Silicon-mediated Alleviation Of Cadmium Toxicity In Tomato And Cucumber Seedlings

Heavy metal contamination in agriculture is increasingly serious because of speedingindustry development, excessive industrial emission, unreasonable usage of chemicalfertilizer and pesticide, and extensive usage of agricultural film in recent years. Due to highsolubility and mobility, cadmium (Cd) toxicity is widely focused by domestic and overseasscientists. Silicon (Si) is only less then oxygen element in terms of content in the earth’s crust,and has been shown to have the ability to alleviate biotic and abiotic stresses in plants. Manypapers reported Si-mediated alleviation of Cd toxicity in plants, but majority of them focusedon cereal plants such as rice and maize, and the mechanisms underlying are not fullyunderstood. There is limited information available on Si effects on Cd stress in vegetableplants. Tomato and cucumber are not only the favorious vegatables by consumers, but alsohave different abilities of Si uptake. In vegetable plants, tomato is a low Si accumulator whilecucumber is a high Si accumulator. In this paper, hydroponic cultivation was used to study theeffects of Si on Cd toxicity in tomato and cucumber seedlings, the aim being to elucidate thephysiological mechanisms for Si-mediated alleviation of Cd toxicity. The main results were asfollows:1. Cd stress induced deleterious Cd toxicity symptoms in both tomato and cucumberplants, such as stumpy roots, small leaves, chlorosis etc., and application of Si significantlymitigated the adverse effects. Compared with Cd treatment, Si+Cd treatment increased thetotal leaf number and biomass in tomato and cucumber. Thus, Si addition alleviated markedlythe inhibition of plant growth under Cd stress in tomato and cucumber.2. Supplement of Si decreased significantly the Cd concentrations in the shoot in tomatoand cucumber, especially in the leaves. However, under Cd stress, supply of Si had nosignificant effects on Cd concentration in the stem of tomato, whereas it increased the Cdconcentration in the stem of cucumber. Though addition of Si increased Cd concentration inthe stem, the Cd existed as Cd-compound with stronger binding force. Si supply resulted inthe decrease of Cd concentration in the root in cucumber, but there was not significantdifference between Cd and Si+Cd treatment in tomato root. The translocation factor (TF, Cdtranslocated from the root to the shoot) in tomato was2-3times higher than that in cucumber,showing the capacity of Cd transloacation is bigger in tomato than that in cucumber. Additionof Si significantly decreased the TF in tomato while it increased the TF in cucumber. In the root and leaf, Si supplement markedly decreased Cd concentration in the symplast in tomatoplants, and decreased both symplastic and apoplastic Cd concentrations in cucumber plants.Therefore, different mechanisms for alleviating Cd toxicity by Si-added existed in tomato andcucumber plants. The main function of Si supply was to restrict root-to-shoot translocation ofCd in tomato whereas it was to decrease the Cd uptake by roots in cucumber.3. Cd toxicity induced oxidative damage in the root and leaf of tomato and cucumberplants such as damage of membrane integrity, increase of lipid peroxidation index and H2O2content, and Si supplement significantly enhanced antioxidant defense capacity and increasedmembrane integrity index and decreased lipid peroxidation content and H2O2content.4. Cell wall plays an important role in compartmenting Cd in the plant cell. In this study,Cd toxicity obviously reduced the peak height in the polysaccharide fingerprint reagion, andSi addition increased the peak values. The phenomena show that Cd toxicity decreased thecontents of some polysaccharide, e.g. pectin, hemicellulose and cellulose, and supply of Siincreased the constituents. These Cd-and Si-induced changes were associated with the rootgrowth. We supposed that increased polysaccharide by Si was to enhance Cd resistancethrough alleviating Cd-induced inhibition of plant growth indirectly, rather than detoxifyingCd toxicity in plants directly.5. In the root, Cd stress increased malic-and citric acids contents and decreased tartaricacid contents in both tomato and cucumber plants. However, application of Si significantlyincreased the tartaric acid content. It was supposed that tartaric acid played mainly as animportant metabolite in plants rather than as a heavy metal ligand. The decrease of tartaricacid content implied that Cd toxicity damaged normal metabolism, whereas Si supplyincreased the tartaric acid content, suggesting that Si mitigated the disorder of metabolism.Under Cd treatment, application of Si decreased the malic-and citric acids contents in tomato,but it further increased the organic acids contents in cucumber. Since organic acids areinvolved in Cd long-distance translocation from the root to the shoot, the results wereconsistent with the decreased TF in tomato and increased TF in cucumber, showingSi-mediated changes in organic acids contents was related to Cd translocation. Cd toxicitydisturbed the nitrogen metabolism in plants, resulting in the changes of amino acids contents,and Si application could modulate the amino acids contents in tomato and cucumber plants,but the changes in amino acids contents were different depending on plants and tissues. Themechanisms of enhanced Cd detoxification by Si in relation to amino acids metabolism stillneed further study.