In old, phosphorus (P)-impoverished habitats, root specializations such as cluster roots efficiently mobilize and acquire P by releasing large amounts of carboxylates in the rhizosphere. These specialized roots are rarely mycorrhizal. We investigated whether Discocactus placentiformis (Cactaceae), a common species in nutrient-poor campos rupestres over white sands, operates in the same way as other root specializations. Discocactus placentiformis showed no mycorrhizal colonization, but exhibited a sand-binding root specialization with rhizosheath formation. We first provide circumstantial evidence for carboxylate exudation in field material, based on its very high shoot manganese (Mn) concentrations, and then firm evidence, based on exudate analysis. We identified predominantly oxalic acid, but also malic, citric, lactic, succinic, fumaric, and malonic acids. When grown in nutrient solution with P concentrations ranging from 0 to 100 μM, we observed an increase in total carboxylate exudation with decreasing P supply, showing that P deficiency stimulated carboxylate release. Additionally, we tested P solubilization by citric, malic and oxalic acids, and found that they solubilized P from the strongly P-sorbing soil in its native habitat, when the acids were added in combination and in relatively low concentrations. We conclude that the sand-binding root specialization in this nonmycorrhizal cactus functions similar to that of cluster roots, which efficiently enhance P acquisition in other habitats with very low P availability.1762345355Afif, E., Barrón, V., Torrent, J., Organic matter delays but does not prevent phosphate sorption by cerrado soils from Brazil (1995) Soil Sci, 159, pp. 207-211. , COI: 1:CAS:528:DyaK2MXkslWltbc%3DAlves, R.J.V., Kolbek, J., Plant species endemism in savanna vegetation on table mountains (Campo Rupestre) in Brazil (1994) Vegetation, 113, pp. 125-139Barrow, N.J., Towards a single-point method for measuring phosphate sorption by soils (2000) Aust J Soil Res, 38, pp. 1099-1113. , COI: 1:CAS:528:DC%2BD3cXot1Wjs7g%3DBates, T.R., Lynch, J.P., Root hairs confer a competitive advantage under low phosphorus availability (2001) Plant Soil, 236, pp. 243-250. , COI: 1:CAS:528:DC%2BD3MXptlKhu7w%3DBates, D., Maechler, M., Bolker, B., (2012) Lme4: linear mixed-effects models using S4 classes, , R Package version 0.99875-6Bolan, N.S., Naidu, R., Mahimairaja, S., Baskaran, S., Influence of low-molecular-weight organic acids on the solubilization of phosphates (1994) Biol Fertil Soils, 18, pp. 311-319. , COI: 1:CAS:528:DyaK2MXjsVCqtbk%3DBrazilian woodland savannah and seasonally dry forest species Persp Plant Ecol Evol Syst, 16 (2), pp. 64-74Cândido, H.G., (2012) Estratégias de aquisição de nutrientes e estequiometria ecológica em comunidades de campos rupestres, , Brasil. Master’s Dissertation, Universidade Estadual de Campinas, Campinas, BrazilChen, S.L., Yang, L.T., Lin, Z.H., Tang, N., Roles of organic acid metabolism in plant tolerance to phosphorus-deficiency (2013) Progress in Botany 74, pp. 213-237. , Lüttge U, Beyschlag W, Rancis D, Cushman J, (eds), Springer, New YorkColwell, J.D., The estimation of the phosphorus fertilizer requirements of wheat in southern New South Wales by soil analysis (1963) Aust J Exp Agric Anim Husbandry, 3, pp. 190-197. , COI: 1:CAS:528:DyaF2cXnvVOhsQ%3D%3DBenites, M.V., Caiafa, A.N., de Mendonça, E.S., Schaefer, C.E., Ker, J.C., Solos e vegetação nos complexos rupestres de altitude da Mantiqueira e do Espinhaço (2003) Flor e Amb, 10, pp. 76-85De