ULTRASTRUCTURAL LOCALIZATION AND EFFECT OF LEAD ON MINERAL ELEMENTS IN CALLUS OF ALTERNANTHERA PHILOXEROIDES

Heavy metal pollution of water bodies has become an increasingly serious environmental problem. Understand the principles of heavy metal contamination and heavy metals in water treatment have become a topic that must be addressed. Heavy metals can cause serious health hazards to animals and humans through the food chain due to the refractory, easy accumulation, toxicity and other characteristics. Therefore, much attention has been devoted to the studies on the mechanisms of heavy metal tolerance in plants and how to improve heavy metal tolerance of plants. Although antioxidative reactions of plants subjected to Pb2+ have been studied by several authors, there are few reports on the research of callus response to heavy metals. In view of this, callus of Alternanthera philoxeroides was cultured to overcome the uncontrollability of light, temperature, moisture and plant growth and development in the natural state, so that the experimental data is more repeated and scientific. Callus of Alternanthera philoxeroides stems was cultured in 1/2 MS medium including 6-BA (3.0 mg/L) and NAA (0.2 mg/L) after the disinfection of 70% ethanol 30s, 5% sodium hypochlorite 10min and 0.1% mercuric chloride 10min, was used as experimental material, and Pb2+ was selected as the stress factor. The toxic effects of Pb gradient concentrations (0, 0.2, 0.4, 0.8 and 1.6 mmol/L) on the accumulation, subcellular distribution, ultrastructural localization and mineral elements were studied. The results indicated that, with the increasing of Pb concentration, (1) Alternanthera philoxeroides callus was able to accumulate lead, the bioconcentration factor was 2341-2681; (2) Subcellular fraction analysis revealed that the contents of Pb increased gradually in cell wall, organelles and soluble fraction, but the distribution ratio was obviously unequal, and the Pb levels occurred in different parts of callus cell with the following sequence cell wall>organelles>soluble fraction. The toxic symptoms of callus showed an evident correlation between dose and effect and the ultrastructural damage was closely related to the distribution of Pb; (3) Ultrastructural localization observation showed that Pb mainly appeared in cell wall, and was also found in cell membrane structure and cell matrix; (4) The effects of lead on mineral elements in three subcellular fractions were different, of which the absorption of macroelements P, K, Mg and microelements Na, Zn, Mn were inhibited; the content of Ca increased at first and then decreased in cell wall, decreased gradually in organelles, increased gradually in soluble fraction; Fe and B increased at first and then decreased in subcellular fractions; the content of Cu in cell wall and organelles displayed with increase trends, increased at first and then decreased in soluble fraction; the content of Si in cell wall decreased gradually, increased at first and then decreased in organelles, while it was not able to be measured in soluble fraction. It could clearly be seen that the ion equilibrium in subcellular fractions was disrupted under Pb stress, resulting in disorder of physiological metabolism. All of these were major performance of lead on callus of Alternanthera philoxeroides.