Study On Differential Tolerances To Mercury In Seedlings Of Two Forage Grass Species With Different Genotypes

This thesis examined the genotypic responses two forage grass species of Medicago sativa and Melilotus officinalis to acute exposure to mercury chloride of various concentrations for germination and seedling growth test. The seedling were analyzed using horizontal starch gel electrophoresis to establish the relationships of allozyme genotypes to the growth of epicotyls and hypocotyls. The measured variables include HgCl2 concentration, genotype frequency, population genetic composition, and resistance ability, etc. Hopfully this study would shed light on the possible applications of allozyme analysis in grass breeding for bioremediation and as environmental biomarker.The seeds of Medicago sativa and Melilotus officinalis were each exposed to HgCl2 at the concentrations of 0 mg·L-1,20 mg·L-1,40 mg·L-1 and 60 mg·L-1 and the germination and seedling growth were observed, the length of epicotyls and hypocotyls were individually recorded. Later the genotypes at the polymorphic loci of Pgi and Me were determined electrophoretically. ANOVA was used to test the differences among the growth parameters within and between different genotypic groups. The results showed high polymorphism of both grass species at Pgi and Me. The mean allele frequency at each locus was 2.5 for Medicago sativa and 3.0 for Melilotus officinalis, and the mean observed heterozygosity was 0.614～0.672 for the former and 0.332～0.342 for the later, suggesting the suitability of the populations being used for alloyme analysis. The fixation index (F) was close to 0 for both species, indicating random mating. The cluster analysis demonstrated the deviation of the genetic structure of control group and the survivors of the treatment group, which indicates the selective effects of HgCl2 stress on the survivorship of the individuals with various genotypes. The measurement data showed that hypocotyls were more susceptible to HgCl2 than epicotyls, and demonstrated by higher inhibitory effects of HgCl2 concentration on the latter's growth in both species. The results of ANOVA show that the seedling with certain genotype(s) demonstrated growth higher growth potential in natural populations of Medicago sativa, while the seedlings with different genotypes in Melilotus officinalis grew more uniformly. Differential responses of the seedlings with different genotypes were observed at both Pgi and Me loci in both species. Medicago sativa seedlings of Me-BB grew faster hypocotyls than other genotypes when exposed to 20 mg·L-1 of HgCl; and Melilotus officinalis seedlings of genotype Me-AA grew faster in hypocotyls than other genotypes when exposed to 40 mg·L-1 of HgCl2. It is suggested that the genotype with better growth potential might be useful as biomarkers for heavy metal pollution. The cluster analysis shows increase in genetic distances between control and the survivors in treatment group indication the differentiations by HgCl2 exposure in the two clover species.