Effects Of Zn Stress On Rhizospheric Mechanism And Zn Accumulation In Different Species And Varieties Of Plant

Plants growing under high levels of heavy metal tend to develop biochemical and molecular mechanisms to tolerate the effect of heavy metal stress, but apparently differences of adaptability and resistance were found among different species and different varieties of plant. The solution cultivation experiment and the pot experiment (root bag method) were carried out to investigate the influence of Zn stress on plant growth, biomarkers, Zn uptake and accumulation, root exudates among four varieties of ryegrass (Lolium perenne L.) (Monocotyledon plant) and six varieties of Chinese cabbage (Brassica campestris L. ssp. Pekinensis (Lour) Olsson) (dicotyledonous plant), and Zn fractions in soil. The main results are as following:1 Different species and varieties of plant showed different response to Zn. Zn (at Znconcentrations of 0-1 mmol/L in solution, at Zn concentrations of 0- 4 mmol/kg in soil) stimulated the growth of ryegrass. The maximums of plant biomass in shoots and roots were increased by 34.3-72.8%, 78.3-151.2% in pot experiment, respectively. Higher Zn concentration (Zn²⁺≥2.0mmol/L in solution;Zn²⁺≥8mmol/kg in soil) began to inhibited the growth of ryegrass. Ryegrass showed high tolerance to Zn stress, the highest tolerance among the varieties of ryegrass was found in Taide, only at high concentration of Zn (at Zn²⁺≥8 mmol/kg in soil ), its growth was inhibited. La solution experiment, the plant growth of Chinese cabbage was significantly decreased under Zn stress (The maximum decreasing rates of plant biomass in shoots and roots were at 58.9-89.8%, 67.0-85.4%, respectively). RTI of the six varieties of Chinese cabbage was at 0.5-1.0. In pot experiment, Zn (at Zn concentrations of 0-0.5mmol/kg in soil) stimulated the plant growth of Chinese cabbage, the maximum increasing rates of plant biomass were at 4.7-30.2%, 10.3-25.1%, respectively, and high tolerance of Chinese cabbage (RTI>1.0) was recorded at Zn ≤ l.0mmol/kg in soil. Fengkang78 and Fengyuangaokangl showed higher tolerance and growth than the other varieties of Chinese cabbage under Zn stress.2 Significant differences of biomarkers of plant were found among different species and varieties of plant. Plants produce defensive response to minimize biochemical change and damage from exposure to heavy metal stress. SOD and POD can protect cellular membrane from being destroyed by heavy metals, phytochelatins (PCs) participates in detoxification to heavy metals, accumulation of proline can keep cellular from dehydration and stabilize the macro-biological molecular structure, and the size of root system vigor can reflect tolerance of plants to heavy metal stress. In our experiments, ryegrass showed high resistance to Zn stress. SOD and POD activity, proline content in shoots, root system vigor of ryegrass were increased with increasing Zn concentrations. SOD and POD activity,proline in shoots, root system vigor of Taide are evidently higher than the other varieties at each treatment of Zn. Chinese cabbage showed low resistance to Zn stress. SOD and POD activity, proline content in shoots, root system vigor of Chinese cabbage were decreased under Zn stress. Higher POD and SOD activity, higher proline content in shoot were the important reason why ryegrass exhibits stronger resistance to Zn stress. Therefore, anti-oxidant enzymes could be used as biomarkers for selecting plants in phytoremediation of low- or moderate-grade heavy metal contaminated soils.3 Different species and varieties of plant showed different trends of nutrient content in plant and Zn absorption by plant under Zn stress. One of toxicity mechanisms of heavy metal to plants was to interfere nutrient absorption by plant and balance in plant. High Zn stress decreased N, P and K contents in plant of ryegrass. Zn contents and transport rates of Taide were higher than the other varieties of ryegrass. The maximums of Zn contents in shoots and roots of Taide were found by 2600 mg.kg"1 DW (in solution experiment), 1036.4 mg.kg"1 DW (in pot experiment), respectively. By reason of higher plant biomass, easily cultivated, grows rapidly up, higher tolerance and resistance to heavy metals, high Zn contents and transport rate in plant, ryegrass is a cheap and effective plant material for phytoremediation of low- or moderate-grade heavy metal contaminated soils. N, P and K contents in plant of Chinese cabbage were recorded by complex changes along with increasing Zn concentrations. Zn contents in shoots of Fengyuangaokangl and Fengkang78 were higher than the other varieties of Chinese cabbage, and Zn contents in roots were lower than the other varieties of Chinese cabbage under Zn stress. Contrarily, Zn contents in roots of Fengkang80 and Fengkang90 were higher, while Zn contents in shoots were lower than the other varieties of Chinese cabbage under Zn stress. Zn