The Stress Responses And Tolerance Thresholds To Soil Lead, Cadmium Zinc Contamination In Centipedegrass Seashore Paspalum

As the rapid development of mining, metallurgy and manufacturing industry, a great deal of heavy metal contaminations has been discharged into the environment, which cause not only toxic effects to plants but also serious harm to human health through the food chain. How to eliminate the heavy metal contaminations in soil has becoming a hot spot and a tough problem worldwide. In recent years, the emergence and development of phytoremediation technology has provided a new approach for the treatement of heavy metal contamined soils by using the hyperaccumulators to stabilize, extract, and remove the heavy metal contaminations from the soil or water. In this study, two turfgrass species, centipedegrass (Eremochloa ophiuroides) and seashore paspalum (Paspalum vaginatum), were used to evaluate the growth and physiological responses under series of concentrations of Pb, Cd and Zn stresses in the pot experiments. The tolerance thresholds of Zn and Cd in two turfgrasses were also discussed. The main results were indicated as follows:(1) The physiological responses to soil lead contamination stresses at 3000mg/kg and 6000mg/kg were studied in a pot experiment for two turfgrass species: centipedegrass and seashore paspalum. The results showed that there were significant differences between two turfgrass species under the two lead stress regimes. Under 3000mg/kg Pb2+ stress, the leaf chlorophyll content, Fv/Fm and osmotic potential in centipedegrass did not show significant changes, while the electrolyte leakage (EL) showed significant increase at 12d, and the leaf relative water content (RWC) showed significant decrease at 24d. The visual turf quality maintained above the acceptable level (6.0) throughout the experiment period although a significant decline occurred after 4d. Under 6000mg/kg Pb2+ stress, all physiological indexes in centipedegrass showed significant changes at 4d, and plants completely dead at 12d. In contrast, under 3000mg/kg Pb2+ stress, seashore paspalum did not show any decreased in leaf Fv/Fm, osmotic potential and chlorophyll content, which were significant higher than the control at 12d, 16d, 24d. There were no significant differences in EL and visual turf quality between 3000mg/kg Pb2+ treatment and the control. Although a significant decrease occurred in leaf RWC at 12d, it recovered at 24d.(2) Two turfgrass species, centipedegrass and seashore paspalum under 8 different cadmium concentrations in a sand culture study were evaluated for their growth and physiological responses. The results showed that visual turf quality, leaf relative water content, electrolyte leakage, osmotic potential did not show significant changes under 180 mg/kg cadmium treatment, and relative growth rate, turf density, leaf chlorophyll content, and Fv/Fm did not show significant changes under 240 mg/kg Cadmium treatment compared to the control after 36d of treatment in centipedegrass. All tested growth or physiological parameters in seashore paspalum did not show the significant changes under 360 mg/kg Cadmium treatment compared to the control after 36d of treatment, except relative growth rate (which was 240 mg/kg). The critical cadmium tolerance concentrations for each parameter were calculated by the regression analysis and Least Significant Difference (P<0.05). The cadmium tolerance thresholds for centipedegrass and seashore paspalum were 182mgCd/kg and 254mgCd/kg, respectively.(3) Two turfgrass species, centipedegrass and seashore paspalum under 7 different zinc concentrations in a sand culture study were evaluated for their growth and physiological responses. The results showed that visual turf quality, vertical growth, turf density, electrolyte leakage, leaf chlorophyll content did not show significant changes under 400 mg/kg Zinc treatment, and Fv/Fm did not show significant changes under 800 mg/kg Zinc treatment compared to the control after 30d of treatment in centipedegrass. All tested growth or physiological parameters in seashore paspalum did not show the significant changes under 800 mg/kg Zinc treatment compared to the control after 30d of treatment. The critical zinc tolerance concentrations for each parameter were calculated by the regression analysis and Least Significant Difference (P<0.05). The zinc tolerance thresholds for centipedegrass and seashore paspalum were 415mgZn/kg and 873mgZn/kg, respectively.(4) The absorption and distribution characteristics to soil cadmium and zinc contamination (single stress) were studied in a sand culture experiment in centipedegrass and seashore paspalum. The results showed that the Cd concentrations in roots of both turfgrass species were higher than that in shoots. The Cd accumulations in roots of centipedegrass were lower than that in shoots, as the results in seashore paspalum were opposite. The Zn concentrations in roots were as well in shoots, and the Zn accumulations in shoots were greater than that in roots in both turfgrass species. The translocating and accumulating abilities of Cd and Zn from root to shoot of centipedegrass were higher than seashore paspalum.