Physiological Aspects of Plant Phosphorus in Relation to Its Acquisition from the Soil by Crop Plants

Keywords: phosphorus acquisition, P efficient crop plants, white lupin (Lupinus albus), phosphorus deficiency, rhizosphere management.

Phosphorus (P) limits crop yields on 30-40% of the world’s arable soils; current estimates indicate that P reserves worldwide will be approximately halved within the next 60 years so that phosphate fertiliser application alone is not a sustainable solution to this problem. There is now enormous interest worldwide in finding ways of increasing phosphorus acquisition by plants from the soil and in developing sustainable managerial practices in the application of phosphate fertilisers. Under conditions of limited phosphorus supply plants have evolved an array of morphological, physiological, biochemical and molecular adaptations to enhance P acquisition. These include modification of root architecture involving the exposure of a higher root surface area per unit volume of soil, mycorrhizal colonisation, the activation of high affinity Pi transporters located in the plasma membranes of the roots, as well as the exudation of carboxylates, protons and various enzymes which mobilise phosphate in the rhizosphere. An understanding of these adaptations is proving valuable in the development of P efficient plants by physiological, biochemical and molecular approaches as well as by traditional plant breeding. Root architecture is particularly important for phosphorus acquisition because of the low mobility of P and the spatial variation in available P accessible to roots. In most non-tilled soils the highest reserves of available P are in the topsoil and the benefits of crop genotypes showing topsoil foraging via shallower basal roots and greater adventitious rooting with lower metabolic expenditure costs have been established. Genotypes of bean, soyabean and maize showing considerably higher yields on low P soils are now being used throughout the world. The particular benefit conferred on cereal cultivars by long root hairs in maintaining stable economic grain yields at lower levels of phosphate fertiliser application has recently been reported. The release of carboxylates from the roots of some crop species can of benefit in increasing P acquisition. White lupin (Lupinus albus) and other plant species which form root clusters under limited P supply are attracting particular attention because of their high efficiency in ‘extracting’ P from soils by a combination of their specialised root structure and exudation of carboxylates. Crop plants better adapted in to P acquisition and to P recycling in crop rotations together with innovative P-resource saving fertilisation strategies will help to achieve success in the coming years as the reserves of phosphatic fertilisers decline on a global scale.

Ernest A Kirkby, Faculty of Biology, University of Leeds, UK, and

Volker Römheld, Institute of Plant Nutrition, University of Hohenheim, Germany.

47 pages, 15 figures, 14 tables, 128 references.