Nitrogen nutrition can influence cluster root fort-nation in many wild species, but the effect of N form on cluster root formation and root exudation by white lupin is not known. In a solution culture study, we examined the effect of N nutrition (ammonium, nitrate, both or N-2 fixation) on cluster root formation and H+ extrusion by white lupin plants under deficient and adequate P supply. The number of cluster roots increased greatly when plants were supplied with 1 muM P compared with 50 muM P, the increase being 7.8-fold for plants treated with (NH4)(2)SO4, 3-fold for plants treated with KNO3 and NH4NO3, and 2.4-fold for N-2-fixing plants. Under P deficiency, NH4+-N supply resulted in production of a greater number and biomass of cluster roots than other N sources. Dry weight of cluster roots was 30% higher than that of non-cluster roots in P-deficient plants treated with (NH4)(2)SO4 and NH4NO3. In plants treated with sufficient P (50 muM) the weight of non-cluster roots was approx. 90% greater than that of cluster roots, Both total (mumol per plant h(-1)) and specific (mumol g(-1) root d. wt h(-1)) H+ extrusions were greatest from roots of plants supplied with (NH4)(2)SO4, followed by those supplied with NH4NO3 and N-2 fixation, whereas plants receiving KNO3 had negative net H+ extrusion between the third and fifth week of growth (indicating uptake of protons or release of OH- ions). The rate of proton extrusion by NH4+-N-fed plants was similar under P-deficient and P-sufficient conditions. In contrast, proton exudation by N-2-fixing plants and KNO3-treated plants was ten-fold greater under P deficiency than under P sufficiency. In comparison with P deficiency, plants treated with 50 muM P had a significantly higher concentration of P in roots, shoots and youngest expanded leaves (YEL). Compared with the N-2 fixation and KNO3 treatments, total N concentration was highest in roots, shoots and YEL of plants supplied with (NH4)2SO4 and NH4NO3, regardless of P supply. Under P deficiency, K concentrations in roots decreased at all N supplies, especially in plants treated with (NH4)(2)SO4 and NH4NO3, which coincided with the greatest H+ extrusion at these P and N supplies. In conclusion, NH4-N nutrition stimulated cluster root formation and H+ extrusion by roots of P-deficient white lupin. (C) 2002 Annals of Botany Company.