Physiological Mechanisms Of Aquatic Plants In Response To Hg²⁺, Cu²⁺ And Ni²⁺ Stress

In this paper,four aquatic plants with different ecotypes(Potamogeton crispus L.and Potamogeton malaianus Miq.,submerged plant;Hydrocharis dubia and Salvinia natans,floating plant)were chosen as experimental materials.Hg²⁺, Cu²⁺and Ni²⁺were selected as the stress factors;the former is an unessential element,the middle is unessential and essential microelement,while the latter is an essential microelement of aquatic plants.These plants were cultivated in solution containing different concentrations of Hg²⁺,Cu²⁺and Ni²⁺under lab condition.The relationship between Hg²⁺,Cu²⁺and Ni²⁺stress as well as the mechisms for the protection of aquatic plants were systemically studied.The results are as follow:(1)The toxic effect of various concentrations of Hg²⁺(0,1,3,5,7 mg·L^-1)on mineral nutrient,Chlorophyll content,soluble protein,MDA content,free proline content,GSH content and the ultrastructure changes in leaf cells of Potamogeton crispus were studied.The results showed that the Chlorophyll content and soluble protein content decreased,MDA content and free proline content increased,GSH content first increased and then decreased,with increasing the concentration of Hg²⁺. The mineral nutrient absorption was also affected by Hg²⁺stress,it increased the absorption of Ca²⁺,Fe³⁺,Mg²⁺,Zn²⁺,but reduced that of P and K⁺.In 5 mg·L^-1and 7 mg·L^-1Hg²⁺-treated fronds:the disappearance of two polypeptides with apparent molecular weights 83600 and 47800.Transmission electron microscopy observation indicates that the higher the pollutant concentration is,the more the damages to the leaf cells serious,such as the thylakoids of Chloroplast swelled,Chloroplast disintegrated.The conclusion could be reached that the death of plant was resulted from destruction of structure foundation of physiological function,unbalance of ion equilibrium and disorder of physiological metabolism.The lethal concentration scope should in 1～3 mg·L^-1.(2)Hydrocharis dubia plants exposed to 0,2,4,6,8 mg·L^-1Cu²⁺for 7d were analyzed with reference to its toxic impact on mineral nutrition adsorption,active oxygen generation,protective enzyme activities and their isozyme expression under lab conditions.The results showed that:the mineral nutrient absorption was affected by Cu²⁺stress,it increased the absorption of Ca²⁺,Fe³⁺,Mg²⁺and Zn²⁺,and reduced that of Mn²⁺,P and K⁺,with increasing the concentration of Cu²⁺.The amount and intensity of protein/peptide band decreased gradually with augment of Cu²⁺ concentration was observed in SDS-PAGE,respectively,and many polypeptides disappeared significantly in fronds treated with Cu²⁺stress.GR activity and H₂O₂ content change tendency assume the single peak curve,when Cu²⁺concentration to 6 mg·L^-1,their datum up to maximize,respectively better 2.51 times and 3.74 times than the control.O₂^.-content and MDA content change assume first slow,and then fast trend of escalation,when Cu²⁺concentration to 8 mg·L^-1,their datum reach the crest,comparing 342.3%and 334.2%to the control;SOD activity assumes the drop tendency,POD activity assumes the trend of escalation,CAT activity only slightly has rise.Their isoenzymic spectrum analysis demonstration,along with Cu²⁺ concentration increasing,the SOD isoenzymic band number and the expression quantity reduces.The CAT isoenzymic band number not obviously change,the expression quantity slightly has the difference.The conclusion could be reached that the death of plant was resulted from destruction of structure foundation of physiological function,unbalance of ion equilibrium and disorder of physiological metabolism.The lethal concentration scope should in 6～8 mg·L^-1.(3)The toxic effect of various concentrations of Cu²⁺(0,2,4,6,8 mg·L^-1)was studied on Chlorophyll content,soluble protein content,protective enzyme activities, active oxygen generation and ultrastructure changes in leaf of Potamogeton malaianus Miq.The results showed that Chlorophyll,Carotenoid and soluble protein content decreased,with increasing the concentration of Cu²⁺.SOD and POD activities generally decreased,CAT activity first increased and then decreased,O₂^.-, H₂O₂,MDA and soluble sugar content always increased,but the data of O₂^.-was down a little at Cu²⁺8 mg·L^-1concentration.In the picture of SDS-PAGE,treating with Cu²⁺concentration,it induced the disappearance or expression amount decrease of four polypeptides with apparent molecular weights 60900,18800,15700,22300 and 30000.Transmission electron microscopy observation indicates that the higher Cu²⁺concentration,the more serious damages to the leaf cells,especially Chloroplast,mitochondria and nucleus.The lethal concentration scope should be in 2～4 mg·L^-1.(4)Salvinia natans plants exposed to 0,5,10,15,20 mg·L^-1Ni²⁺for 10d were analyzed with reference to its stress impact on mineral nutrition adsorption, Chlorophyll content,soluble protein,protective enzymes system,reactive oxygen species levels,and submicroscopic structure under laboratory conditions.The results revealed that,with the rise of Ni²⁺concentration in culture medium,the addition of Ni²⁺affected the absorption of mineral nutrients,it mainly increased the absorption of Ca²⁺,Na⁺,Zn²⁺,Fe³⁺and Mg²⁺,but reduced that of Mn²⁺,Mo²⁺,P and K⁺.The content of Chlorophyll,Carotenoid,soluble protein,soluble sugar,superoxide dismutase(SOD)and catalase(CAT)decreased gradually,while that of peroxidase (POD),superoxide(O₂^.-),hydrogen peroxide(H₂O₂)and monodehydroascorbate (MDA)increased.In the leaves of Ni²⁺-treated fronds,the amounts of three polypeptides with apparent molecular weights 94000,was became visible in SDS-PAGE.The amount and intensity of protein/peptide band increased gradually with augment of Ni²⁺was also observed,the polypeptide with apparent molecular weight 35000 and＜17200 increased significantly in fronds.Transmission electron microscope observation indicated that Ni²⁺also imposed injury action on submicroscopic structure of leaf cells,disaggregation of nucleolus,agglutination and disappearance of chromatin of nucleus,disruption of nuclear membrane,swelling of thylakoids and breakage of Chloroplast envelope,decreasing of cristae quantity and vacuolization of mitochondria.To sum up,the stress of heavy metals on aquatic plants is very complex,with its coercive effect of both types of the metal,and the concentration of the metal ions. At the same time,heavy metals on aquatic plants in the destruction of a certain kind of coercion are not a single enzyme or the contents of the substances,but the entire aquatic plant physiological and biochemical reactions and harm the overall structure of the cell.