《Table 3–The low molecular weight organic acid (LMWOA) concentrations (mg/L) under Pb stress.》

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《Woody species Rhus chinensis Mill. seedlings tolerance to Pb:Physiological and biochemical response》

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As is to know，Pb is not essential element in plant metabolism（Zaier et al.，2010）.In the present study，R.chinensis can grow normally under the lower Pb stress（25 to 50 mg/L Pb）.However，the physiological processes were inhibited under the higher Pb stress（≥100 mg/L）and the visual damage（leaf dehydration and chlorosis）to the seedlings was observed.Malar et al.（2014b）also reported that under the heavy metal stress，biomass is a good indicator for plant growth.The similar phenomenon was observed in this study.For example，the concentration of Pb in roots was strongly negative correlation with the root biomass（Pearson r=-0.753，p<0.001，N=18）.And these symptoms were also reported by Islam et al.（2007），Malar et al.（2014a）and Zhou et al.（2017）This phenomenon was due to the oxidative stress induced by Pb+（Mahdavian et al.，2016）.In this study，the concentrations of H2O2，O2·-and MDA in leaves and roots were increased apparently compared to control，especially at the higher Pb concentration treatments（Appendix A Table S1）and showed the negative correlation with plant biomass.These results suggested that R.chinensis is tolerant to lower and moderate Pb concentrations in this growth medium.It also demonstrated that R.chinensis can actively react to the environmental changes via a series of physiological and biochemical responses.In the current study，most of the Pb was sequestered in the roots，which suggested that the high bioavailability of heavy metals could be limited once inside the plant.This is because the endodermis Casparian strips in the roots would block Pb transport（Zhou et al.，2016a，2017）Furthermore，endothelial cells of the root tissue also blocked Pb transport to aboveground parts（Kopittke et al.，2008；Sharma and Dubey，2005）.Therefore，we speculated that the Pb accumulation in the roots is one of the Pb tolerance manifestations of R.chinensis seedlings.