Cluster-root formation and carboxylate release in three Lupinus species as dependent on phosphorus supply, internal phosphorus concentration and relative growth rate.
OBJECTIVE
Some Lupinus species produce cluster roots in response to low plant phosphorus (P) status. The cause of variation in cluster-root formation among cluster-root-forming Lupinus species is unknown. The aim of this study was to investigate if cluster-root formation is, in part, dependent on different relative growth rates (RGRs) among Lupinus species when they show similar shoot P status.


METHODS
Three cluster-root-forming Lupinus species, L. albus, L. pilosus and L. atlanticus, were grown in washed river sand at 0, 7·5, 15 or 40 mg P kg(-1) dry sand. Plants were harvested at 34, 42 or 62 d after sowing, and fresh and dry weight of leaves, stems, cluster roots and non-cluster roots of different ages were measured. The percentage of cluster roots, tissue P concentrations, root exudates and plant RGR were determined.


RESULTS
Phosphorus treatments had major effects on cluster-root allocation, with a significant but incomplete suppression in L. albus and L. pilosus when P supply exceeded 15 mg P kg(-1) sand. Complete suppression was found in L. atlanticus at the highest P supply; this species never invested more than 20 % of its root weight in cluster roots. For L. pilosus and L. atlanticus, cluster-root formation was decreased at high internal P concentration, irrespective of RGR. For L. albus, there was a trend in the same direction, but this was not significant.


CONCLUSIONS
Cluster-root formation in all three Lupinus species was suppressed at high leaf P concentration, irrespective of RGR. Variation in cluster-root formation among the three species cannot be explained by species-specific variation in RGR or leaf P concentration.
School of Plant Biology and Institute of Agriculture, The University of Western Australia, 35 Stirling Highway, Crawley (Perth), WA 6009, Australia.
Xing
X
Wang
Stuart J
SJ
Pearse
Hans
H
Lambers
D018533:Biomass; D002264:Carboxylic Acids / Q000378:metabolism*; D031297:Lupinus / Q000254:growth & development* / Q000378:metabolism*; D008954:Models, Biological; D010758:Phosphorus / Q000378:metabolism*; D018517:Plant Roots / Q000254:growth & development*; D018520:Plant Shoots / Q000378:metabolism; D058441:Rhizosphere
D002264:Carboxylic Acids; D010758:Phosphorus
Cluster roots; L. atlanticus; L. pilosus; Lupinus albus; net assimilation rate; phosphorus acquisition; relative growth rate