Autotoxic substances in the root exudates from continuous tobacco cropping

Effects of continuous cropping on the growth of tobacco seedlings were investigated in different soils of Central China. The continuous cropping of tobacco suppressed the tobacco growth and also drastically decreased the root activity than in rice/tobacco rotation in Liuyang. To explore the autotoxicity mechanism, allelochemicals in the root exudates of tobacco seedlings were measured by a gas chromatograph coupled to a mass spectrometer (GC-MS). In the root exudates of continuous cropping and rotation cropping of tobacco, 18 and 20 compounds were found respectively. Fourteen compounds were in higher amounts in continuous cropping than in rotation cropping. Phytotoxic effects of phthalate esters [dibutyl phthalate, diisooctyl phthalate and diisobutyl phthalate] were evaluated using tobacco seeds as test material. The phthalate esters at high concentrations (above 0.5 mM) inhibited the seed germination. At low concentrations, the inhibitory effects of diisobutyl phthalate were more serious than dibutyl phthalate and diisooctyl phthalate but there were no differences at high concentrations. Thus. phthalate esters from tobacco root exudates might play major role in tobacco allelopathy, in soils of the continuous cropping of tobacco.