Effects Of Growth Medium And Localized Phosphorus Supply On Cluster Root Formation And Citrate Exudation By Lupinus Albus

Cluster root is one of the major plant adaptations for P acquisition under P deficient conditions. White lupin has the capacity to form cluster roots and exude organic acid anions (dominantly citrate) and protons to mobilize sparingly soluble P that is unavailable to other plants under P-deficiency. The effects of growth medium, localized phosphorus supply and phosphorus sources on the formation of cluster roots and citrate exudation by Lupinus albus (Lupinus albus L. cv. Kiev Mutant) were investigated. The main results were as following:1. In a sand/solution split-root system, phosphorus was supplied at 1 (low-P plants) or 50 (high-P plants) μM P as KH₂PO₄ to the solution compartment. In contrast to the high-P plants, P concentration in shoots of the low-P plants was lower; there was more cluster roots formed on low-P plants. Phosphorus mobilized from hydroxyapatite supplied to the sand compartment by the low-P plants showed a saturable increase with increasing P (hydroxyapatite) supply in the sand compartments. For low-P plants, cluster root formation was less in sand compartment than in solution compartment; the citrate exudation increased in the sand compartment and decreased in the solution compartment with time, showing a lack of synchronization in citrate exudation by two root halves grown in different media. These indicated that cluster root formation and citrate exudation can be significantly affected by root growth medium and P supply in addition to being regulated by shoot P status.2. The percentage of root biomass allocated to cluster roots increased in the soil layer supplemented with P in a split-root system. When lowering the P-applied layer in one compartment in the pot, the increase became more evident, concurrently accompanied by a decrease in cluster root formation in other layers in the root system. These indicated that cluster roots formation could be modified by heterogeneous distribution of phosphorus in soil.3. In a four-compartment split-root system, phosphorus was supplied just to one compartment as hydroxyapatite (Ca-P), iron phosphate (Fe-P), phytate (Phy-P) and KH₂PO₄ (K-P). It was found that Ca-P was an effective P source for white lupin under P-deficient conditions, and Phy-P was the least available one for white lupin. Compared to the compartment without P application, the percentage of dry root biomass allocated to cluster roots was greater in the P-supplemented compartments. P concentration in shoots of Phy-P plants was significantly lower than that of Fe-P plants, but the total cluster root percentage was similar between the two treatments. Citrate was the dominant organic acid anion detected in the root exudates, irrespective of P supply by different sources. Compared with the compartment without P applied, citrate exudation rate only increased significantly in Fe-P applied compartment, with no increase in other sources of P applied compartments. These indicated that cluster root formation could be enhanced by localized P application and affected by different P sources. P source influences citrate exudation.