Effects of Chronic Stress of Cadmium and Lead on Anatomical Structure of Tobacco Roots

Abstract

Abstract Cadmium (Cd 2+) is one of the major widespread environmental pollutants, and can cause serious problems to all organisms. Lead (Pb 2+) is another wide spread dangerous heavy metal. Tobacco is a popular growing economic crop in China. Most tobacco growing region soils contain excessive Cd 2+ and Pb 2+. To assess anatomic changes of tobacco roots under Cd 2+, Pb 2+, and Cd 2++Pb 2+ chronic stress, a pot experiment was carried out in field. The tobacco seedlings with 6 leaves were transplanted to pots in which soil was placed. The amounts of Cd 2+ added to soil were 0, 3, 6, 10, 30, 60, and 100 mg kg −1 dry soil. The amounts of Pb 2+ added to soil were 0, 150, 300, 450, 600, 750, and 1 000 mg kg −1 dry soil. The amounts of Cd 2++Pb 2+ added to soil were 0+0, 3+150, 6+300, 10+450, 30+600, 60+750, and 100+1 000 mg kg −1 dry soil. The contents of Cd 2+ and Pb 2+ in root systems were determined by inductively coupled plasma, and the anatomical structure was studied by method of paraffin sectioning. The results revealed that the amounts of exchangeable Cd 2+ and Pb 2+ and carbonate bound Cd 2+ and Pb 2+ in soil increased with the amounts of Cd 2+ and Pb 2+ added to soil, and the contents of both Cd 2+ and Pb 2+ in roots were significantly increased along with stress time and the amounts of Cd 2+ and Pb 2+ added to soil. The growing of tobacco in Cd 2+ and Cd 2++Pb 2+ polluted soil for 50, 100, and 150 d resulted in some abnormal external morphological and anatomical changes in ripe region of lateral roots. All the abnormal roots had abnormal vascular cylinders, and the ratio of abnormal external morphological and anatomical changes of roots positively correlated with the Cd 2+ contents in roots and stress time. While, there were no abnormal external morphological and anatomical changes of roots under Pb 2+ stress. It was suggested that Cd 2+ stress could cause abnormal anatomic changes of roots, but Pb 2+ stress could not.