Soil pH influences the organic acid metabolism and exudation in cluster roots of Protea species from the Mediterranean-type fynbos ecosystem, Western Cape, South Africa

Phosphorus (P) is limiting for crop yield on > 30% of the world's arable land and, by some estimates, world resources of inexpensive P may be depleted by 2050. Improvement of P acquisition and use by plants is critical for economic, humanitarian and environmental reasons. Plants have evolved a diverse array of strategies to obtain adequate P under limiting conditions, including modifications to root architecture, carbon metabolism and membrane structure, exudation of low molecular weight organic acids, protons and enzymes, and enhanced expression of the numerous genes involved in low-P adaptation. These adaptations may be less pronounced in mycorrhizal-associated plants. The formation of cluster roots under P-stress by the nonmycorrhizal species white lupin (Lupinus albus), and the accompanying biochemical changes exemplify many of the plant adaptations that enhance P acquisition and use. Physiological, biochemical, and molecular studies of white lupin and other species response to P-deficiency have identified targets that may be useful for plant improvement. Genomic approaches involving identification of expressed sequence tags (ESTs) found under low-P stress may also yield target sites for plant improvement. Interdisciplinary studies uniting plant breeding, biochemistry, soil science, and genetics under the large umbrella of genomics are prerequisite for rapid progress in improving nutrient acquisition and use in plants. Contents I. Introduction 424 II. The phosphorus conundrum 424 III. Adaptations to low P 424 IV. Uptake of P 424 V. P deficiency alters root development and function 426 VI. P deficiency modifies carbon metabolism 431 VII. Acid phosphatase 436 VIII. Genetic regulation of P responsive genes 437 IX. Improving P acquisition 439 X. Synopsis 440.

Nitrogen (N) tends to limit plant productivity on young soils; phosphorus (P) becomes increasingly limiting in ancient soils because it gradually disappears through leaching and erosion. Plant traits that are regarded as adaptations to N- and P-limited conditions include mycorrhizas and cluster roots. Mycorrhizas 'scavenge' P from solution or 'mine' insoluble organic N. Cluster roots function in severely P-impoverished landscapes, 'mining' P fixed as insoluble inorganic phosphates. The 'scavenging' and 'mining' strategies of mycorrhizal species without and non-mycorrhizal species with cluster roots, respectively, allow functioning on soils that differ markedly in P availability. Based on recent advances in our understanding of these contrasting strategies of nutrient acquisition, we provide an explanation for the distribution of mycorrhizal species on less P-impoverished soils, and for why, globally, cluster-bearing species dominate on severely P-impoverished, ancient soils, where P sensitivity is relatively common.

Plant and Soil, 205(1), 25-44