Study On Physiological Mechanism Of Tolerance To Manganese Toxicity In Phytolacca Acinosa

Phytolacca acinosa is a recently discovered manganese (Mn) hyperaccumulator from Southern China. In this study, the biomass, root growth and Mn accumulation of P. acinosa were measured to investigate its response to Mn stress. The photosynthesis, antioxidant enzyme activities and the content of mineral elements including potassium (K), sodium (Na), magnesium (Mg), calcium (Ca), zinc (Zn), iron (Fe) under Mn stress were also investigated. The main results are as follows:1. The effects of excess Mn on the growth of P. acinosaNo visual toxic symptoms were observed until the second week, although Mn was significantly accumulated in shoots. Compared with the control, plants treated with excess Mn kept high aboveground biomass in earily period and were only slightly affected in root activities. Both the root biomass and the growth rate were even promoted in the first two weeks under 2 mmol/L Mn level. These results indicated that P. acinosa exhibited high Mn tolerance and accumulated Mn within shoots to keep root in high activities.2. Response of photosynthesis to Mn stress in P. acinosaCompared with the control, the photosynthesis rate in 2 mmol/L Mn-treated plants was higher at the first week, and kept 80%-90% of the control in the second week and third week for both the 2 mmol/L and 8 mmol/L Mn-treated plants. Pn was not remarkablely changed in P. acinosa when accumulating excess Mn. As for the fluorescence parameters, Fv'/Fm' decreased and NPQ increased under high Mn, while Fv/Fm and q_p changed little. These results indicated that there was no damage in the PS II action centre and the excitation energy absorbed by PS II antenna complex could be dissipated before arriving PS II action centre.3. Response of antioxidant enzymes to Mn stress in P. acinosaMDA content in the Mn treated plants was not significantly increased in the first two weeks, suggesting that no membrane damage was caused by excess Mn which might be attributed to the balance between the oxidant and antioxidant systems. CAT and APX activities increased remarkably only under 8 mmol/L Mn while SOD activity were elevated in both treatments. These results indicated that P. acinosa could up-regulate the activity of SOD,CAT and APX to scavenge active oxygen free radicals.4. Mineral contents in P. acinosa under Mn stress Potassium content in P. acinosa kept high when treated with the excess Mn. Zinc content in treated plants was lower than that of the control. Compared with the control, the content of magnesium in plants under Mn stress kept above 4 g/kg which was in normal level. Besides, both calcium and iron kept in a steady level in plants under Mn stress. Mineral absorbtion was less affected in the plants treated with 2 mmol/L Mn than those with 8 mmol/L Mn. Keeping minerals absorption in balance may be one of the response mechanisms for P. acinosa to resist high Mn concentrations.