Toxic effects of Ni2+ on growth of cowpea (Vigna unguiculata)

Despite the importance of Ni-polluted soils throughout the world, comparatively little is known about the activity of Ni2+ required to reduce plant growth and the effects that Ni2+ toxicity has on the plant. Cowpea (Vigna unguiculata (L.) Walp. cv Caloona) was grown in dilute nutrient solutions to investigate the effect of Ni2+ activity on shoot and root growth. A Ni2+ activity of 1.4 mu M was found to cause a 10% reduction in the relative fresh mass of the root and shoots. The primary site of Ni2+ toxicity was the shoots, with the younger leaves displaying an interveinal chlorosis (possibly a Ni-induced Fe deficiency) at Ni2+ activities N1.7 mu M. Lateral root formation was inhibited in the two highest Ni2+ treatments (3.3 and 5.1 mu M), and the roots growing at the highest Ni2+ activity were short and stubby and brown in color. However, no other symptoms of toxicity were observed on the roots at lower Ni2+ activities.