Study On Physiological Response And The Detoxification Mechanisms Of As By Eucalyptus

In the study,Eucalyptus as experimental materials,through the hydroponic experiment,the effects of different concentrations of As stress on the biomass of Eucalyptus,on the contents of chlorophyll and malondialdehyde(MDA),and on contents of antioxidant system(superoxide dismutase(SOD),catalase(CAT)and peroxidase(POD))and partial thiol pools(non-protein thiols(NPT),glutathione(GSH)and phytochelatins(PCs))were studied;the subcellular distribution and chemical form of As in roots and leaves of Eucalyptus were also studied,the detoxification mechanism of As of Eucalyptus was discussed from the aspects of subecellular distribution and chemical form of As,and antioxidant enzyme system and the chelation of sulfhydryl groups of Eucalyptus.The main results are as follows:(1)High concentration of As could significantly inhibit the growth of Eucalyptus,Eucalyptus has strong oxidative damage.At low concentrations of As stress(5 mg/L and 10 mg/L)of 7 d and 14 d,the biomass in roots,stems and leaves and the contents of MDA in roots and leaves of Eucalyptus had insignificant differences compared to the controls,the results showed that low concentrations of As stress had no obvious toxic effect on Eucalyptus.At high concentrations of As stress(20 mg/L and 30 mg/L),biomass and the contents of chlorophyll decreased,the contents of MDA significantly increased,the results showed that high concentrations of As inhibited the growth of Eucalyptus,and Eucalyptus strongly suffered from the oxidative damage.(2)The effect of root retention and limited absorption on As is an important mechanism of detoxification to As of Eucalyptus.At As stress,the order of accumulation of As in different parts of Eucalyptus was roots>stems>leaves,most of the As was stored in the roots(76.9%-88.9%),which limits the transported of As to the shoot,reducing toxicity of As to the shoot of Eucalyptus.At As stress,with increasing the concentration of As,the accumulation of As in roots of Eucalyptus firstly increased and then remained unchanged,which showed a certain limited absorption.(3)The important mechanisms of detoxification to As of Eucalyptus included the regional effect of soluble fraction,the fixation of cell wall fraction,and the change of partial As in roots to sodium chloride extraction.At As stress,the As in roots and leaves of Eucalyptus was mainly stored in the soluble fraction(distribution ratio were 66.3%-79.9%and 50.6%-53.8%,respectively),followed by the cell wall fraction(distribution ratio were 14.6%-23.2%and 35.6%-39.9%,respectively).At As stress,As in roots of Eucalyptus was mainly extracted by ethanol(29.5%-40.0%),deionized water(28.3%-32.3%)and sodium chloride(24.1%-33.8%),with increasing of As stress concentration,the proportion of ethanol extraction was basically unchanged and then decreased,deionized water extraction firstly increased and then remained unchanged,sodium chloride extraction decreased before increased.Partial As in roots integrated with pectate and protein,increasing the ratio of sodium chloride extraction could reduce the toxicity of As to Eucalyptus.On the other hand,a large amount of As could not be changed to more stable chemical form,indicated that Eucalyptus was more susceptible to As toxicity.(4)Under low concentrations of As stress,the antioxidant enzyme system and chelation of thiol pools of Eucalyptus could alleviate the toxic effects of As on Eucalyptus.At low concentrations of As stress of 7 d,the activities of SOD,CAT and POD in roots of Eucalyptus increased,the accumulation of hyperoxide and membrane peroxidation alleviated;at high concentrations of As stress,the activities of SOD and CAT in roots significantly decreased compared with the highest,the activities of CAT and POD in leaves significantly decreased compared to the controls,much hyperoxide was accumulated,the alleviating effects of the antioxidant enzyme system on the oxidative damage were deeply reduced.At low concentrations of As stress,the contents of NPT,GSH and PCs in roots and leaves increased,it is great significance to chelation of thiol pools in As poisoning.At As stress of 14 d,the levels of PCs in roots decreased with increasing As concentration,the chelation and detoxification of PCs to As decreased.