| Copper is an essential micronutrient for plant growth,playing an important role in plants.However,copper is also one of heavy metals in environment contamination, elevated concentration in plant of copper can destroy the plasma membrane,disrupt the physiological and biochemical processes,restrict the absorption of the nutrients,and lead to the inhibition of the growth of plants.Plants possess homeostatic mechanisms to maintain the correct concentrations of essential metal ions.These include roles for the following:for mycorrhiza,for extracellular exudates,for binding to cell wall,for reduced uptake or efflux pumping of metals at the plasma membrane,for chelation of metals in the cytosol,for the compartmentation of metals and for the antioxidative defence system.Rice(Oryza sative L.) is considered the main staple food for the population not only of China but also of many other countries.A high concentration of Cu in the soil not only inhibits rice growth and decreases grain yield,but also causes health hazards to man and animals through the food chain.To study the tolerance of rice(Oryza sativa L.) to excessive Cu,167 varieties were screened for Cu tolerance or sensitivity in this research.Based on the elongation of the roots,B1139 and B1195 were chosen as Cu-tolerant and Cu-sensitive variety respectively. This research was carried out to compare growth of rice seedlings and some physiological characters between Cu-tolerant and -sensitive rice varieties when exposed to elevated levels of copper under controlled environments.The aim was to investigate the copper tolerance mechanisms in rice.To identify the copper tolerance of Cu-tolerant or -sensitive rice varieties,effects of Cu on the growth of them were compared.Compared with the control(0.32μmol 1-1 Cu), treatments with 5 and 8μmol 1-1 Cu for 10 days had no significant effect on the roots and shoots dry masses of Cu-tolerant variety B1139,but significantly decreased those of variety B1195.Cu at concentration of 5 and 8μmol 1-1 also significantly decreased the contents of chlorophyll and carotenoid and the ratio of variable to maximal chlorophyll fluorescence (Fv/Fm) of Cu-sensitive variety B1195 leaves.In addition,5 and 8μmol 1-1 Cu also increased the electrolyticleakage,MDA contents and proline contents of shoots and roots in Cu-sensitive B1195,but had no significant effect on those of B1139.The results showed that variety B1139 had higher Cu tolerance than variety B1195.Excess Cu can cause an increased formation of reactive oxygen species(ROS). However,plants have evolved antioxidative defence systems to keep ROS under control. The H2O2 contents of shoots and roots of variety B1195 were both increased significantly under Cu treatments,which had no effects on variety B1139.Copper stress increased the activities of CAT,GR,APX,POD and SOD of shoots of variety B1139,while decreased those of variety B1195 except GR and SOD.In roots,the same Cu treatments had no significant effect on the activities of CAT,GR,POD and SOD of variety B1139,but significantly increased the activities of APX;while in variety B1195,the activities of CAT were decreased,and those of GR,APX,SOD and POD were not affected.The ratio of GSH/GSSG of shoots and roots of the two varieties were both decreased with increasing concentrations of copper,and the decrease of variety B1195 was much more than that of variety B1139.5 and 8μmol 1-1Cu had no effect on the ratio of AsA/DHA of shoots of variety B1195,but increased that of variety B1139.And the ratio of AsA/DHA of roots of the two varieties were both decreased,and the decrease of variety B1195 was also much more than that of variety B1139.The results showed that the capacity of the antioxidative defence system in variety B1139 was obviously higher than that in variety B1195.In addition,copper stress increased the NPT content of shoots and the NPT and PBT contents of roots in variety B1139,which indicated the role of the thiols in the copper tolerance of rice.As the copper concentrations in nutrient solutions increasing,the content of Cu in shoots and roots in rice increased significantly,and most of Cu was accumulated in roots. Roots and shoots of variety B1139 contained significantly higher the concentration of Cu than variety B1195 exposed to 5 and 8μmol 1-1 Cu,and variety B1139 had higher Cu proportions on the cell walls in shoots than variety B1195.The results showed that about 50%of Cu in shoots of rice was bound to cell wall and the other existed in soluble state,but 70-90%of Cu in roots was bound to cell wall.And the capacities of the cell walls of roots and shoots of variety B1139 to bind Cu were more effective than those of B1195.The results indicated that the cell walls of rice shoots and roots played an important role in the copper tolerance of rice,and it was more prominent in roots.To further understand the roles of cell walls in copper tolerance of rice,changes in cell wall properties of rice roots under copper treatments were examined.The results showed that Cu treatments significantly increased the total sugars contents in cell wall fractions of root tips of variety B1139,while only increased the contents in pectin of variety B1195 and decreased those in other cell wall fractions.The total sugars contents in cell wall fractions of root bases of two rice varieties were both decreased by Cu treatments.Cu treatments did not influence the neutral sugar composition of cell wall fractions of root tips and bases of variety B1139,but increased the contents of arabinose,xylose and galactose of variety B1195.There are increases in the uronic acid contents of cell wall fractions both of root tips and of root bases of variety B1139 after Cu treatments,and it was more significant in root tips,but those of variety B1195 were not affected,this showed the important role of uronic acid in copper tolerance in rice roots.In addition,pectin,hemicellulose-â… and cellulose of root tips of variety B1139 contained significantly higher concentrations of Cu than variety B1195 exposed to 5 and 8μmol 1-1 Cu.And among the cell wall fractions,the Cu contents in pectin were the highest.The results indicated that most of Cu that accumulated in the roots of Cu-tolerant variety B1139 was tightly bound to the cell wall pectin of root tips.The efflux rates of xylem sap of Cu-tolerant rice variety B1139 were not affected exposed to Cu treatments,but those of variety B1195 were inhibited obviously.After Cu treatments,the contents of Cu in the xylem sap of variety B1139 were significantly higher than variety B1195,and the transportation rates of Cu in the xylem of variety B1139 were also increased greatly.These results indicated that there must be some detoxicity mechanisms of Cu in the cells of variety B1139.The amino acid composition of xylem sap of rice was also analyzed.Contents of amino acids in xylem sap of rice were all increased, and the increase of the serine content was the most prominent among them.The serine content in xylem sap of variety B1139 increased 2.02 fold,and the transportation rate of which increased 2.28 fold after Cu treatment.It needs further studies to testify whether the increases of serine contents induced by copper are related to the copper transportation and detoxification in the xylem of rice.
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