Physiological Mechanisms Of The Absorption And Translocation Of Cadmium In Peanut Plants

This study aimed to investigate the physiological mechanisms of the absorptionand translocation of cadmium (Cd) in peanut plants. The responses of the rootmorphology, dissolved organic matters in the roots and leaves as well as xylem andphloem saps to Cd exposure, and the subcellular distribution of cadmium in the rootsand leaves of five peanut cultivars, including Haihua1, Luhua8, Qishan208, Xvhua13and Zhenghong3, were determined by hydroponics and pot experiments. Theirrelationships with the translocation of Cd within plants were analyzed. The main resultsare shown as follows:(1) A hydroponics experiment was conducted to determine the plant biomass, Cdaccumulation and root morphology of five peanut cultivars, at three Cd levels (0,2and20μM CdCl2). The results showed that, there are differences between plant growth androot morphology and Cd accumulation of different cultivars of peanut. Exceed Cdconsiderably in decreased the root lengths (RL), surface area (SA), specific root length(SRL) and number of root tips but increased the root diameters (RD). Cd significantlydecreased RL and SA in the0–0.2and0.2–0.4mm diameter classes. Peanut cultivarsdiffer in Cd accumulation and root morphological parameters. A positive correlationwas observed between RL and Cd amount in shoots. RD negatively correlated to Cdconcentrations in roots and shoots. Positive correlations were also found between RL vs.shoot Cd concentration, SA vs. Cd amount in shoots, SRL vs. root Cd concentration,SRL vs. shoot Cd concentration, and SRL vs. Cd amount in shoots. The fine roots play acrucial role in determining Cd accumulation in peanut plants. Cultivars with more fineroots in their root system (i.e. Haihua1and Zhenghong3) have high capability of Cdaccumulation.(2) To understand the translocation and distribution of Cd in peanut plants, thekinetics of Cd subcellar distribution in the roots of two contrasting peanut cultivarsdiffering in Cd accumulation. The results showed that for both cultivars, the kinetics ofCd in cell walls and symplasm (organelle and soluble fractions) could be described by Michaelis-Menten plus linear model, which is comparable to those of Cd uptake andaccumulation in plants. The kinetics for the proportion of Cd in soluble fractions alsofitted the Michaelis-Menten plus linear model, while for the proportion of Cd in cellwalls, it is characterized by the exponential linear combination model. The high-Cd-accumulating cultivar (Haihua1) showed a higher Vmax value for Cd uptake than thelow-Cd-accumulating cultivar (Qishan208). However, translocation factors were higherin Qishan208than that in Haihua1. This study confirms the existence of a low affinitytransport system, and verifies that a high proportion of Cd in the soluble fraction mayresult in a decrease of Cd translocation. The high shoot Cd concentration in Haihua wasdue to the higher Cd influx rather than Cd translocation.(3) A hydroponics experiment was conducted to determine the influence of Cd onthe dissolved organic matter in the roots and xylem sap of five peanut cultivars. Theresults showed that peanuts varied among cultivars and Cd concentrations in the solubleprotein, soluble sugar and free amino acids in the root. Free amino acids and solublesugars are positively correlated with shoot and root Cd concentrations, negativelycorrelated with TF. It is concluded that Cd may be combined with amino acids andpolysaccharides in root cells, which are fixed in the cell walls or transported and storedto vacuole. The main organic acids of roots and xylem sap are oxalic acid and EDTA.Positive correlations were found between roots Cd concentrations vs. oxalic acid andEDTA concentration of root, and xylem sap Cd concentrations vs. oxalic acid and EDTAconcentration of xylem sap. There are no significant correlations between amino acidsconcentration of xylem sap and shoot Cd concentrations. The results indicated thatoxalic acid and EDTA maybe bind the Potentially Poisonous Cd ions, reduce the activityof metal Cd ions in the metabolically active cell compartments. And this amino acidsinvolved in the Cd transport long-distance, but the complex is not the main form.(4) For the purpose of revealing the key processes that determine the variation ofseed Cd concentrations among peanut cultivars, Cd concentrations in different plantparts, the dissolved organic matter in the leaves and phloem of five peanut cultivarswere investigated different developmental stages. The results showed that shoot Cdcontent is reached the maximum at the pegging stage, and then, decreased, while TF isgradually declined. This indicates that the ability of uptake and translocation of Cd isdepending on peanut cultivars and developmental stages. There are correlations betweenCd concentration of seeds and soluble protein, free amino acids, soluble sugar, non-protein thiol concentrations of leaves and phloem sap. Positive correlation was observed between Cd concentration of seeds and the oxalic acid concentrations of leaves andphloem sap. This show that shoot Cd content, root Cd content, Cd content in phloem sapand oxalic acid will produce important influence for Cd accumulation in the grain.