Research Of Mucilage Of Root And Its Border Cells Of Pea (Pisum Sativum) Under Aluminum Toxicity At Different Boron Levels

Root border cells are recently defined alive population sloughed from root tips and loosely adhered in the water. Border cells, a series of degradation cell wall fragment from root cap cells and orgnic acid,saccharide and giant molecule organics makes up the root cap mucilage. Mucilage plays a major role to suffer the stress of disadvantage environmental factors.But it has argument wether mucilage has roles of protecting roots from aluminium injury. The research aimed at the possible role and mechanism of mucilage under Al stess, as well as the effect of B through mist culture. Border cell viability, cell number and the relative area of mucilage and the effect of the Al contents and callose contents in root tip were studied under Al and B treatments with or without mucilage. Gel chromatography was used to separate mucilage polysaccharide - Al compound, and the complexation of polysaccharide with Al was confirmed by the addition of exonous polysaccharides. The main results were shown as follows:1. The Al content and the callose content were determined in the mist condition or solution condition, we found the damage is similar at 4mmol/L aluminum for 40h mist culture to 20μmol/L for 6h solution Culture. The proper Al concentration for mist culture was 4 mmol/L.2. Our research provided evidence that border cell viability at Al stress was reduced significantly (p＜0.05),but the number and the relative area of mucilage of border cells were higher than contrast, which showed that Al induced cell death as well as the thickening of mucilage and increasing of border cell number. B could increase the border cell viability significant under Al stess (p＜0.05), which indicated that B could alleviate Al toxicity through inhibiting the cell death.3. The elongation of root tip and the Al content and the callose content with or without mucilage were determined in mist condition. we found the Al content and callose content without mucilage in root tip were relatively higher than that with mucilage under Al stress. The relative root elongation rate was relatively lower without mucilage than that with mucilage. The results showed that mucilage could prevent roots from Al toxicity. The relative root elongation was relatively higher and root Al content relatively lower after adding B, which indicated that B can alleviate Al toxicity.4. With gel chromatography ( Sephacryl^TM S-200),all the mucilage polysaccharide was separated and the molecular weight was determined. Root tip mucilage was separated to two fractions, a high molecular mass polysaccharide and a lower molecular mass polysaccharide. The molecular weight of the high molecular mass polysaccharide was over 500KD, the lower molecular mass polysaccharide was 27.75KD in the contrast; while the molecular weight of the high molecular mass polysaccharide was over 500KD, the lower molecular mass polysaccharide was 450KD at the Al stress,and the peak time was in advance than contrast, and Al could bound the two fractions. These proved the mucilage polysaccharide could pound Al to form mucilage polysaccharide - Al compound. At Al stress,the mucilage polysaccaride content was higher than contrast (p＜0.05), molecular weight and the area of the peak at Al stress were increased. These indicated that Al promoted the increasing of polysaccharide which could bound Al. The polysaccharide content was lower than contrast and the peak time was in advance after adding B,indicated that B could promote the binding of higher molecular polysaccharide with Al.5. External polysaccharide could relatively decrease the Al content of root tip at Al stress. The Al content of root tip significantly decreased after the increasing of the external pectin (p＜0.05);and the Al content of root tip was decreased after the addition of the external T-40 dextran at Al stress which was significant at 0.1mg/mL (p＜0.05). Thus indicated external polysaccharide can protect root tip from Al injury, but the protecting mechanism is different because different polysacchaide has different characterization.In conclusion, Al stress could promote the production of border cell and its mucilage while induced its death. Mucilage polysaccharide could bound Al and become the barrier of Al entering into the root, thus alleviate Al toxicity. These proved mucilage could protect root tip from Al injury. The cell viability increased significantly and the higher content and larger molecules of mucilage polysaccaride indicated under Al stress after B addtion indicated, B could alleviate Al toxicity.