Previous studies in our laboratory showed that vetiver grass (Vetiveria zizanioides L.) can accumulate upto 23,000 mg lead (Pb) /kg tissue in a hydroponics set-up. We were able to identify, characterize, and quantify a range of phytochelatins (PC_n), a class of metal-binding peptides, in vetiver using high pressure liquid chromatography coupled with electrospray ionization tandem mass spectrometry (HPLC-ESI-MS). In addition to PC_n, low molecular weight thiols have been reported in literature as antioxidants that facilitate metal detoxification. Following the basic incubation study, a greenhouse experiment was conducted to elucidate the efficiency of vetiver grass (with or without chelating agents) in remediating Pb-contaminated soils from actual residential sites where Pb-based paints were used. Because the primary factor affecting Pb phytoavailability in soils is soil pH, we used two soil types widely varying in pH that have total Pb concentrations above 1500 mg /kg soil. Lead-contaminated, low pH, acidic soils were collected from residential sites in Baltimore, MD and high pH, alkaline soils were collected from residential sites in San Antonio, TX. Based on the soil characterization results, two most appropriate soils (one from each city, having similar Pb levels but variable soil physico-chemical properties) were selected for this study. The levels of PC_n and total thiols were measured in vetiver root and shoot following Pb exposure. Results indicate that PC_n are involved in the tolerance mechanism of Pb toxicity in vetiver grass. Biosynthesis of PC_n and thiols shows a linear correlation with Pb concentration in vetiver tissues, which in turn, is a function of soil properties.

Keywords: Chelate-aided phytoremediation, Lead, Metal-binding proteins, Thiols, Soil properties