Technical Abstract: Induction of ferric-reductase activity in dicots and non-grass monocots is a well-recognized response to Fe deficiency. Recent evidence has shown that Cu deficiency also induces plasma-membrane iron reduction. In this study, we investigated whether other nutrient deficiencies could also induce ferric-reductase activity in roots of pea seedlings (Pisum sativum L. cv Sparkle). Of the nutrient deficiencies tested (K, Mg, Ca, Mn, Zn, Fe, and Cu), only Cu and Fe deficiencies elicited a response. Cu deficiency induced an activity intermediate between Fe-deficient and control plant activities. To ascertain if the same reductase is induced by Fe and Cu deficiency, concentration- and pH-dependent kinetics of root ferric reduction were compared in plants grown under control, -Fe and -Cu conditions. Additionally, rhizosphere acidification, another process induced by Fe deficiency, was quantified in pea seedlings grown under the three regimes. Control, Fe-deficient, and Cu-deficient plants exhibited no major differences in pH optima or Km for the kinetics of ferric reduction. However, the Vmax for ferric reduction was dramatically influenced by plant nutrient status, increasing by 16 to 38 fold under Fe deficiency and 1.5 to 4 fold under Cu deficiency, compared with that of control plants. These results are consistent with a model in which varying amounts of the same enzyme are deployed on the plasma membrane in response to plant iron or copper status. Rhizosphere-acidification rates in the copper-deficient plants were similarly intermediate between those of the control and Fe- deficient plants. These results suggest that Cu deficiency induces the same suite of responses induced by Fe deficiency in peas.