Arsenic-contaminated soils. Phytotoxicity studies with sunflower and sorghum

Environmental pollution caused by arsenic (As) is a major ecological problem. There has been intense worldwide effort to find As-hyperaccumulating plants that can be used in phytoremediation - the green-plant-assisted removal of chemical pollutants from soils. For phytoremediation, it is natural to prefer cultivated rather than wild plants, because their agriculture is well known. This study was conducted to evaluate the tolerance of common sunflower (Helianthus annuus L.) and sugar sorghum (Sorghum saccharatum Pers.) for soil-As contents of 10-100 mg As kg-1 soil, with sodium arsenite as a model contaminant. Plants were grown in a growth chamber for 30 days. Microfield experiments were conducted on experimental plots. To study the phytoremediation effect of the auxins indole-3-acetic acid (IAA) and 2,4-dichlorophenoxyacetic acid (2,4-D), we treated 1- and 3-day-old plant seedlings with water solutions of the auxins (concentrations 10-5, 10-7, and 10-9 g 1-1). The soil and plant-biomass samples were analyzed for total As by using the color reaction of ammonium molybdate with As. Phytotoxicity studies showed that 100 mg As kg-1 soil poisoned sunflower and sorghum growth by 50%. There was a linear correlation between soil-As content and As accumulation in the plants. Laboratory experiments showed that the soil-As content was reduced two- to threefold after sunflower had been grown with 10-100 mg As kg-1 soil for 30 days. Treatment of sunflower and sorghum seedlings with IAA and 2,4-D at a concentration of 10-5 g 1-1 in microfield experiments enhanced the phytoremediation two- to fivefold as compared with untreated control plants. The best results were obtained with 3-day-old seedlings. ] The conclusions are: (a) Sunflower and sorghum are good candidates to remediate As-polluted soils. (b) Phytoremediation can be improved with IAA or 2,4-D. (c) Mixed cropping of sorghum and sunflower may be another way of improving phytoremediation.