Toxicity of metals to roots of cowpea in relation to their binding strength.

Metal phytotoxicity is important in both environmental and agricultural systems. A solution culture study examined the toxicity of 26 metals to roots of cowpea (Vigna unguiculata (L.) Walp.); new data were collected for 15 metals and published data for 11 metals. Metal toxicity, calculated as causing a 50% reduction in root elongation rate, was determined based on either the measured concentration in the bulk solution (EC50b) or the calculated activity at the outer surface of the plasma membrane (EA50o0 ). The EC50b values ranged from 0.007 mM for Tl to 98,000mM for K, with the order of rhizotoxicity to cowpea, from most to least toxic, beingTl¼Ag>Cu>Hg¼Ni¼Ga¼Ru¼In Sc¼Cd¼Gd¼La¼Co¼Cs¼Pb>Zn¼Al¼H>Mn>Ba¼Sr>Li>Mg>Ca¼ b>Zn¼Al¼H>Mn>Ba¼Sr>Li>Mg>Ca¼ Na>K. The EA50o0 values suggest that the binding of metals to hard ligands is an important, general, nonspecific mechanism of toxicity, a hypothesis supported by the similar toxicity symptoms to roots of cowpea by many metals. However, additional mechanisms, such as strong binding to soft ligands, substantially increase rhizotoxicity of some metals, especially Tl, Ag, and Cs. Besides direct toxic effects, osmotic effects or reduced activity of Ca2þ at the outer surface of the root plasma membrane (and resultant Ca deficiency) may decrease short-term root growth. [ABSTRACT FROM AUTHOR]