Studies On Physi-Ecological Responses And Accumulation Characteristics Of Swamp Cabbage (Ipomoea Aquatica Forsk.)to Lead And Cadmium

Lead (Pb) and Cadmium (Cd) are important contaminants in soil and bring great threaten to agro-ecological environment with the development of industry and agriculture. It had definitely put great threaten to the agricultural production in many areas. Pb, Cd uptake and accumulation in the stem and leaf for Swamp cabbage under different types of soils with added Pb and Cd were studied using potted experiment, and their environmental critical values for Swamp cabbage (Ipomoea aquatica Forsk.) production area with high security were analyzed as well. A further research on the effect of Pb, Cd stress on the physiological and biochemical characteristics were also studied. The objectives of this study were for acquaint the knowledge on the physiological response of Swamp cabbage to Pb and Cd, and offer the theoretics for the green vegetable production and the assessment of agricultural producing area. The main results were as followings:1. The growth of Swamp cabbage was significantly inhibited under high Pb concentrations (600and900mg·kg-1). The biomass in the900mg·kg-1Pb treatment was only32.4%of that in the control. Soil Cd stress significantly inhibited root growth, but did not affect the aboveground biomass. The net photo synthetic rate (Pn), stomatal conductance (Gs), transpiration rate (Tr), and water use efficiency (WUE) decreased with the increasing Pb concentrations. However, the intercellular CO2concentration (Ci) showed an increasing trend, indicating that the reduced net photo synthetic rate of Swamp cabbage under Pb stress was caused mainly by non-stomatal factors. Nevertheless, the responses of these parameters to Cd stress were just opposite to those of Pb. Pb and Cd stresses decreased the contents of total cholrophyⅡ, cholrophyⅡ a, and cholrophyⅡ b in leaf, which could be an important factor for photosynthesis inhibition under Pb and Cd stresses. The contents of total cholrophyⅡ, cholrophyⅡ a, and cholrophyⅡ b in the900mg·kg-1Pb treatment decreased by26.34%,24.43%, and31.40%compared to the control, up to26.59%,28.42%, and24.61%in the7mg·kg-1Cd treatment, respectively. The decreasing of chlorophyll content may be one reason for the decline of Pn of Swamp cabbage. Furthermore, Pb and Cd stresses significantly reduced the potential efficiency of photosystemⅡ (Fν/Fm) and the photosystemⅡ quantum efficiency (ΦPSII), suggesting that the destruction of the structure and function of photosystemⅡ (PSII) in Swamp cabbage’s leaf may be another key reason for the restraint of leaf photosynthesis under soil Pb and Cd stresses.2. The content of malondialdehyde (MDA) in leaf increased with increasing Pb and Cd concentrations. The MDA contents under the highest concentrations of Pb and Cd increased by12.9%and29.5%compared to the control, respectively, indicating that swamp cabbage was more resistant to Pb than Cd. The activities of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) in leaf of swamp cabbage increased initially and then declined with the growing Pb concentration. Our results indicated that the three anti-oxidative enzymes showed remarkably synergistic effects under low concentrations of Pb than Cd, and increased the adaptation of swamp cabbage to heavy metal stresses.3. The cumulative amounts of Pb and Cd in the stem were significantly higher than those in the leaf. For example, the average levels of Pb in the stem were3.86and2.02times higher than those in the leaf, and the average levels of Cd in the stem were3.58and4.73times higher than those in the leaf, respectively, in the two soils. Pb and Cd in the Paddy soil were easily enriched by the crop than in the Alluvial soil. For instance, the Pb concentrations in the stem and leaf of Swamp cabbage in the Paddy soil were4.88and2.56times higher than those in the Alluvial soil, and Cd concentration were3.04and4.02times higher than those in the Alluvial soil, respectively, under the same treatments. Based on the National Standards for Maximum Levels of Contaminants in Foods (GB-2762-2005), the environmental critical values of soil Pb in the Paddy field was calculated as57.83mg-kg-1, and the environmental critical values of soil Pb and Cd concentrations in the Alluvial field were calculated as77.02mg-kg-1and0.47mg-kg-1for Swamp cabbage safely cropping, respectively. The critical values were higher than those in National Standards of Soil Security (HJ332-2006).