| In recent years,the rapid development of industrialization and urbanization has led to increasingly serious environmental problems,especially in heavy metal Cd pollution.As a non-essential element in plants,Cd is characterized of high mobility,strong migration ability and difficult degradation,which causes a serious threat to animals,plants and even human beings through the food chain and food web.As safe and green environmental remediation materials,plants are widely used in Cd pollution remediation and prevention.Although a large number of studies have reported the response of different plants to Cd stress,due to the influence of plant variety,habitat and degree of environmental pollution,there are still some problems in the studies on the effects of Cd stress that have not been reported or less reported,and the tolerance mechanism is still unclear.In view of this,this research used the Elymus sinosubmuticus S.L.Chen（Hereafter referred to as SIN）,Elymus tangutorum（Nevski）Hand.-Mazz.（Hereafter referred to as TAN）and Elymus sibiricus L.（Hereafter referred to as SIB）as materials which were less well studied and have characteristics of cold tolerance,drought tolerance and saline-alkali tolerance.This paper uses the hydroponics method to simulate Cd stress treatment(the concentration was 0,5,10,20,40 mg·L-1),and investigate the seed germination,seedling root,stem,leaf and biomass growth,photosynthetic pigment content,malondialdehyde content,antioxidant enzyme activity and accumulation of carbon and nitrogen metabolites of these three alpine herbage plants ang also screened Cd-tolerant plant materials.The physiological regulation mechanism of Cd tolerance in three alpine herbage seedlings was revealed.The main results are as follows:（1）Cd stress inhibited seed germination and seedling growth of three alpine herbage species.The seed germination and seedling growth of the three alpine herbage species were different under low and medium concentration Cd treated(5～20 mg·L-1),and Cd toxicity was obvious at the concentration reached 40 mg·L-1.For example,in the study of SIN,the germination rate,relative damage rate,germination potential and germination index under 40 mg·L-1Cd treatment were decreased by 14.67%,16.67%and 15.38%compared with the control group.Long root length,short root length,long leaf length,short leaf length,stem length and plant height were decreased by 1.66,0.59,4.33,3.76,and 1.57 cm,respectively.（2）Cd treatment significantly reduced the chlorophyll content of these three alpine herbage species.Compared with the control group,chlorophyll a,chlorophyll b,chlorophyll a+b and carotene of the SIN were significantly decreased（P<0.05）,and the values decreased by 1.23,0.43,1.64 and 0.16μg·mL-1 in the 5 mg·L-1Cd treatment group,compared with control group.chlorophyll a and chlorophyll b,and chlorophyll a+b carotenet of TAN was significantly reduced（P<0.05）in 40 mg·L-1Cd treatmen,and its value were 4.80,2.54,7.34 and 0.84μg·mL-1.In the study of SIB,the minimum values of chlorophyll a,chlorophyll b,chlorophyll a+b and carotene were 5.68,2.59,7.86 and 0.91μg·mL-1 when Cd treated with 5 mg·L-1.（3）Under Cd stress,MDA content and antioxidant enzyme activity of three kinds of alpine herbage increased significantly,but the variation trend was slightly different.Compared with the control group,MDA content of the three herbage increased significantly when Cd concentration was 5 mg·L-1（）P<0.05,and SIN、TAN and SIB increased by 7.15,8.99 and 2.99 g·FW-1.With the increase of Cd concentration,the activities of SOD,POD,APX and CAT enzymes of SIN awort increased gradually,and reached the maximum when the concentration of Cd was 40 mg·L-1(compared with the control group,its increased by 6.99,2976.71,55.93 and 1166.67 FW·min-1,respectively).（4）Cd stress can affect the contents of free proline,nitrate nitrogen,ammonium nitrogen,soluble sugar,sucrose and starch in three alpine herbage species.For example,With increasing Cd concentration,free proline content of SIN and TAN increased by 0.95 and 4.08μg·g-1 compared with the control group,under 40 mg·L-1Cd treatment reached maximum.Under 20 mg·L-1Cd treatment,the highest content of free proline was 8.61μg·g-1 higher than that of control group.The lowest nitrate nitrogen content decreased by 4.48μg·g-1 compared with control group,at 40mg·L-1Cd of SIB.The lowest nitrate nitrogen content of SIN increased by 2.23μg·g-1compared with the control group,at 40 mg·L-1Cd of nitrate nitrogen of TAN.Under 20mg·L-1 Cd concentration maximize at 11.47μg·g-1.The maximum nitrate nitrogen of SIB increased by 1.77μg·g-1 compared with the control group,when Cd concentration was 5 mg·L-1.（5）There were differences in the response of three alpine herbage species to Cd stress at germination and seedling growth stage.For wheat in seed germination tolerance size TAN>SIB>SIN.Major judgment basis for three kinds of herbage relative damage rate of TAN in 0～20 mg·L-1 under Cd stress difference was not significant,the SIB in 0～10 mg·L-1 under Cd stress difference was not significant,and the SIN each stress relative damage rate have significant difference between groups and the cadmium concentration increases.In the seedling growth period,SIN and TAN all have good resistance to Cd.In SIN’S growth change,5 mg·L-1 Cd stress promoted the growth of long leaf and 10～20 mg·L-1 Cd stress promoted the growth of short leaf.In the growth of TAN change,the plant height and fresh weight were promoting at 5mg·L-1 Cd stress.However Under all Cd stress treatments,the growth of SIB was inhibited.In addition,the three kinds of photosynthetic pigment content in the response of alpine forage grass,TAN was less interference.Therefore,Cd resistance of three kinds of alpine pasture seedlings growth period for TAN>SIN>SIB.In conclusion,Cd stress result in different degrees of damage to the three alpine herbage species,mainly caused lipid membrane peroxidation damage,reduced seed vitality,inhibited the growth of plant roots,stems,leaves and biomass,and weakened plant photosynthesis.At the same time,this three species of alpine herbage initiated a series of mechanisms to reduce body damage.Firstly,the activity of antioxidant enzymes was activated to remove excessive accumulation of reactive oxygen species in the body.The results of this study showed that the activities of SOD,POD,APX and CAT increased with the increase of Cd stress concentration.Secondly,the osmotic substances in the body were increased to reduce the damage.The results of this study showed that the content of free proline increased with the increase of Cd stress concentration.And the contents of nitrate nitrogen,ammonium nitrogen,soluble sugar,sucrose and starch of three alpine herbage species had significant changes under different concentrations of Cd stress.To sum up,above results can provide reference for the ecological remediation of Cd contaminated soil on the Qinghai-Tibet Plateau by using native plants. |
