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

Cadmium (Cd2+) is one of the major widespread environmental pollutants, and can cause serious problems to all organisms. Lead (Pb2+) is another wide spread dangerous heavy metal. Tobacco is a popular growing economic crop in China. Most tobacco growing region soils contain excessive Cd2+ and Pb2+. To assess anatomic changes of tobacco roots under Cd2+, Pb2+, and Cd2++Pb2+ 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 Cd2+ added to soil were 0, 3, 6, 10, 30, 60, and 100 mg kg−1 dry soil. The amounts of Pb2+ added to soil were 0, 150, 300, 450, 600, 750, and 1 000 mg kg−1 dry soil. The amounts of Cd2++Pb2+ 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 Cd2+ and Pb2+ 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 Cd2+ and Pb2+ and carbonate bound Cd2+ and Pb2+ in soil increased with the amounts of Cd2+ and Pb2+ added to soil, and the contents of both Cd2+ and Pb2+ in roots were significantly increased along with stress time and the amounts of Cd2+ and Pb2+ added to soil. The growing of tobacco in Cd2+ and Cd2++Pb2+ 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 Cd2+ contents in roots and stress time. While, there were no abnormal external morphological and anatomical changes of roots under Pb2+ stress. It was suggested that Cd2+ stress could cause abnormal anatomic changes of roots, but Pb2+ stress could not.