Investigation Of Mechanisms Of Chromium Accmulation And Tolerance In Different Varieties Of Pakchoi(Brassica Chinensis L.)

This study were carried out to investigate the physiological mechanisms of Chromium(Cr)uptake,accmulation and tolerance in Brassica chinensis L.based on the research on varietal difference in Cr uptake,accmulation and tolerance and effects of Cr6+ on growth and physiological metabolism in Brassica chinensis L,The main results are as follows:1.Investigation of different varieties of the growth and Cr accumulation in Brassica chinensis L.The difference in tolerance and uptake of Cr6+ by pakchoi(Brassica chinensis L.)of 41 cultivars was studied with hydroponic culture.The results showed that with 100?mol·L-1 Cr6+,the Cr uptake,accumulation and tolerance of varietal difference were significantly different.Among the green vegetable cultivars,the inhibition percentage of dry weight in shoots varied from 10.3%to 48.3%,but the inhibition percentage in roots varied from 12.0%to 58.4%.The Cr concentration in roots varied from 403 to 970.6 mg·Kg-1,while the Cr concentration in shoots was in a range of 19.9 to 72.6 mg·Kg-1.The Cr accumulation in shoots varied from 7.1 to 38.4 ?g pot-1,but the Cr accumulation in roots varied from 13 to 42.3 ?g·pot-1.2.The effects of Cr on the growth and some physiological characteristic in Brassica chinensis L.The effects of chromium(Cr)on the dry matter weight andsome physiological charact-eristic were studied in two pakchoi variety,Meijiahua(Cr6+-stress tolerant)and Sanyueman(Cr6+-stress sensitive).The results showed that Cr6+ treatments at all concentrations significantly decreased the dry matter weight of both cultivars,but the decrement in Meijiahua was significantly lower than in Sanyueman.The relative electrical conductivities and MDA content increased gradually with Cr concentration.The relative electrical conductivities and MDA content of Sanyueman were significantly higher than those in Meijiahua.Under low concentration of Cr6+(10 ?mol·L-1),the activities of SOD,POD and CAT were significantly increased,but the extent of increase was significantly higher in Meijiahua than Sanyueman.The activities of SOD,POD and CAT were significantly downregulated under high concentration of Cr6+(100 ?mol L-1),but the magnitude of downregulation in Meijiahua was significantly lower than that in Sanyueman.The contents of soluble sugar content and soluble protein content were increased firstly and then decreased in two varieties of pakchoi with Cr6+ concentration.The contents of soluble sugar content and soluble protein content were significantly higher in Meijiahua than Sanyueman.Cr6+ treatments at all concentrations significantly increased the concentrations of non-protein thiol and phytochelatins(PCs)in shoots of both cultivars,but the increment in Meijiahua was significantly higher than in Sanyueman.The free amino acid content was significantly lower in Meijiahua than Sanyueman under the same Cr6+ treatment.3.Investigation of mechanism of uptake in Brassica chinensis L.The characteristics of Cr uptake by pakchoi roots for both Cr3+ and Cr6+ were discussed with hydroponic culture and different exposure time with the variety Meijiahua(Cr6+= stress tolerant)and Sanyueman(Cr6+-stress sensitive).Effects of metabolic inhibitors and low temperature,ion channel inhibitor and nutritive element on Cr uptake by pakchoi roots for both Cr3+ and Cr6+ were analyzed.The results showed that the concentration of Cr and treatment time was positively correlated with the concentration of Cr.After 12h,the Cr3+ concentration tended to be saturated,and the concentration of Cr6+ increased continuously.After lh,the concentration of Cr in roots was up to 989mg·Kg-1 with 100?mol·L-1 Cr3+and the concentration was only 220ng·Kg-1 with 100 ?mol·L-1 Cr6+.During 1-120h with supplying Cr3+ and during 48-120h with supplying Cr6+,Cr concentration were significantly higher in Meijiahua than Sanyueman.The differences increased with the exposure time.The ATPase inhibitor(NaVO4)and SO42-inhibited the absorption of Cr6+into plant and Cr3+uptake did not significantly inhibit.Ca2+ channel inhibitor LaCl3 and Fe inhibited the uptake of Cr3+ into plant,but there was no obvious inhibition to the absorption of Cr6+.Low temperature inhibited the uptake of Cr3+ and Cr6+ into plant.4.Role of organic acids in Cr6+tolerance of Brassica chinensis L.The changes of organic acid contents in root exudation by different genotypes of pakchoi for Cr6+ were studied with hydroponic culture in the variety Meijiahua(Cr6+-stress tolerant)and Sanyueman(Cr6+-stress sensitive).Effects of exogenous inhibitors on growth index of pakchoi and Cr uptake,organic acid contents in pakchoi and organic acid contents in root exudation were analyzed.The results showed that the organic acid contents in root exudation increased with the Cr6+ concentration.The concentration of oxalate and malic acid were much higher than others.Under 10 and 50?moIL-1 of Cr6+,the contents of oxalate and malic acid in root exudation were significantly higher in Meijiahua than Sanyueman.When addition of A9C under the Cr6+ stress,the relative root length and relative plant height,total root length inhibited significantly,but the extents of decrease were significantly higher in Meijiahua than Sanyueman.The content of Cr in pakchoi was increased,but the extents of increase were significantly higher iin Meijiahua than Sanyueman.The contents of oxalate and malic acid in pakchoi root and in root exudation were decreased.5.Distribution characteristics and chemical form of Cr in pakchoi plants under different Cr stressThe contents of Cr in pakchoi and subcellular distribution and chemical forms of Cr were studied with the variety Meijiahua(Cr6+-stress tolerant)and Sanyueman(Cr6+-stress sensitive).The results showed that the contents of Cr in shoots were significantly lower than those in the roots in both cultivars.The Cr contents and Cr retention rate in roots of Meijiahua were significantly higher than those in Sanyueman.It was found that Cr in the root cells of pakchoi plants exposed to Cr stress was mainly localized in cell walls,and Cr in shoots in Meijiahua were mainly present in cell walls,but in Sanyueman were mainly localized in vacuoles.The Cr in the organelle to total Cr in shoots of Meijiahua was significantly higher than those in Sanyueman.The Cr ions in pakchoi tissues exist predominantly in the forms extracted by 80%ethanol and d-H2O under lower Cr levels.The amount of Cr in the chemical forms extracted by 2%HAc,0.6M HCl and in residues in shoots and the percentage of Cr in the chemical forms extracted by 2%HAc,0.6 M HCI,and in residues in shoots was increased under the highest Cr levels.The Cr ions in pakchoi exist predominantly in the forms extracted by 2%HAce 0.6 M HCl,residues in roots under all Cr levels.Meijiahua had a lower percentage of Cr in the chemical forms extracted by 80%ethanol and d-H2O and had a higher percentage of Cr in the chemical forms extracted by 2%HAc,0.6M HCl and in residues compared to Sanyueman in roots under all Cr levels.