Is there a critical level of shoot phosphorus concentration for cluster-root formation in Lupinus albus?

This study examined the effects of localised phosphorus (P) supply on cluster-root formation and citrate exudation in white lupin (Lupinus albus L. cv. Kiev Mutant). White lupin plants were grown in nutrient solutions with a range of P supplies in a split-root system with one root half deprived of P and the other root supplied with 0, 2, 5, 8, 10 or 75 μm P. Plants were also grown in soil with or without organic matter added to the top layer. The proportion of cluster roots as a percentage of the total root biomass decreased similarly on both root halves with increasing P supply in the hydroponic experiments. More than 18% of the P taken up by the P-supplied root halves was incorporated into the P-deprived halves. Irrespective of the P supply or organic matter addition in the experiments, the proportion of cluster roots and the rate of citrate exudation decreased sharply with increasing P concentration in the shoots up to a critical level of 2-3 mg P g^-1 dry weight. In contrast, the rate of proton release was higher in P-deprived root halves than in P-supplied ones. The formation of cluster roots is regulated by shoot P concentration with a critical level of 2-3 mg g^-1. Citrate exudation is predominantly governed by shoot P status, whereas proton release strongly responds to local P supply.