There has been a growing interest in essential oils (EO) as natural botanical herbicides. However, the EOs also display phytotoxic effects against crops. Hence, the study aimed to assess the phytotoxic potential of the essential oil obtained from the hyssop's herb (Oleum hyssopi officinalis) concerning the germination and initial growth of seedlings of two crops: spring wheat (Triticum aestivum L.) and white mustard (Sinapis alba L.). The EO was obtained by steam distillation and subjected to component analysis using the GC/MS method. Two series of laboratory experiments were performed. Oil in water (o/w) solutions of EO with an admission of ethyl alcohol (2.0% v/v) were used in five doses: 0.004; 0.007; 0.01; 0.02, 0.03 g. liter-1 (w/w). Twenty seeds of the tested crops were placed on the Petri dishes lined with a sterile filter paper, previously soaked with the o/w solution. The dishes were left in the dark at room temperature for seven days. The seedlings were then counted, and the length of their roots and stems (in mm) was measured. Statistical analysis was performed based on the one-way analysis of variance; the ED50 dose was calculated using the R program's drc package (version 3.5.3).

In the hyssop herb's EO, 37 chemical compounds (93.4% in total) were identified, among which terpenes (camphor and β-pinene) dominated. Higher doses of the EO completely inhibited the germination of wheat and white mustard. The ED50 was 0.027 g liter^-1 for mustard and 0.016 g liter^-1 for wheat, respectively. Similarly, the growth of wheat seedlings was inhibited to a greater extent than that of mustard seeds. Simultaneously, the level of tissue damage in both species was independent of the EO dose. In conclusion, wheat is more sensitive than mustard to the effects of hyssop EO. The physiological basis of the reaction of seedlings of both species to hyssop OE will be further investigated.