Comparative Study On Toxicological Effects Of Pb Or Cd Pollution Stress On Three Mosses

At present,heavy metal pollution is becoming an increasingly aggravating environmental issue with the rapid growth of industrial and agricultural activities.It has been found that excessive heavy metals accumulated in the roots,stems,leaves,and fruits of various plants.This bioaccumulation of heavy metals may not only stunt plants,but also pose great threat to human health even life via food chains.Therefore,it is a very important subject to investigate damage and tolerance mechanism of heavy metals pollution to plants.Because of their unique biological properties,mosses have been admittedly regarded as indicator plants for heavy metal pollution.In addition,their relatively uncomplicated developmental stage and plant structure facilitate unveiling damage mechanism of heavy pollution to plants with respect to growth, ultrastructure,and related gene.In this study,under stress of two heavy metals Pb and Cd,bioaccumulation of metals,content of photosynthetic pigment,cell damage,and activities of antioxidant enzymes in three mosses involving Dolichomitriopsis diversiformis,Plagiomnium acutum, and Brachythecium procumbens were studied by means of water culture experiments,physiological and biochemical assays,and microscopical observation.The results showed as follows:1.After Pb or Cd stress,an obvious external damage was that mosses lost green and became yellow from top to bottom,and this damage got severe with elevated heavy metal concentrations as well as extended duration.In comparison,D.diversiformis exhibited least damages,followed by P.acutum,and B.procumbens showed heavy damages,and observation results of leaf cell damage were in good agreement with external damage.2.Total chlorophyll contents of three mosses showed an approximate identical change after Pb or Cd stress,low concentrations of Pb or Cd had no effects on total chlorophyll contents,high concentrations of Pb or Cd significantly reduced total chlorophyll contents.The greatest reduction of total chlorophyll contents occurred in B.procumbens.No significant changes of chlorophyll a/b were observed at low concentrations of Pb or Cd,however,at high concentrations of Pb or Cd,chlorophyll a/b decreased sharply with exception of D. diversiformis,which showed an obviously increased chlorophyll a/b, and the greatest decrease occurred in B.procumbens.The contents of carotenoid in three mosses decreased with increasing Pb or Cd concentrations,the greatest decrease occurred in P.acutum under Pb stress,and that occurred in B.procumbens under Cd stress,in both cases, the least decrease occurred in D.diversiformis.3.Three mosses showed high accumulation capacity to Pb or Cd, and they could accumulate much more Pb than Cd.In addition,there was distinct interspecific difference with respect to accumulation capacity,D. diversiformis had greatest accumulation capacity to Pb or Cd,and B. procumbens had least accumulation capacity.4.In the antioxidant enzymes of three mosses,SOD was more sensitive to Pb or Cd stress than POD and CAT.SOD played a pivotal role in the resistance to reactive oxygen species.D.diversiformis showed greatest SOD activities under Pb or Cd stress.5.When integrated observations including external damage,cell damage,content of photosynthetic pigment and SOD activities,it was concluded that the order of resistance to Pb or Cd stress was D. diversiformis＞P.acutum＞B.procumbens,which was positively correlated with accumulation of metals.