| Soil heavy mental pollution has become one of the current hot topics in environmentalscientific community. Pb pollution in environmental pollution of heavy metal is most prominent.Soil Pb pollution can enter into human body through the food chain quickly, this affectmetabolism of human body seriously. Besides the process which human body release Pb by itselfis very slow, the damage to organs in the body is irreversible. Therefore, Pb pollution treatmenthas become an urgent subject currently. Traditional method of treatment to heavy metal pollutionin soil mainly use mixing of soil, leaching method, chemical modifiers ect. Such physical andchemical methods which are not only expensive and cannot apply in small areas but also causedsecond pollution, such methods cannot solve the Pb pollution in soil fundamentally. In recentyears, people have found the bioconcentration ability of some plants to heavy metals. It is a highefficiency, environmental protection and cost control measures to use such plants asphytoremediation of heavy metal pollution. This treatment measure has a very broad applicationprospects.The prerequisite to the application of phytoremediation to heavy metal contaminated soilswas the discovery of heavy metal hyperaccumulators. Pb is not easy be absorbed by plant for thehigher electronegativity of Pb, and easy form a covalent bond with the soil organic matter andFe-Mn oxide. Currently rare hyperaccumulators were found at home and abroad. Pbhyperaccumulator plant—Pogonatherum crinitum was found by our research teem previous forthe first time in China, which can grow normally in the lead-zinc mining area of high content ofPb up to17496mg·kg-1. This plant has strong tolerance and accumulation ability of Pb, and overof Pb hyperaccumulator standard. But currently it is still not clear of the tolerance andaccumulation mechanism of Pogonatherum crinitum to Pb, especially not so clear howPogonatherum crinitum react by physiological ecology characteristics under high Pbconcentration. These problems limit the application of Pogonatherum crinitum in the remediationof Pb polluted soil.Based on these researches and studies, according to the principle and method of plantresistance physiology, toxicology, root biology, taking Pb hyperaccumulator Pogonatherumcrinitum which was first found by our research group as the research object, meanwhile selecting Paspalum natatu which is included in the same grass family as a control, simulation Pb stress testin the lab was taken, designed homogeneity and heterogeneity Pb stress test with manyinstruments including BTC root dynamic monitor, root image analyzer, Lci portablephotosynthetic apparatus, atomic absorption spectrophotometer, ect.Morphology and physiologyreaction was tested under different Pb stress condition, accumulation and transport capacity of Pbby Pogonatherum crinitum and Paspalum natatu was compared, the possible physiologicalecology way of Pogonatherum crinitum adopt to enrionment was revealed. This can also providescientific basis to clarify the hyperaccumulator plants resistant to Pb and Pb rich mechanism. Ithas important theoretical and practical significance for accelerating the repair of soil Pb pollutionand discover potential rich Pb characteristics of Pb Hyperaccumulator. The main results are asfollows:（1） Low concentration of Pb stress (50mg·L-1and150mg·L-1) had positive effect onPogonatherum crinitum seeds germination potential, germination rate, germination index andvigor index. But with the increase of Pb stress concentration, the germination of the seed indexwas gradually decreased. Pogonatherum crinitum still had certain germination ability under Pbstress concentration of800mg·L-1. The effect on the shoot, root and seedling growth inhibition ofPb stress of Pogonatherum crinitum was not obvious.（2） Homogeneous Pb stress had no significant inhibitory effect on the growth of Pogonatherumcrinitum. Seedings of Pogonatherum crinitum rapid growth in July and August, the growth inOctober was smallest. Pogonatherum crinitum could increase root biomass allocation adapt to Pbstress environment.Pb stress could accelerate the sprout updation of Pogonatherum crinitum, thisprobably was one mechanism of Pogonatherum crinitum adapt to high concentration.