| In this paper, with rape as experimental subject, hydroponic culture in Hoagland nutrient liquid and soil pot experiment were carried out under the stresses of cadmium chloride and cadmium carbonate, respectively, to study the distribution, accumulation of cadmium in the rape and their mechanisms. Through soil culture experiments, the author studied the change and distribution of soil cadmium speciations and their effects of environmental factors in karst region, and accumulation effects of cadmium and phenols in the rape and its toxic effects. In hydroponic experiments, simulated were the effects of low concentrations of cadmium on physiology, biochemistry and accumulation of cadmium of oilseed rape seedlings in different growth stages. Finally, discussed is the mechaniss how cadmium speciations transform in the soil, migrate in the soil-root interface, and transport and accumulate in rape under cadmium carbonate/chloride stresses to provide a theoretical basis for improvement of the quality of rape planted in the karst area soil. The main findings are as follows:1. Under the stress of cadmium carbonate, the exchangeable and carbonate-bound cadmium are main whose conversion are mainly affected by the pH, the available calcium, humic acid in the soil and etc. The transformation among water soluble, the exchangeable, humic acid-bound, and the residual has a significant correlation with pH, carbonate-bound cadmium correlates negatively with pH(coefficient-0.517), and the CEC and Cd forms have negative correlations.2. With the increase of concentrations of cadmium carbonate in the soil, cadmium in rape increase too. The cumulative amount of root is from31.41to87.00μg/g, of stems, from20.89to92.01μg/g, of rapeseed, from6.12to29.91μg/g and pod skin, from13.15to49.77μg/g. So, the cadmium contents in rapeseed are more than30times than the national standard’s, but the content of the contrast is below the standard.3. With the increase of the concentrations of cadmium carbonate in the soil, the moisture contents in rape grow, which are up to75.61%～85.83%in root,83.23%～86.75%in stem and75.79%～82.04%in seed-pod skin, respectively. Under the same concentrationof stress, the stem has the maximal of water content, while the seed-pod skin, the lowest. Addition of water in rape may partly alleviate the toxic effects of cadmium on the rape.4. With the increase of the concentrations of cadmium carbonate in the soil, the cumulative coefficients of cadmium in rape roots continue to lower, from4.90to3.02. Migration coefficients of cadmium in stem increase from0.66to1.06, and the cumulative coefficients,3.26,3.47,3.20, respectively. In rapeseeds, migration and cumulative coefficients are0.19,0.37,0.34, and0.95,1.54,1.04, respectively. Migration coefficients are0.42,0.60,0.57, and the cumulative coefficients,2.05,2.52,1.73, respectively, in pod skin.5. Phenol complex accumulated in the rape are tannins, flavonoids and polyphenols and etc. Whose magnitude order is seeds (including pod skin)＞stems＞roots. Flavonoids in rape root are5.71%to10.81%in amount, in stems,6.94%to11.28%, in pod skin,13.36%to16.01%and in seeds,27.01%to34.81%, which have significant or very significant difference level with the contrast under10mg/kg of cadmium carbonate. Tannin contents are from1.70%to1.99%in root, from4.44%to4.81%in stems, from6.44%to7.37%in pod skin, and from7.08%to7.67%in seeds, respectively. Rape roots, stems, rapeseeds and pod skins have different contents of tea polyphenols,0.33%to0.61%,0.61%to1.66%,5.85%to8.67%, and3.96%to5.81%, respectively. Therefore, flavonoids are highest, tannins, hgher and tea polyphenols, lowest in rape.6. Rape seedlings differentially accumulate cadmium in four growth stages. During two leaves-one stage, the contents of cadmium in the stems are5.75to115.46mg/kg, in the leaves,11.57to109.06mg/kg, and in the roots,24to442.70mg/kg. During four leaves-one stage, contents of cadmium are19to98mg/kg in the stems,15to75mg/kg in the roots, and65to666mg/kg in the leaves, respectively. During six leaves-one stage, the stems, roots and leaves have the cadmium contents of24to158mg/kg,26to97mg/kg, and36to315mg/kg, respectively. During eight leaves-one stage, the contents of cadmium are up to19.46to22.98mg/kg in the stems,13.55to19.02mg/kg in the roots, and86to125mg/kgin the leaves, respectively.7. In stages of rape seedlings, the correlations exist between protective enzyme activities inside cells and membrane permeability of the leaves due to cadmium in which CAT and MDA, SOD and the protein have significantly negative correlation(-0.342,-0.728,respectively), proline and SOD, POD significantly negatively correlate(-0.355,-0.201, respectively).8. Under cadmium carbonate stress, the carbonate-bound and humic acid-bound are converted to the exchangeable, and migrate to the rhizosphere and near-rhizosphere soil to increase available cadmium in soil. The water-soluble and the exchangeable in the rhizosphere soil are absorbed by the roots to be decreased through soil water or ion replacement. Those available cadmium absorbed by rape roots, mainly exchangeable, migrate towards the above-ground of rape through water and transpiration, and accumulate in the stems, pod skins and rapeseed. Hydroponic culture experiments can simulate soil pot experiments below low cadmium contents which showed that rape seedlings had been damaged by cadmium, the activities of protective enzymes had been reduced, cell membrane permeability had been increased and the contents of chlorophyll had been decreased so as to mahe some leaves yellow. This phenomenon was all confirmed and observed in soil culture and hydroponic experiments.9. Abundant phenolic compounds were stored in rapeseed and pod skin. Cadmium can replaced hydrogen atoms of hydroxyl, carboxyl functional groups in phenols to form complexs which will reduce accumulated or distributed cadmium in the organs of rape. Maybe this is one of countermeasures for rape to response to cadmium stress. |
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