contents in edible parts (shoots) of Chinese cabbage were at 7.5-17.1 mg.kg'1 DW (the safe limit standard of Zn in edible parts was within 20 mg.kg"1 DW) in pot experiment. The maximum or high Zn contents in edible parts were found in Fengyuangaokangl and Fengkang78 (Zn contents in shoots were at 16.7 or 17.1 mg.kg"1 DW in pot experiment). It suggested that Fengyuangaokangl and Fengkang78 can be used as vegetables of high Zn accumulating to plant in no adding Zn areas, while Fengkang80 and Fengkang90 recorded lower Zn transport rates and accumulations can be used to plant in heavy metal pollution areas.4 Different changes of available N, P and K contents in rhizosphere or non-rhizosphere were found under Zn stress. Available N, P and K contents in ryegrass rhizosphere, available N contents in ryegrass non-rhizosphere were all decreased, available P and K contents in ryegrass non-rhizosphere were decreased at first, then increased with increasing Zn concentrations in soil. It suggested that pollution of Zn decreased bio-available nutrient contents in rhizosphere. Available N and K contents in Chinesecabbage rhizosphere and non-rhizosphere were decreased at first, and then increased with increasing Zn concentrations in soil. Available K contents in Chinese cabbage rhizosphere and non-rhizosphere were distinctly increased possibly caused by decreasing of nutrient absorption by plants under Zn stress. Available P contents in Chinese cabbage rhizosphere and non- rhizosphere were increased or increased at first, and then decreased with increasing Zn concentrations in soil. The higher of available N, P and K contents in rhizosphere were found at the treatments of the maximum plant biomass or higher plant biomass in pot experiments. These results were corresponding to the changes of N, P and K contents in plant with increasing Zn concentrations in soil.5 Zn fractions and contents in soil were changed under Zn stress. In the Zn treatments, the contents and the proportions in total Zn of exchangeable, Mn-oxide, organically bound, carbonate bound and residual fraction were all increased, the contents and the proportions of crystalline Fe-oxide were decreased. Contents of amorphous Fe-oxide bound were all lower than 0.01. Residual fraction was the major fraction of Zn in soil. The total Zn in rhizosphere or in non- rhizosphere was followed a tendency of Airuisi>Taide, Fengkang80> Fengyuangaokangl.The sum of exchangeable, Mn-oxide and organically bound (available Zn) in rhizosphere was followed a tendency of Taide > Airuisi, Fengyuangaokangl> Fengkang80. It indicated that these varieties of plant possessed higher Zn absorption have high efficiency to Zn in soil, and high bio-available Zn reserve in their rhizosphere. The sum of exchangeable, Mn-oxide and organically bound determined by method of Jiang (1990) was in accordance with available Zn in soil by distilled with 0.1 mmol. L'1 HC1, and assayed by atomic absorb method. The three fractions of Zn can be treated as bio-available Zn in soil.6 Zn stress promoted or inhibited roots of plant to excrete low-molecular-weight organic acids and amino acids. The different levels and species of root exudates of plants were found among different species and varieties of plant under Zn stress. Oxalic acid, tartaric acid, malic acid and succinct acid (low-molecular-weight organic acids) were detected from root exudates of ryegrass (monocotyledon plant). Major amino acids of root exudates of ryegrass were poline, glutamic acid, aspartic acid ( C:>500u g/lOOml). Low-molecular-weight organic acids of root exudates of Chinese cabbage (dicotyledonous plant) were menstruated of oxalic acid, tartaric acid and malic acid. Major amino acids of root exudates of Chinese cabbage were serine, isoleucine, valine and lysine ( C^400 u g/lOOml). Total contents of low-molecular-weight organic acids and amino acid in ryegrass rhizosphere under Zn stress were higher than in ryegrass rhizosphere of no Zn treatment, while total contents of low-molecular-weight organic acids and amino acid in solution of Chinese cabbage under Zn stress were lower than in solution of no Zn treatment in Chinesecabbage. It directly interpreted why Zn absorption and efficiency to Zn in soil by ryegrass were higher than Chinese cabbage. Whatever at Zn treatments or no Zn treatments in soil, in rhizosphere or in non- rhizosphere, total contents of low-molecular-weight organic acids and amino acids were followed a tendency of Taide > Airuisi, whatever at Zn treatments or no Zn treatments in solution, the low-molecular-weight organic acids and amino acids were followed a tendency of Fengyuangaokangl > gongguan5. Root exudates can affect fractions and bioavailability of heavy metal in soil by chelating and dissolving with heavy metals. Low-molecular-weight organic acids and amino acid play an important role in available Zn contents in soil and Zn absorption by different species and varieties of plant.