（3） Pogonatherum crinitum seedling height, sprout tiller had some inhibitory effect at early stageof heterogeneous stress （30d）, but with the time increasing of Pb stress, inhibition is less and lessunder heterogeneous Pb stress condition. Under heterogeneous Pb stress condition Pogonatherumcrinitum seedling height increase to the maximum in July, minimum in October, seedling height inJune and August are same. Biomass allocate more to root part under low Pb concentration stresscondition, but with the increase of Pb concentration, biomass allocation to shoot more, whichmore Pb could transfer to shoot. （4） Pb stress had inhibitory effect on root length and surface area of Pogonatherum crinitum underHomogeneous Pb stress condition. Along with the increase of Pb concentration, total root lengthand surface area of the root were increased first and then decreased. Root length grows up tomaximum of621.05cm at Pb stress concentration500mg·kg-1treatment, root surface area growsup to a maximum of20.29cm2at Pb stress concentration2500mg·kg-1treatment. Pb stress had noobvious inhibition on the root average diameter and volume of Pogonatherum crinitum.Pogonatherum crinitum could avoid Pb stress by root growth to adapt to the higly Pbconcentration stress environment.（5） Total root length and surface area of Pogonatherum crinitum on left compartments was greaterthan right compartments with Pb stress concentration increased under heterogeneous Pb stresscondition. Root length of Paspalum natatu in left compartments without Pb stress under differentPb stress condition were greater than Pogonatherum crinitum, but root length of Pogonatherumcrinitum in right compartments were greater than Paspalum natatu. Along with the increase of Pbconcentration, the root surface area and average diameter of Pogonatherum crinitum increasedunder higly Pb concentration condition.（6） Root could be observated in the treatment of Pb concentration of150mg·kg-1and2500mg·kg-1at homogeneity of Pb stress conditions by BTC dynamic monitoring system, theprobability of root growing into Pb stress plaque was28.57%. In Higly Pb concentration (2500mgkg-1) plaque, root surface area and root volume increased more than the low Pb concentration(150mg kg-1) plaque from August to September, the the changes of root length and diameter wereantithetical. Root could be observated at Pb concentration of1000mg·kg-1treatment. Theprobability of root growing into Pb stress plaque was14.28%under heterogeneous Pb stresscondition in the season of September and October, this means Pogonatherum crinitum hasdcertain avoidance of Pb stress.（7） Photosynthesis rate （Pn） and stomatal conductance （Gs） of Pogonatherum crinitum weresignificant inhibited in medium test term （September） under homogeneous Pb stress conditions,but less inhibitory effect in Pb stress test initial stage and last phase, and quickly increased atinitial stage of Pb concentration of150mg·kg-1and1500mg·kg-1treatments. Photosynthesis rate（Pn） and stomatal conductance （Gs） of Paspalum natatu were smaller than control at different testtime. Intercellular CO2concentration （Ci） of Pogonatherum crinitum had no significant difference with control at different stress time under homogeneous Pb stress conditions.Transpiration rate （Tr）of Pogonatherum crinitum decreased. Pogonatherum crinitum could reduce the transpiration inorder to ensure its growth.（8）Various photosynthetic characteristics of Pogonatherum crinitum were inhibited underheterogeneous Pb stress conditions, except photosynthetic rate of Pogonatherum crinitum wasinhibited significantly in stress medium term（September）, other stress times inhibitory effect werenot obvious.（9） Leaf photosynthesis rate （Pn）, stomatal conductance （Gs） and transpiration rate（Tr） ofPogonatherum crinitum were first decreased and then increased, reached a maximum value at12:00, then decreased gradually, while the intercellular CO2concentration（Ci） first decreased andthen increased, reached a minimum value at12:00under homogeneous and heterogeneous Pbstress condition.（10） With increased of stress time, Pogonatherum crinitum leaf variable fluorescence Fv and themaximal photochemical efficiency of PSⅡ first decreased then increased, PSII potential activityincreased first and then decreased under homogeneous Pb stress:. In different heterogeneous Pbstress concentration and different stress time, PSⅡmaximal photochemical efficiency and PSⅡpotential activity were more than control which without Pb stress. This means that Pb stresspromoted the fluorescence properties of Pogonatherum crinitum.（11） Leaf green degree of Pogonatherum crinitum decreased substantially with the increase ofstress time under homogeneous and heterogeneous Pb stress conditions. With the increase of Pbstress concentration, leaf green degree of Pogonatherum crinitum was W–shape changed, butdifferent Pb stress treatments had no significant difference with the control （P <0.05）. Pb stresshad a certain inhibitory effect on leaf green degree.