Phytotoxicity of black cumin (Nigella sativa), dragonhead (Dracocephalum moldavica), dill (Anethum graveolens), and soybean (Glycin max) residues on emergence and establishment of wheat

Allelopathic effects of plant residues is an important research avenue regarding optimization of rotation systems in agronomy. The aim of this study was to investigate the allelopathic effects of four plant residues, namely, black cumin, dragonhead, dill, and soybean on the germination and growth of wheat (Triticum aestivum) in different cropping systems. Results showed that application of organic manure for previous crops reduced the residue phytotoxicity and consequently alleviated the adverse effect of plant residues on the leaf area, length, and dry weight of the wheat root, affecting chlorophyll a, chlorophyll b, and carotenoids of wheat seedling leaves. In the presence of plant residue, the length and dry weight of the wheat roots were more negatively affected in comparison with shoots. The greatest allelopathic inhibition was observed for the wheat cultivated in the residue of black cumin, but soybean, dill, and dragonhead residues also potentially showed inhibition effects. It can be concluded that agroecosystems in which autumn wheat is in the rotation should be avoided where there are residues of soybean, black cumin, dragonhead, and dill. The tillage system in the same condition may not be agronomically suitable because of the allelopathic effects of previous crops.