Toxic effects of low concentrations of Cu on nodulation of cowpea (Vigna unguiculata)

Although Cu is phytotoxic at Cu2+ activities as low as 1–2 μM, the effect of Cu2+ on the nodulation of legumes has received little attention. The effect of Cu2+ on nodulation of cowpea (Vigna unguiculata (L.) Walp. cv. Caloona) was examined in a dilute solution culture system utilising a cation exchange resin to buffer solution Cu2+. The nodulation process was more sensitive to increasing Cu2+ activities than both shoot and root growth; whilst a Cu2+ activity of 1.0 μM corresponded to a 10% reduction in the relative yield of the shoots and roots, a Cu2+ activity of 0.2 μM corresponded to a 10% reduction in nodulation. This reduction in nodulation with increasing Cu2+ activity was associated with an inhibition of root hair formation in treatments containing ≥0.77 μM Cu2+, rather than to a reduction in the size of the Rhizobium population.