Impact of phosphorus mineral source (Al–P or Fe–P) and pH on cluster-root formation and carboxylate exudation in Lupinus albus L.
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Lupinus albus L. were grown in rhizoboxes containing a soil amended with sparingly available Fe–P or Al–P (100 μg P g−1 soil/resin mixture). Root halves of individual plants were supplied with nutrient solution (minus P) buffered at either pH 5.5 or 7.5, to assess whether the source of mineral-bound P and/or pH influence cluster-root growth and carboxylate exudation. The P-amended soil was mixed 3:1 (w/w) with anion-exchange resins to allow rapid fixation of carboxylates. Treatments lasted 10 weeks. Forty percent and 30% of the root mass developed as cluster roots in plants grown on Fe–P and Al–P respectively, but cluster-root growth was the same on root-halves grown at pH 5.5 or 7.5. Mineral-bound P source (Al– or Fe–P) had no influence on the types of carboxylates measured in soil associated with cluster roots—citrate (and trace amounts of malate and fumarate) was the only major carboxylate detected. The [citrate] in the rhizosphere of cluster roots decreased with increased shoot P status (suggesting a systemic effect) and also, only for plants grown on Al–P, with decreased pH in the root environment (suggesting a local effect). In a separate experiment using anion exchange resins pre-loaded with malate or citrate, we measured malate (50%) and citrate (79%) recovery after 30 days in soil. We therefore, also conclude that measurements of [citrate] and [malate] at the root surface may be underestimated and would be greater than the 40- and 1.6-μmol g−1 root DM, respectively estimated by us and others because of decomposition of carboxylates around roots prior to sampling.
KeywordsCitrate P-deficiency Proteoid roots Split-root design Systemic signal White lupin
Helpful comments by anonymous reviewers were greatly appreciated. MWS was the recipient of a Research Fellowship at the Institute Phytosphere Forschungszentrum, Jülich, Germany (kindly provided by Professor U. Schurr) and a Janice Klumpp award from the University of Western Australia. Many thanks to Carola Mohl for the ICP-MS analysis, Manfred Michulitz for the N and S measurements and to Marion Roeb for the help in setting-up the rhizoboxes and growing the plants. Also, thanks to Drs. Siegfried Jahnke, Peter Minchin, Björn Thiele and Achim Walter for the helpful discussions. Many thanks to Dr. Walter Schröder for help with the SEM image in Fig. 1 (all FZ Jülich). This research was supported in part by the Australian Research Council.
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