Cluster roots, P and growth rate in Lupinus

Some Lupinus species produce cluster roots in response to low plant phosphorus (P) status. Wang et al. measure relative growth rate (RGR) among three lupin species, L. albus, L. pilosus and L. atlanticus, with similar shoot P status and find that cluster-root formation is suppressed at high leaf P concentration, irrespective of RGR. Variation in cluster-root formation among these species cannot be explained by species-specific variation in RGR or leaf P concentration. This goes against the expectation that, even when P-uptake rates are high, plants with fast growth rates might not accumulate shoot P and hence a correlation between RGR and cluster-root formation could be anticipated.