Effect of coumarin on Sorghum sudanense seed germination and seedling growth

Coumarin is the main allelochemical of Melilotus officinalis, and can inhibit seed germination and seedling growth in a variety of weeds. In this study, we aimed to explore the allelopathy mechanism of coumarin and its inhibition of grass. Petri dish experiments were conducted on Sorghum sudanense to study the effect of coumarin on the main growth and physiological and biochemical indices, and to observe ultrastructural changes in the roots and stems by transmission electron microscopy. The following results were obtained. Coumarin concentrations in the range of 20 to 100 μg·mL-1 significantly (P<0.05) inhibited the early germination of S. sudanense and the degree of inhibition increased with an increase in coumarin concentration. Coumarin solution significantly (P<0.05) inhibited root length, and with an increase in treatment time and coumarin concentration, the roots became more coarse and distorted. There were concentration and time effects on stem growth. At 4 d, a coumarin concentration of 20~100 μg·mL-1 promoted stem length, whereas at 6 and 8 d, a low coumarin concentration promoted stem length and a high concentration inhibited stem length. A 100 μg·mL-1 coumarin solution significantly (P<0.05) inhibited the growth of S. sudanense, and the germination rate, root length, and stem length were all significantly (P<0.05) lower than in the control group; Coumarin solution significantly (P<0.05) increased the activity of catalase and reduced the activity of superoxide dismutase, whereas the activity of the peroxidase initially decreased and then significantly (P<0.05) increased. Coumarin solution significantly (P<0.05) enhanced the content of soluble sugars and reduced the content of chlorophyll. Furthermore, treatment with coumarin initially decreased and then increased the content of starch, and the activity of indole acetic acid oxidase in coumarin-treated plants was lower than that in the control group. Treatment with coumarin solution destroyed the ultrastructure of Sudan grass root cells, with vacuole volume becoming larger, the number and types of organelles decreasing, the nuclear membrane being distorted, and the nuclei becoming smaller or even disappearing. Coumarin treatment also significantly (P<0.05) increased the number of starch granules in the chloroplast of S. sudanense and their volume became larger. The above results indicate that 100 μg·mL-1 coumarin solution can regulate the growth of S. sudanense by changing antioxidant enzyme systems, increasing osmotic adjustment substances, decreasing chlorophyll content, affecting enzyme activity, and destroying cell ultrastructure.