Conservative shoot P content under varying supply

The Australian legume Viminaria juncea forms both cluster roots and mycorrhizal associations. de Campos et al. manipulate P supply over a range of 0 to 50 mg P kg^–1 dry soil in order to investigate if these root specializations are expressed at different shoot P concentrations [P]. Remarkably, they find that shoot [P] over the entire range of supplies is constant. They conclude that to maintain stable shoot [P] values, V. juncea must down-regulate its growth rate when very little P is supplied; conversely, it down-regulates its P-uptake capacity very tightly at higher P supply, when its maximum growth rate has been reached. The persistence of cluster roots and mycorrhizal colonization up to the highest P treatments is probably a consequence of tightly controlled shoot [P].