Comparative Evaluation of Zinc and Lead and their Synergistic Effects on Growth and Some Physiological Responses of Hassawi Okra (Hibiscus esculentus) Seedlings

Soil contamination with heavy metals has become a worldwide problem, leading to a reduction of plant growth and productivity. The objective of this study was carried out to compare the effect of different levels of zinc (Zn2+) or lead (Pb2+)and their interactions on growth and physiological changes of Hassawi okra seedlings. These seedlings were grown in a soil, to which different levels (0, 5, 10, 20, 40, 80 mM) of Zn2+ or Pb2+ were added with water (control), singly or in combinations. The results revealed that, okra seedlings growth were reduced gradually with increasing Zn2+ or/and Pb2+. These parameters were completely inhibited at 80 mM of Zn2+ or Pb2+, when they added in combination. While, 5 mM Zn2+ had a favorable effect on these growth parameters. The contents of chl. a and chl. b were significantly reduced as a result of exposure to Zn2+ or/and Pb2+. However, carotenoids content was increased at lower levels as compared with control. The increased of Zn2+ or/and Pb2+ levels inhibited sugars accumulation while the contents of protein, total free amino acids and proline were promoted. Pb2+ had more toxic effect than Zn2+. The toxicity caused by each metal alone was lower than that caused when they were added in combinations. Our data suggested that Zn2+ and Pb2+ exhibited synergistic effects on the growth and physiological responses of Hassawi okra seedlings, leading to injurious effects followed by death of these seedlings at 80 mM. Combined exposure to Zn2+ and Pb2+ enhanced or inhibited some of the effects, that were induced when only one metal was applied to growth medium.