select (1) the least aggressive inoculation method, (2) to determine the most aggressive Diaporthe isolate, and (3) to determine the most tolerant soybean cultivar to this isolate. Based on the present malondialdehyde (MDA) content, the main end product of the lipid peroxidation process, and the biomarker for oxidative stress, the mycelium contact method was chosen as the least aggressive inoculation method, compared to the toothpick method and plug method. The activity of the antioxidant enzymes (superoxide–dismutase (SOD), catalase (CAT), and peroxidase (PX)), the reduced glutathione (GSH) content, and the level of lipid peroxidation (LP) were measured in soybean cv. Sava infected by five different Diaporthe species (DPM1F—D. aspalathi, DPC/KR19—D. caulivora, DPC004NY15—D. eres, 18-DIA-SOY-14—D. gulyae, and PL157A—D. longicolla). The most pathogenic Diaporthe species to cv. Sava was D. eres. The screening of the antioxidant enzymes activity in the leaves of 12 different soybean cultivars (Altona, Atlas, Capital, Chico, CX134, Favorit, Lakota, McCall, Morsoy, Strain, Rubin, and Victoria) infected with D. eres by the mycelium contact inoculation method showed that Capital, McCall, and Morsoy were the cultivars with the highest tolerance to D. eres, followed by Chico, Favorit, Lakota, and Rubin. The most sensitive cultivars were Atlas, CX134, Victoria, and Strain.

Oxidative stress in soybean seedlings and the length of the soybean stem lesions infected with the fungus Macrophomina phaseolina (Tassi) Goid were evaluated to determine the most tolerant soybean cultivar to this pathogen. The level of superoxide anion radical (O₂^•−) production, the activity of the antioxidant enzyme superoxide-dismutase (SOD), and the intensity of lipid peroxidation (LP) were measured in four soybean cultivars: Favorit, Atlas, Victoria, and Rubin. Results showed that O₂^•− radical production and SOD activity were the most elevated in the cv. Favorit inoculated with M. phaseolina, while the level of lipid peroxidation intensity was the lowest compared to the control. This indicates that the soybean cv. Favorit has managed to prevent infection with M. phaseolina. Furthermore, higher O₂^•− radical production and lower SOD enzyme activity were measured in cv. Victoria, with enhanced lipid peroxidation. This means that the cv. Victoria was infected with M. phaseolina, and was the most sensitive. None of the tested oxidative stress parameters showed a significant difference in the cvs. Atlas and Rubin compared to the control. Furthermore, the highest lesion length was measured in the cv. Victoria, followed by cv. Favorit, while the lowest lesion length was measured in the cv. Atlas followed by the cv. Rubin; and thus, the cv. Atlas followed by the cv. Rubin, were the most tolerant soybean cultivars to this pathogen.

Diaporthe spp. are among the most serious pathogens of soybean. Many different Diaporthe species can infect soybean plants. The species differ in their aggressiveness or virulence and in the severity of the damage they cause. Resistance breeding in soybean has been performed for only two Diaporthe species, so far. It would be very advantageous to identify soybean cultivars with resistance against other Diaporthe species as well, both as sources of resistance for breeding and to inform farmers which cultivars should be planted when a given Diaporthe species shows high incidence. We performed greenhouse experiments to differentiate levels of resistance using the Stem Cut and Stem Wound methods for inoculation of the plants with Diaporthe. Symptom severity was rated visually, and at 5 dpi the level of lipid peroxidation (LP), activity of superoxide dismutase (SOD), total phenolics and total flavonoids were measured. Among the four Diaporthe species tested, D. caulivora was most aggressive, followed by D. longicolla. Of the cultivars evaluated, Magnolia exhibited the highest level of resistance with no significant differences observed among the other cultivars. Although biochemical responses could be observed, it was impossible to determine the specific response responsible for elevated resistance in Magnolia.

Maize (Zea mays L.) and sunflower (Helianthus annuus L.) seedlings were treated with sodium cholate (NaC, concentrations 20, 40, 60, and 80 mg L−1), and the content of free phenolic acids in leaf samples was determined. The hypothesis of this work was that NaC causes oxidative stress in these seedlings and thus activates the plant’s defense response in the form of increased synthesis of secondary biomolecules, phenolic acids, which participate in the removal of free radicals and the mitigation of oxidative stress. The idea was that bile salt, with elicitor activity, could influence the preparation of the plant’s defense system for a subsequent stress. The determination of nine phenolic acids (gallic acid—GA, protocatechuic acid—PCA, chlorogenic acid—CHA, p-hydroxybenzoic acid—PHBA, caffeic acid—CA, vanillic acid—VA, syringic acid—SA, p-coumaric acid—p-CA, ferulic acid—FA) was performed by the HPLC method. The results showed increased concentrations of GA, PCA, VA, SA, p-CA, and FA in maize leaves treated with 60 and 80 mg L−1 NaC compared to the control, after the 5th and 7th days. In sunflower leaves, this pattern only appears in PCA and PHBA. It was also shown that CHA was the most abundant free phenolic acid in maize treated with NaC, and CHA, PHBA, and SA in sunflower. Concentrations of 40 and 60 mg L−1 NaC induced the highest amount of free phenolic acids in maize, and 20 and 40 mg L−1 NaC in sunflower.