Formation of cluster roots and citrate exudation by Lupinus albus in response to localized application of different phosphorus sources

It is well known that lateral roots of some plants can respond to heterogeneous nutrient supply, but it is unknown how the cluster roots of white lupin, the special root structure induced by phosphorus (P) deficiency, respond to P-rich patches of different P sources in soil. The present study examined the effect of localized P supply on cluster root formation and root exudation by white lupin (Lupinus albus L. cv. Kiev Mutant) grown in a P-deficient sandy soil. Phosphorus was applied only to one compartment in a four-compartment split-root system as either hydroxyapatite (Ca-P), iron phosphate (Fe-P), phytate (Phy-P) or KH2PO4 (K-P). The proportion of dry root biomass allocated to cluster roots in P-supplemented compartments was greater than that in the compartments without P application. The shoot P concentration of Phy-P plants was much lower than that of Fe-P ones, but there was no difference in the proportion of cluster roots in these two treatments. In comparison with Phy-P and Fe-P, there was a lower proportion of cluster roots in Ca-P and K-P plants. Citrate exudation rate increased significantly in the Fe-P-supplemented compartment, but did not respond to other P sources, compared with the compartment without P application. It is concluded that localized P application significantly stimulates cluster root formation by white lupin, and cluster root formation and citrate exudation are greatly dependent on the form of P supplied.