Allelopathic role of essential oils in sunflower stubble on germination and seedling growth of the subsequent crop

The phytotoxic effect of sunflower extracts prepared by different plant parts on germination and seedling growth of potentially crop rotational plants like wheat (Triticum aestivum L.), barley (Hordeum vulgare L.), lentil (Lens culinaris Medik.) and sunflower (Helianthus annuus L.) were studied. Stem and head extracts of sunflower cv. Sanbro at 2.5, 5 and 10% concentrations were obtained from sunflower residuals after harvest. Germination percentage, mean germination time, root and shoot length, seedling fresh and dry weight were investigated. Also, electrical conductivity (EC) and pH values were measured at each concentration and essential oil compounds of both stem and head extracts were determined. Results showed that germination and seedling growth of the species were significantly influenced by sunflower extract doses. Barley appeared to be more tolerant to phytotoxicity of sunflower, while sunflower was the most susceptible. Higher doses of extract inhibited ruination and seedling growth of the investigated plants. EC values of both extracts were raised by increasing level of extracts without significantly changing pH. The main essential oil compound of head extract was alpha-pinene with 54.70% followed by Calarene with 10.82% while Benzofuran (13.65%), palmitic acid (13.52%) and trans-verbenol (11.89%) were mostly found in stem extract. There were apparently differences between stem and head extracts of sunflower in terms of toxicity and stem extract had less inhibitory effect on germination and seedling growth of target plants than head. It was concluded that the inhibitory effect of head extract was mainly due to higher EC values and alpha-pinene in essential oil composition and removing the head of sunflower should be beneficial for alleviating the allelopathic effect.