Campos, M.C.R., Phosphorus-acquisition and phosphorus-conservation mechanisms of plants native to south-western Australia or to Brazilian rupestrian fields (2012) PhD thesis, , University of Western Australia, PerthDinkelaker, B., Hengeler, C., Marschner, H., Distribution and function of proteoid roots and other root clusters (1995) Bot Acta, 108, pp. 183-200Dong, D., Peng, X., Yan, X., Organic acid exudation induced by phosphorus deficiency and/or aluminium toxicity in two contrasting soybean genotypes (2004) Physiol Plant, 122, pp. 190-199. , COI: 1:CAS:528:DC%2BD2cXovVyns7g%3DFox, T.R., Comerford, N.B., Influence of oxalate loading on phosphorus and aluminum solubility in spodosols (1992) Soil Sci Soc Am J, 56, pp. 290-294. , COI: 1:CAS:528:DyaK38XitlCmsbg%3DGardner, W.K., Boundy, K.A., The acquisition of phosphorus by Lupinus albus L. IV. The effect of interplanting wheat and white lupin on the growth and mineral composition of the two species (1983) Plant Soil, 70, pp. 391-402. , COI: 1:CAS:528:DyaL3sXktVCltLg%3DGardner, W.K., Barber, D.A., Parbery, D.G., The acquisition of phosphorus by Lupinus albus L. III. The probable mechanism by which phosphorus movement in the soil/root interface is enhanced (1983) Plant Soil, 70, pp. 107-124. , COI: 1:CAS:528:DyaL3sXhsF2ls7k%3DGiovanetti, M., Mosse, B., An evaluation of techniques for measuring vesicular arbuscular mycorrhizal infection in roots (1980) New Phytol, 84, pp. 489-500Graetz, D.A., Nair, V.D., Phosphorus sorption isotherm determination (2009) Methods of phosphorus analysis for soils, sediments, residuals, and waters, pp. 33-37. , Kovar JL, Pierzynski GM, (eds), Virginia Tech University, VirginiaGrierson, P.F., Attiwill, P.M., Chemical characteristics of the proteoid root mat of Banksia integrifolia L (1989) Aust J Bot, 37, pp. 137-143. , COI: 1:CAS:528:DyaK3cXhvFShtg%3D%3DHayes, P., Turner, B.L., Lambers, H., Laliberté, E., Foliar nutrient concentrations and resorption efficiency in plants of contrasting nutrient-acquisition strategies along a 2-million-year dune chronosequence (2014) J Ecol, 102, pp. 396-410. , COI: 1:CAS:528:DC%2BC2cXjt1Shsro%3DHinsinger, P., Bioavailability of soil inorganic P in the rhizosphere as affected by root-induced chemical changes: a review (2001) Plant Soil, 237, pp. 173-195. , COI: 1:CAS:528:DC%2BD38XovVWlsQ%3D%3DHoffland, E., van den Boogaard, R., Nelemans, J., Findenegg, G., Biosynthesis and root exudation of citric and malic acids in phosphate-starved rape plants (1992) New Phytol, 122, pp. 675-680. , COI: 1:CAS:528:DyaK3sXhs1ygs70%3DHoffland, E., Wei, C., Wissuwa, M., Organic anion exudation by lowland rice (Oryza sativa L.) at zinc and phosphorus deficiency (2006) Plant Soil, 283, pp. 155-162. , COI: 1:CAS:528:DC%2BD28XpvVGgtro%3DHopper, S.D., OCBIL theory: towards an integrated understanding of the evolution, ecology and conservation of biodiversity on old, climatically buffered, infertile landscapes (2009) Plant Soil, 322, pp. 49-86. , COI: 1:CAS:528:DC%2BD1MXhtVWisrnNJauregui, M.A., Reisenauer, H.M., Dissolution of oxides of manganese and iron by root exudate components (1982) Soil Sci Soc Am J, 46, pp. 314-317. , COI: 1:CAS:528:DyaL38Xkt1aksr4%3DJohansen, D.A., (1940) Plant microtechnique, , McGraw-Hill, New YorkJohnson, S.E., Loeppert, R.H., Role of organic acids in phosphate mobilization from iron oxide (2006) Soil Sci Soc Am J, 70, pp. 222-234. , COI: 1:CAS:528:DC%2BD28Xht1Wnsrw%3DJones, D.L., Organic acids in the rhizosphere – a critical review (1998) Plant Soil, 205, pp. 25-44. , COI: 