Plant Responses to Phosphorus Availability

Abstract

Phosphorus (P) is an essential macronutrient for normal plant growth and function. However, acquisition of sufficient P from the soil environment is often problematic for the plant owing to the tendency for P to react with the principal cations in most soils and its low rates of diffusive supply. Thus, a marked P depletion zone forms around the root. Plants can enhance their P acquisition by modifying their root system architecture through increasing the number and length of lateral roots and root hairs. Plants can also chemically modify the soil environment close to their roots through the release of protons and organic acids. These chemical modifications are particularly well developed in ‘cluster‐rooted’ plant species and help these plants extract P from otherwise poorly available sources. Finally, plants can form symbiotic associations with mycorrhizal fungi, an ancient symbiosis that likely evolved to help solve the P problem for plants.

Key Concepts

  • Phosphorus (P) is an essential macronutrient for healthy plant growth and development.
  • Phosphorus ions react with a number of charged surfaces in the soil environment and phosphate ions are poorly mobile in soils.
  • Small, but sharply defined, phosphate depletion zones rapidly develop at the root surface.
  • Mycorrhizal hyphae and/or root hairs can extend outside these depletion zones to explore a larger soil volume for P uptake.
  • Mycorrhizal fungi can also down‐regulate the root (i.e. direct) P uptake pathway.
  • In extremely nutrient impoverished soils, some plant species have developed cluster roots to aid P acquisition.
  • Cluster roots release compounds, notably citrate, into the soil to help chemically extract P from charged surfaces and their large surface area further aids uptake of the P forms released.

Keywords: phosphorus; depletion zones; mycorrhizal fungi; root system architecture; cluster roots

Figure 1. Autoradiographs of the roots of Brassica napus in a soil labelled with the radioactive isotope 33P after 7‐day plant growth. The depletion zones around the roots are clearly visible by the white areas. Reproduced with permission from Bhat and Nye (1973) © Springer International Publishing Switzerland.
Figure 2. Proliferation of primary and secondary laterals by barley (Hordeum vulgare L.) plants grown in solution culture with the middle zone (i.e. between the two black lines) exposed to a 100‐fold greater concentration of phosphate, nitrate, ammonium or potassium ions compared to roots above or below the lines. Controls were supplied with a uniformly high level of nutrient ions in all zones. Abbreviations: H, high; L, low referring to the nutrient concentration experienced by the top, middle and bottom sections of the root system. Reproduced with permission from Drew (1975) © John Wiley & Sons.
Figure 3. (a) Confocal image of an arbuscule structure in a Plantago lanceolata root. (b) Cross section of an ectomycorrhizal tropical legume, Dicymbe, root. The fungal sheath or mantle has been stained pink, whereas the Hartig net structure is stained blue. (c) Proliferation of arbuscular mycorrhizal fungal hyphae in a patch of organic matter. The hyphae have been stained pink with acid fuchsin. Images (a) and (c) by Angela Hodge and (b) Reproduced from Fitter AH, Hay RKM with courtesy of Professor I. J. Alexander (2002) © Elsevier.
Figure 4. Images of (a) dauciform roots of Tetraria sp. showing extensive production of root hairs, (b) simple type cluster roots of Lupinus albus L. and (c) compound cluster roots of Banksia prionotes. Images courtesy of Dr M. W. Shane (a) Reproduced with permission from Lambers et al. (2006) © Oxford University Press and (b–c) Reproduced with permission from Herrera et al. (2015) © John Wiley & Sons Ltd.
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Further Reading

Fitter AH and Hay RKM (2002) Environmental Physiology of Plants, 3rd edn. London: Academic Press.

Lambers H, Raven JA, Shaver GR, et al. (2008) Plant nutrient‐acquisition strategies change with soil age. Trends in Ecology and Evolution 23: 95–103.

Miguel MA, Postma JA and Lynch JP (2015) Phene synergism between root hair length and basal root growth angle for phosphorus acquisition. Plant Physiology 167: 1430–1439.

Smith FA and Smith SE (2015) How harmonious are arbuscular mycorrhizal symbioses? Inconsistent concepts reflect different mindsets as well as results. New Phytologist 205: 1381–1384.

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Hodge, Angela(Nov 2015) Plant Responses to Phosphorus Availability. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0021258]