Allelopathic Potential of Sweet Sorghum Root Exudates and Identification of the Relevant Allelochemicals

This study determined the influence of cattle manure compost, chemical fertilizers, and mulch on the growth of weeds, sugar content, and growth of sweet sorghum (Sorghum bicolor (L.) Moench). The inhibitory potential of root exudates from two sweet sorghum cultivars (A; K1151 and B; K3351) was also evaluated. Chemical fertilizers increased the plant height, stem weight, biomass production, and sugar content of sweet sorghum. The total phenolic contents in the root exudates were 22.93 mg gallic acid equivalent per g dry weight (GAE/g DW) for cultivar A and 15.66 mg GAE/g DW for cultivar B. The total flavonoid contents in the root exudates were 14.77 mg rutin equivalent per g dry weight (RE/g DW) for cultivar A and 12.44 mg RE/g DW for cultivar B. The leaf extracts contained a higher amount of total phenolics and flavonoids than that of the stem and root. The inhibitory level of the root exudates from cultivar A on the seed germination and shoot growth of lettuce was greater than for cultivar B. Six phenolic acids, including protocatechuic, p-hydroxybenzoic, syringic, sinapic, p-coumaric, and benzoic acids, were detected from root exudates, root, stem, and leaf of both cultivars. The amount of p-coumaric acid in root exudates was greater than the other plant parts; however, protocatechuic acid was only found in the root exudates. p-Coumaric and protocatechuic acids may play an important role in the allelopathy of sweet sorghum to help reduce the dependence on synthetic herbicides in agricultural practice. This study indicates that cultivation methods and fertilization are important to increase both agronomic and economic values of sweet sorghum in agricultural production.