The crucial role of sulfur in a phytoremediation process : Lessons from the Poaceae species as phytoremediants : a review

Among living organisms plants are the most tolerant to pollution. This fact, emphasizes their utility for phytoremediation, a promising technology for environmental cleanup. Many botanical families include multiple species which exhibit a notable phytoremediation potential. Each of these species enjoys certain advantages, but at the same encounters some limitations with respect to its application as phytoremediants. Careful selection of the appropriate family and genotype to match the particular pollutant and environment is crucial for successful phytoremediation; the Poaceae family seems to be one of the most important for this technology. Phytoremediation of inorganic and organic pollutants largely depends on sulfur (S) metabolism. Therefore, S sufficient conditions and efficient S metabolism constitute the prerequisites for the effectiveness of the process. In the modern societies, heavy-metal pollution of soils is causing ever greater problems, exacerbated by the fact that most heavy metals accumulated in plants may, either directly or indirectly, find their way into animals and human beings. Wheat, rice and maize are among the world’s most prominent crops, whilst cadmium (Cd) and nickel (Ni) are among the most toxic metals. Therefore, the physiological adaptations induced in cereals, by Cd for example, are alarming and subsequently, the responsiveness of these species has been studied extensively as model plants. Studies with Arundo donax as phytoremediant have proven that this species is tolerant to increased concentrations of Cd and Ni in its rhizosphere, and as a consequence, it can be cultivated in contaminated areas for phytoremediation and energy production purposes. S-deficiency of crops is frequently reported, especially during the past two decades, worldwide. The main reasons for this deficiency are: the reduction of sulfur dioxide emission from power plants and various industrial sources, the ever increasing use of high-analysis low-S-containing fertilizers and the decreasing use of S-containing fertilizers, S-containing fungicides, pesticides and high-yielding varieties. A sufficient S supply of cereal crops is required in early growth stages, which cannot be fully compensated by S fertilization during later growth. Elemental S proved to contribute continuously to the crop’s S nutrition due to its constant release after oxidation. Therefore, the early diagnosis of S deficiency, the profiling of S forms in the soil of the phytoremediation site and the prediction of S-fertilizer requirements are of key importance for achieving a successful process.