Absorption of different sorghum accessions on heavy metals in polluted soil

Sweet sorghum, which is used to produce alcohol and energy, hardly enters human food chain. This purpose combined with powerful root uptake ability makes sweet sorghum a low-cost, safe and effective plant in removing heavy metals (HMs) from polluted soils. In this study, eight sweet, two forage and one grain sorghum accessions planted in polluted soil were evaluated and compared with those planted in unpolluted soil. Eight HMs were evaluated in different plant organs to characterize HMs uptake ability of different sorghum accessions. The results showed that contents of Hg, Cd, Mn and Zn in sweet sorghum accessions were significantly different in polluted and unpolluted soil. However, the contents of Co, Cr, Pb and Cu were not significantly different. Mn content in sweet sorghum accessions in unpolluted soil was higher than that in polluted soil. Zn content was much higher in leaves and spikes than in stems and roots of plants from unpolluted soil, much different from that of plants from polluted soil. HMs contents varied in different sweet sorghum organs. For instance, there were high contents of Hg, Cd, Co, Cr and Zn in plant root while high contents of Cu, Mn and Pb were in plant spike in polluted soil. Significant differences existed in uptake, transfer and accumulation of HMs both among different accessions and HMs. Forage sorghum 'Wancao 2' had more Cr and Zn in leaves while 'Jinzhong 0823' grain sorghum accession had more heavy metals in stems than other accessions. High HMs enrichment coefficients were existed in the root of sorghum, which ranged from 0.02 (for Pb) to 0.23 (for Cd), and transfer coefficients were 0.21 (for Co) to 3.42 (for Pb). Significant differences existed among accessions in terms of certain HM uptake. Enrichment coefficients of Co, Cr, Cu, Mn, Pb and Zn in root of 'Ximeng' sweet sorghum were high. Also enrichment coefficients of Hg, Cd, Mn, Pb and Zn in stem of 'Jinzhong 0823' grain sorghum were high. The ability of HMs enrichment and transfer was not consistent in sorghum accessions, such as 'Lüneng No.1' sweet sorghum had a high transfer coefficient for several HMs, only Zn had high transfer coefficient in 'Jinzhong 0823' grain sorghum. Thus the result above suggested that the enrichment and transfer of HMs varied with HM types in sweet sorghum. Sweet sorghum first absorbed Zn and then accumulated it in leaves and spikes, and then in stems and roots as Zn content of the plant was high enough. Mn uptake of sweet sorghum competed with other HMs. Hg was seldom transferred to above-ground parts after its uptake by sweet sorghum, while Cu, Mn and Pb mainly accumulated in sweet sorghum spikes. 'Ximeng' sweet sorghum had a rich accumulation of several HMs in roots, while large amounts of various HMs accumulated in stems of 'Jinzhong 0823' grain sorghum accession. There was no specific pattern of HMs uptake in forage and sweet sorghum accessions. Therefore, sorghum plant could be used to remove HMs from polluted soils. This could be more efficiently done by employing accessions with high HM enrichment and transfer ability.