1:CAS:528:DyaK1MXhtlGjs78%3DKeerthisinghe, G., Hocking, P.J., Ryan, P.R., Delhaize, E., Effect of phosphorus supply on the formation and function of proteoid roots of white lupin (Lupinus albus L.) (1998) Plant Cell Environ, 21, pp. 467-478. , COI: 1:CAS:528:DyaK1cXlt1elsrg%3DKendall, M.G., (1970) Rank correlation methods, , Griffin, LondonKoske, R.E., Gemma, J.N., A modified procedure for staining roots to detect VA mycorrhizas (1989) Mycol Res, 92, pp. 486-488Laliberté, E., Turner, B.L., Costes, T., Pearse, S.J., Wyrwoll, K., Zemunik, G., Lambers, H., Experimental assessment of nutrient limitation along a 2-million-year dune chronosequence in the south-western Australia biodiversity hotspot (2012) J Ecol, 100, pp. 631-642Lambers, H., Shane, M.W., Cramer, M.D., Pearse, S.J., Veneklaas, E.J., Root structure and functioning for efficient acquisition of phosphorus: matching morphological and physiological traits (2006) Ann Bot, 98, pp. 693-713. , PID: 16769731Lambers, H., Chapin, F.S., Pons, T.L., (2008) Plant physiological ecology, , Springer, New YorkLambers, H., Raven, J.A., Shaver, G.R., Smith, S.E., Plant nutrient-acquisition strategies change with soil age (2008) Trends Ecol Evol, 23, pp. 95-103. , PID: 18191280Lambers, H., Brundrett, M.C., Raven, J.A., Hopper, S.D., Plant mineral nutrition in ancient landscapes: high plant species diversity on infertile soils is linked to functional diversity for nutritional strategies (2010) Plant Soil, 334, pp. 11-31. , COI: 1:CAS:528:DC%2BC3cXhtVaqtbnJLamont, B., The biology of dauciform roots in the sedge Cyathochaete avenacea (1974) New Phytol, 73, pp. 985-996Lamont, B., Mechanisms for enhancing nutrient uptake in plants, with particular reference to mediterranean South Africa and Western Australia (1982) Bot Rev, 48, pp. 597-689. , COI: 1:CAS:528:DyaL3sXntVymtA%3D%3DLenth, R.V., (2014) Lsmeans: least-Squares Means, , R Package version 2:05Lynch, J.P., Brown, K.M., Topsoil foraging—an architectural adaptation of plants to low phosphorus availability (2001) Plant Soil, 225, pp. 225-237Ma, J.F., Hiradate, S., Matsumoto, H., High aluminum resistance in buckwheat. II Oxalic acid detoxifies aluminum internally (1998) Plant Physiol, 117, pp. 753-759. , COI: 1:CAS:528:DyaK1cXkvVylsbs%3DMachado, M., Braun, P., Taylor, N.P., Zappi, D., (2013) Discocactus placentiformis, , IUCN 2013 IUCN Red List Threatened Species Version 2013.2Martin, L.A., James, G., Unusual habitats, unusual plants (2009) Cactus Succul J, 81, pp. 106-112McCully, M.E., Roots in soil: unearthing the complexities of roots and their rhizospheres (1999) Annu Rev Plant Physiol Plant Mol Biol, 50, pp. 695-718. , PID: 15012224, COI: 1:CAS:528:DyaK1MXkt1yktrg%3DMiller, R.M., The nonmycorrhizal root: a strategy for survival in nutrient-impoverished soils (2005) New Phytol, 165, pp. 655-658. , PID: 15720676Motomizu, S., Wakimoto, T., Tôei, K., Spectrophotometric determination of phosphate in river waters with molybdate and malachite green (1983) Analyst, 108, pp. 361-367. , COI: 1:CAS:528:DyaL3sXitVCjtrw%3DMuler, A.L., Oliveira, R.S., Lambers, H., Veneklaas, E.J., Does cluster-root activity benefit nutrient uptake and growth of co-existing species? (2014) Oecologia, 174, pp. 23-31. , PID: 23934064Myers, N., Mittermeier, R.A., Mittermeier, C.G., da Fonseca, G.A.B., Kent, J., Biodiversity hotspots for conservation priorities (2000) Nature, 403, pp. 853-858. , PID: 10706275, COI: 1:CAS:528:DC%2BD3cXhs1Olsr4%3DNagarajah, S., Posner, A.M., Quirk, J.P., Competitive adsorption of phosphate with polygalacturonate and other organic anions on kaolinite and