Effects Of Low Molecular Weight Organic Acids On La,Gd And Y Accumulation And Transportation In Tomato And Phytolacca Plants

Recently, many advances had been obtained in researches on the effects of low molecular weight organic acids (LMWOAs) on heavy metals tolerance or hyperaccumulation in plants. It is well known that LMWOAs not only played important role in metals detoxification, but also participated in metals uptake, transportation and accumulation in plants which were important biological mechanisms of plant tolerance or hyperaccumulation of heavy metals. Little research had focused on the mechanisms of plants tolerance to rare earth elements (REEs), although many further studies were about heavy metals. To further study the mechanisms of LMWOAs on plants tolerance to REEs and to provide theoretic evidences on the phytoremediation of REEs contamination. We had designed a series of cultivated experiments, and mainly focused on:(1), Roles of REEs on organic acids concentrations in tomato plants (Lycopersicon esculentum);(2), Effects and relative mechanisms of extraneous LMWOAs on REEs accumulation and fractions and LMWOAs concentrations in tomato plants;(3), Roles of REEs on LMWOAs concentrations in phytolacca plants (Phytolacca americana L.);(4), Effects and relative mechanisms of extraneous LMWOAs on REEs accumulation and fractions and LMWOAs concentrations in phytolacca plants. The main results were shown as below:1. Concentrations of correlative LMWOAs in tomato and phytolacca plants tissues increased when exposed in rare earth elements solutions. Concentrations of malic, citric and succinic acids in phytolacca plants increased obviously than in tomato plants when plants were exposed at the same REE level. Higher concentrations of organic acids in phytolacca plants provided the possibility for REEs hyperaccumulation. Pearson correlation coefficients were calculated to determine the relationships between REE concentrations and LMWOAs concentrations in tomato and phytolacca plants tissues, separately. The results indicated that malic, citric and succinic acids played important roles in REEs accumulation and transportation in plants. 2. The fractionations of REEs in plants were investigated using solution culture of tomato and phytolacca plants with extraneous mixed REEs. The results showed that significant fractionations of REEs exist in plants organs. Gd (representative of middle REEs) enrichment was observed in roots, La (representative of light REEs) enrichment and Y (representative of heavy REEs) enrichment were found in stems and leaves, respectively.3. The effects of extraneous LMWOAs on accumulation, transportation and fractionation of the REEs were studied. The results indicated that extraneous LMWOAs could reduce the REEs accumulations in tomato and phytolacca roots under hydroponic condition. The REEs should be absorbed by plant roots mostly in the form of free ions. Extraneous LMWOAs could form steady complexes with REE ions, straightly holding back the diffusion of REE ions to plants roots, and finally lead to the obvious reduction of REE ions uptake in roots. While compared to tomato plants, extraneous LMWOAs additions promoted REEs transportation to aerial parts, especially accumulation in leaves in phytolacca plants more effectively. The results above revealed some hyperaccumulation feature of phytolacca plants.4. Through the influences of extraneous LMWOAs on the concentrations of LMWOAs in tomato and phytolacca plants, we concluded the mechanisms of REEs fractionations in plants:The fractionation pattern in plant root system was caused by cell walls phosphate precipitation. Under the neutral pH conditions, the precipitation process may take place easily between Gd (representative of middle REEs) ions and phosphates, also succinic acid had an important role in REE accumulation in roots. The transfer of REE ions to the aerial parts was mainly through the xylem vessels. Combination with ligands was an ideal way for REEs to transfer upwards. There was a large amount of LMWOAs in xylem vessels, and the complexation with REE ions was the main transportation way for REE ions in the xylems vessels. Based on our research, malic acid or citric acid can combine with Y (representative of heavy REEs) ions preferentially in xylem vessels of tomato and phytolacca plants and tranferred Y to the aerial parts of plants more conveniently. When Y ions were unloaded from the xylem to the leaf cells, succinic acid contributed to Y ions accumulation in leaves. While in these processes, more La (representative of light REEs) ions can be easily fixed in transportation route stems.The researches mentioned above had showed that LMWOAs played important roles in REEs accumulation, transportation and fractionation not only in nonhyperaccumulator tomato plants but also in hyperaccumulator phytolacca plants.