Nutrient uptake, cluster root formation and exudation of protons and citrate in Lupinus albus as affected by localized supply of phosphorus in a split-root system

White lupin (Lupinus albus L. cv. Amiga) plants were grown in nutrient solution, using a split-root system with two compartments to examine the effect of localized phosphorus (P) supply on nutrient uptake, cluster root formation and root exudation. Phosphorus was supplied at 0 and 250 μM P in the −P and +P compartments, respectively, with three treatments of −P+P, −P−P and +P+P. Shoot dry weight in −P+P was higher than that in −P−P, but not significantly different from that in +P+P. Shoot P concentration [P] in −P+P was lower than that in +P+P, but higher than that in −P−P. In the −P+P treatment, root [P] was lower in the −P than in the +P root half and the similar trend was observed for root dry weight. Compared with −P−P, 19% of P uptake in the +P root half in −P+P was translocated to the −P root half. Phosphorus deficiency decreased K, Mg and S uptake in −P−P, and localized P deficiency reduced S uptake in the −P root half in −P+P. The number of cluster roots and exudation rates of protons, citrate and acid phosphatase (APase) in both root halves were higher in −P−P than in +P+P. The localized P supply increased the total number of cluster roots in the +P root half compared with the −P root half in −P+P. Compared with −P−P, the localized P supply significantly increased the shoot [P] and decreased the exudation rate of citrate and APase in both root halves in −P+P. Exudation rate of citrate in the −P root half in −P+P was higher than that of the +P root half in +P+P. Localized P supply reduced proton release in the +P and even −P root halves in −P+P, but the pH was lower in the −P than +P compartments for the individual plant, indicating a significant effect of the localized P supply on proton release. The results suggest that cluster root formation and citrate exudation are regulated by the shoot [P] and affected by localized supply of external P, and that proton release is inhibited by localized P supply by altering the balance of anion and cation uptake.