oxide surfaces (1970) Nature, 228, pp. 83-85. , PID: 16058419, COI: 1:CAS:528:DyaE3MXht1agtw%3D%3DNambiar, E.K.S., Uptake of Zn65 from dry soil by plants (1976) Plant Soil, 44, pp. 267-271. , COI: 1:CAS:528:DyaE28XntlSguw%3D%3DNeumann, G., Martinoia, E., Cluster roots: an underground adaptation for survival in extreme environments (2002) Trends Plant Sci, 7, pp. 162-167. , PID: 11950612, COI: 1:CAS:528:DC%2BD38XjtVKnsbY%3DNeumann, G., Massonneau, A., Langlade, N., Dinkelaker, B., Hengeler, C., Römheld, V., Martinoia, E., Physiological aspects of cluster root function and development in phosphorus-deficient white lupin (Lupinus albus L.) (2000) Ann Bot, 85, pp. 909-919. , COI: 1:CAS:528:DC%2BD3cXjs12hu7o%3DNieuwenhuis, R., te Grotenhuis, M., Pelzer, B., Influence.ME: tools for detecting influential data in mixed effects models (2012) R J, 4, pp. 38-47Nishi, A.H., Vasconcellos-Neto, J., Romero, G.Q., The role of multiple partners in a digestive mutualism with a protocarnivorous plant (2013) Ann Bot, 111, pp. 143-150. , PID: 23131297, COI: 1:CAS:528:DC%2BC38XhvFShsrvPNorth, G.B., Nobel, P.S., Drought-induced changes in hydraulic conductivity and structure in roots of Ferocactus acanthodes and Opuntia ficus-indica (1992) New Phytol, 120, pp. 9-19Oburger, E., Kirk, G.J.D., Wenzel, W.W., Puschenreiter, M., Jones, D.L., Interactive effects of organic acids in the rhizosphere (2009) Soil Biol Biochem, 41, pp. 449-457. , COI: 1:CAS:528:DC%2BD1MXit1Omtr8%3DOlde Venterink, H., Does phosphorus limitation promote species-rich plant communities? (2011) Plant Soil, 345, pp. 1-9. , COI: 1:CAS:528:DC%2BC3MXptFymtbg%3DOlsen, S., Cole, C.V., Watanabe, F.S., Dean, L.A., Estimation of available phosphorus in soils by extraction with sodium bicarbonate (1954) USDA Circular, (939), pp. 1-19Palomo, L., Claassen, N., Jones, D.L., Differential mobilization of P in the maize rhizosphere by citric acid and potassium citrate (2006) Soil Biol Biochem, 38, pp. 683-692. , COI: 1:CAS:528:DC%2BD28XivVaqsbs%3DPearse, S.J., Veneklaas, E.J., Cawthray, G.R., Bolland, M.D.A., Lambers, H., Carboxylate release of wheat, canola and 11 grain legume species as affected by phosphorus status (2006) Plant Soil, 288, pp. 127-139. , COI: 1:CAS:528:DC%2BD28XhtFaksL7FPereira, C.G., Almenara, D.P., Winter, C.E., Fritsch, P.W., Lambers, H., Oliveira, R.S., Underground leaves of Philcoxia trap and digest nematodes (2012) Proc Natl Acad Sci USA, 109, pp. 1154-1158. , PID: 22232687, COI: 1:CAS:528:DC%2BC38XislWksrk%3DPeterson, R.L., Farquhar, M.L., Root hairs: specialized tubular cells extending root surfaces (1996) Bot Rev, 62, pp. 1-40Playsted, C.W.S., Johnston, M.E., Ramage, C.M., Edwards, D.G., Cawthray, G.R., Lambers, H., Functional significance of dauciform roots: exudation of carboxylates and acid phosphatase under phosphorus deficiency in Caustis blakei (Cyperaceae) (2006) New Phytol, 170, pp. 491-500. , PID: 16626471, COI: 1:CAS:528:DC%2BD28XlsVensL4%3D(2012) R: a language and environment for statistical computingRaghothama, K.G., Phosphate acquisition (1999) Annu Rev Plant Physiol Plant Mol Biol, 50, pp. 665-693. , PID: 15012223, COI: 1:CAS:528:DyaK1MXkt1yktrs%3DRoelofs, R.F.R., Rengel, Z., Cawthray, G.R., Dixon, K.W., Lambers, H., Exudation of carboxylates in Australian Proteaceae: chemical composition (2001) Plant Cell Environ, 24, pp. 891-903. , COI: 1:CAS:528:DC%2BD3MXntVKgtr4%3DShane, M.W., de Vos, M., de Roock, S., Cawthray, G.R., Lambers, H., Effects of external phosphorus supply on internal phosphorus concentration and the initiation, growth