A Study On Differential Tolerance Of Seedlings Among Genotypes To Heavy Metal Exposure

Population genetic composition is closely related to adaptation and the dynamic alternations of genetic structure at population level may be useful as an indicator of environmental stress.This thesis,using horizontal gel electrophoresis,studied allozyme allele frequencies at three polymorphic loci of the three plant seedlings of alfalfa(Medicago sativa),sweet clover (Melilotus Adans) and kohlrabi(Brassica oleracea),and explored the relationships between allozyme genotypes and tolerance to heavy metals (mercury chloride or cadmium sulfate) exposure.The allozyme analysis data showed that both alfalfa and sweet clover are polymorphic at loci of Pgi and Me,and kohlrabi is polymorphic at Pgi and Pgm.Among all the species the mean number of alleles per locus is 2.5～3.0 and the mean heterozygosity is 0.346～0.655.The allele frequency at most loci are consistent with the Hardy-Weinberg's expectations.The data suggested the three samples are good for population genetics study.The germination of alfalfa and sweet clover was conducted in HgCl₂ of various concentrations and that of kohlrabi in CdSO₄ solutions.The length of root and stem of each individual was measured to obtain the mid-concentration for further exposure study.The concentrations were determined as:alfalfa:40mg/L(HgCl₂),sweet clover:30mg/L(HgCl₂),and kohlrabi 30mg/L(CdSO₄).After 6-day exposure test the allozyme genotypes of each individual were identified and the length of seedling root and stem measured,to compare the average length difference among the genotype groups. The data showed that the exposure to HgCl₂(40mg/L) inhibited both root growth more than stem in alfalfa seedlings.The ANOVA showed that in control group the average root length in Me-AB genotype group(43.41±1.63mm) was significantly greater than that in Me-BB(52.83±2.99mm). However,in exposure group such difference was not observed,suggesting that Me-BB has root growth advantages over other genotypes,but more sensitive to acute exposure to HgCl₂.Similarly the seedlings of sweet clover showed root growth advantage in Me-AA genotype group in control group, while the HgCl₂ exposure imposed nearly uniform inhibition to all genotypes at Me locus.At Pgi locus no growth difference was found among the genotypes groups of sweet clover,but under acute HgCl₂ exposure the root length of Pgi-AC(19.75±0.73mm) was significantly longer than that of Pgi-BB(14.67±1.04mm),implying higher tolerance of Pgi-AC genotype to HgCl₂ exposure.As for kohlrabi seedling exposure to CdSO₄,neither root nor stem growth difference was observed among the genotypes at Pgi and Pgm in both control and exposure groups,which indicated the uniform inhibitive effects across all genotypes in this kohlrabi sample.The results from the present study demonstrated that under normal conditions certain genotypes of alfalfa and sweet clover are correlated to seedling growth,and acute HgCl₂ exposure showed differential inhibition to root growth among the certain allozyme genotypes.However this difference was not obvious in kohlrabi. This thesis demonstrates the usefulness of allozyme analysis in determining the relationships of plant population genetic structure to growth and/or tolerance of heavy metals,which is potentially applicable to genetic breeding and breeding,as well as ecological rehabilitation.