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 (Zn super(2+)) or lead (Pb super(2+))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 Zn super(2+) or Pb super(2+) were added with water (control), singly or in combinations. The results revealed that, okra seedlings growth were reduced gradually with increasing Zn super(2+) or/and Pb super(2+). These parameters were completely inhibited at 80 mM of Zn super(2+) or Pb super(2+), when they added in combination. While, 5 mM Zn super(2+) 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 Zn super(2+) or/and Pb super(2+). However, carotenoids content was increased at lower levels as compared with control. The increased of Zn super(2+) or/and Pb super(2+) levels inhibited sugars accumulation while the contents of protein, total free amino acids and proline were promoted. Pb super(2+) had more toxic effect than Zn super(2+). The toxicity caused by each metal alone was lower than that caused when they were added in combinations. Our data suggested that Zn super(2+) and Pb super(2+) 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 Zn super(2+) and Pb super(2+) enhanced or inhibited some of the effects, that were induced when only one metal was applied to growth medium.