and exudation of cluster roots in Hakea prostrata R (2003) Br. Plant Soil, 248, pp. 209-219. , COI: 1:CAS:528:DC%2BD3sXhtFCqs7w%3DShane, M.W., de Vos, M., de Roock, S., Lambers, H., Shoot P status regulates cluster-root growth and citrate exudation in Lupinus albus grown with a divided root system (2003) Plant Cell Environ, 26, pp. 265-273. , COI: 1:CAS:528:DC%2BD3sXhslOgur8%3DShane, M.W., Dixon, K.W., Lambers, H., The occurrence of dauciform roots amongst western Australian reeds, rushes and sedges, and the impact of phosphorus supply on dauciform-root development in Schoenus unispiculatus (Cyperaceae) (2005) New Phytol, 165, pp. 887-898. , PID: 15720700, COI: 1:CAS:528:DC%2BD2MXisFGjtbw%3DShane, M.W., Cawthray, G.R., Cramer, M.D., Kuo, J., Lambers, H., Specialized “dauciform” roots of Cyperaceae are structurally distinct, but functionally analogous with “cluster” roots (2006) Plant Cell Environ, 29, pp. 1989-1999. , PID: 16930324, COI: 1:CAS:528:DC%2BD28Xht1aktLnMShane, M.W., McCully, M.E., Canny, M.J., Pate, J.S., Lambers, H., Development and persistence of sandsheaths of Lyginia barbata (Restionaceae): relation to root structural development and longevity (2011) Ann Bot, 108, pp. 1307-1322. , PID: 21969258Sharpley, A.N., Kleinman, P.J.A., Weld, J.L., Environmental soil phosphorus indices (2008) Soil sampling and methods of analysis, pp. 141-159. , Carter MR, Gregorich EG, (eds), Taylor & Francis, Boca RatonShishkova, S., Rost, T.L., Dubrovsky, J.G., Determinate root growth and meristem maintenance in angiosperms (2008) Ann Bot, 101, pp. 319-340. , PID: 17954472, COI: 1:CAS:528:DC%2BD1cXjsl2jt7w%3DSkene, K.R., Cluster roots: some ecological considerations (1998) J Ecol, 86, pp. 1060-1064Smith, R.J., Hopper, S.D., Shane, M.W., Sand-binding roots in Haemodoraceae: global survey and morphology in a phylogenetic context (2011) Plant Soil, 348, pp. 453-470. , COI: 1:CAS:528:DC%2BC3MXht1Gnt73F(2007) STATISTICA (data analysis software system)Turner, B.L., Condron, L.M., Pedogenesis, nutrient dynamics, and ecosystem development: the legacy of T.W. Walker and J.K. Syers (2013) Plant Soil, 367, pp. 1-10. , COI: 1:CAS:528:DC%2BC3sXnvVOitb4%3DUllah, M.H., Jabbar, A., Khan, M.A., The influence of soil pH and texture on the adsorption of phosphorus by soils (1983) Pakistan J Agric Res, 4, pp. 41-46Viani, R.A.G., Rodrigues, R.R., Dawson, T.E., Oliveira, R.S., Savanna soil fertility limits growth but not survival of tropical forest tree seedlings (2011) Plant Soil, 349 (12), pp. 341-353. , COI: 1:CAS:528:DC%2BC3MXhsFKntb%2FPViani, R.A.G., Rodrigues, R.R., Dawson, T.E., Lambers, H., Oliveira, R.S., Soil pH accounts for differences in species distribution and leaf nutrient concentrations of Brazilian woodland savannah and Seasonally Dry forest species (2014) Perspect Plant Ecol Evol Syst, 16, pp. 64-74Walker, T.W., Syers, J.K., The fate of phosphorus during pedogenesis (1976) Geoderma, 15, pp. 1-19. , COI: 1:CAS:528:DyaE28Xht1Cltro%3DWatt, M., Evans, J.R., Linking development and determinacy with organic acid efflux from proteoid roots of white lupin grown with low phosphorus and ambient or elevated atmospheric CO2 concentration (1999) Plant Physiol, 120, pp. 705-716. , PID: 10398705, COI: 1:CAS:528:DyaK1MXks1amtL0%3DWitkowski, E.T.F., Mitchell, D.T., Variations in soil phosphorus in the fynbos biome, South Africa (1987) J Ecol, 75, pp. 1159-1171Zheng, S.J., Ma, J.F., Matsumoto, H., High aluminum resistance in buckwheat. I. Al-induced specific secretion of oxalic acid from root tips (1998) Plant Physiol, 117, pp. 745-751