（12） Pb stress had a certain inhibitory effect on the chlorophyll content of Pogonatherum crinitum,except lower Pb concentration of150mg·kg-1treament could promote Chlorophyll a, chlorophyllb, total chlorophyll and chlorophyll a/b of Pogonatherum crinitum under homogeneous Pb stress.With the increase of Pb stress concentration the inhibition increased.（13） The content of POD、SOD and MDA in Pogonatherum crinitum leaf is more than Paspalumnatatu while CAT in Pogonatherum crinitum leaf is less than Paspalum natatu both underhomogeneous and heterogeneous Pb stress conditions. This indicates that Pogonatherum crinitum under Pb stress condition increase SOD and POD activity to increase membrane lipid reaction toadapt Pb stress environment, this is one of the physiological mechanism of Pogonatherumcrinitum resistant to Pb sress.（14） Main organic acids in root part of Pogonatherum crinitum include Oxalic acid, citric acid andmalic acid, and showed gradually increased trend along with increased of Pb stress concentrationunder homogeneous Pb stress conditions. Under heterogeneous Pb stress conditions, root andshoot in Pogonatherum crinitum had low molecular organic acid mainly include oxalic acid,malonic acid, malic acid and citric acid, the oxalic acid content is the highest, followed by malicacid and citric acid. Pogonatherum crinitum can reach to detoxification of Pb stress mainlythrough the effect of oxalic acid chelate.（15） Total Pb content in shoot under higher Pb stress condition significantly greater than othertreatments and control, and Pb content in shoot and Pb transfer index were exceed the standard ofPb hyperaccumulation. Pogonatherum crinitum has strong transport of soil Pb, and based on thepart of the litter make Pb out of body, this may be one of the mechanisms of Pogonatherumcrinitum resistance Pb stress.（16） The form of Pb in shoot and root of Pogonatherum crinitum under different Pb stresstreatments in both homogeneous and heterogeneous Pb stress condition were mainly HClextraction, ethanol extraction and residual Pb existed certain difference. HCl extraction Pbaccounted for70.61%-85.92in root and79.04%-94.37%in shoot under homogeneous Pb stresscondition.The residual Pb content in plant increased but ethanol extraction Pb dcreased, whichshows that Pb in Pogonatherum crinitum could component chelating, lower ability of migration,thereby reducing the toxicity of Pb.（17）Ethanol and HCl extractable Pb content and total Pb content increased first and thendecreased, the residue Pb content increased with the increase of Pb concentration underhomogeneous Pb stress condition. Ethanol extractable Pb content increased first then decreased,HCl extractable Pb content increased with the increase of Pb concentration under heterogeneousPb stress condition. Total Pb and different form Pb content in apoptosis body of Pogonatherumcrinitum were higher than Paspalum natatu, which drawed the results Pogonatherum crinitumcould remove Pb out of body through the apoptosis of plant tissue, in order to detoxification.In summary response mechanism of Pb hyperaccumulator Pogonatherum crinitum under Pb stress mainly include: firstly, Pogonatherum crinitum through the rapid elongation of root, find noPb patch, to avoid the Pb stress, so as to adapt to the high Pb concentration environment, but rootwill absorb Pb from soil after a period when can not avoid Pb stress. Then Pogonatherum crinitumcan increase photosynthetic rate （Pn） and stomatal conductance （Gs）, decrease the transpirationrate （Tr）, promote chlorophyll synthesis, improve the variable fluorescence Fv, the maximalphotochemical efficiency of PSⅡ and PSⅡ potential activity, increase synthesis of biomass,improve root biomass allocation adaptive Pb stress environment under Pb concentrationconditions, by improving the biomass distribution of shoot, transfer Pb in soil to overground part,then accelerate sprouting update rate, through fallen litter take Pb out off body, realize their owndetoxification under High concentration Pb stress. In addition, Pogonatherum crinitum canincrease POD and SOD activity, increased membrane lipid peroxidation reaction, by increasingsecrete more citric acid and malic acid, chelation of Pb in body, transform Pb into insoluble state,to achieve detoxification of Pb stress. |
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