Carboxylate release of wheat, canola and 11 grain legume species as affected by phosphorus status

Abstract

The capacity of plant roots to increase their carboxylate exudation at a low plant phosphorus (P) status is an adaptation to acquire sufficient P at low soil P availability. Our objective was to compare crop species in their adaptive response to a low-P availability, in order to gain knowledge to be used for improving crop Pacquisition efficiency from soils that are low in P or that have a high capacity to retain P. In the present screening study we compared 13 crop species, grown in sand at either 3 or 300 lM of P, and measured root mass ratio, cluster-root development, rhizosphere pH and carboxylate composition of root exudates. Root mass ratio decreased with increasing P supply for Triticum aestivum L., Brassica napus L., Cicer arietinum L. and Lens culinaris Medik., and increased only for Pisum sativum L., while the Lupinus species and Vicia faba L. were not responsive. Lupinus species that had the potential to produce root clusters either increased or decreased biomass allocation to clusters at 300 lM of P compared with allocation at 3 lM of P. All Lupinus species acidified their rhizosphere more than other species did, with average pH decreasing from 6.7 (control) to 4.3 for Lupinus pilosus L. and 5.9 for Lupinus atlanticus L.; B. napus maintained the most alkaline rhizosphere, averaging 7.4 at 300 lM of P. Rhizosphere carboxylate concentrations were lowest for T. aestivum, B. napus, V. faba, and L. culinaris than for the other species. Exuded carboxylates were mainly citrate and malate for all species, with the exception of L. culinaris and C. arietinum, which produced mainly citrate and malonate. Considerable variation in the concentration of exuded carboxylates and protons was found, even with a genus. Cluster-root forming species did not invariably have the highest concentrations of rhizosphere carboxylates. Lupinus species varied both in P-uptake and in the sensitivity of their cluster-root development to external P supply. Given the carbon cost of cluster roots, a greater plasticity in their formation and exudation (i.e. reduced investment in cluster roots and exudation at higher soil P, a negative feedback response) is a desirable trait for agricultural species that may have variable access to readily available P.

Grain yield and phosphorus use efficiency of wheat and pea in a high yielding environment

“…The high P uptake per unit root length shown by pea (2.3 fold) indicates that this crop acquires its P through a mechanism other than root extension. Other mechanisms of P acquisition include higher root exudation, longer root hairs and higher activity and quantity of P transporters (Watt and Evans, 2003;Nuruzzaman et al, 2006;Pearse et al, 2006). In addition, the fungal community in the rhizosphere of legumes and cereals has shown to be different with potential effect in arbuscular mycorrhizal (AM) colonization and P mobilization (Wang et al, 2012).…”

“…In addition, the fungal community in the rhizosphere of legumes and cereals has shown to be different with potential effect in arbuscular mycorrhizal (AM) colonization and P mobilization (Wang et al, 2012). We did not measure these mechanisms; however, in previous studies legumes have shown to have higher organic exudation than wheat (Nuruzzaman et al, 2006;Pearse et al, 2006). Nonetheless, studies comparing root hairs of these crops at field conditions are needed since this trait has been an important mechanism behind crop differences in P acquisition (Gahoonia et al, 1999).…”

“…In general, temperate cereals, such as wheat and barley have higher root length and specific root length than legumes under different water regimens (Gregory and Eastham 1996). However, legumes could compensate for their lower root length through a higher P uptake per unit of root length, since legumes have shown higher organic exudation than wheat (Nuruzzaman et al, 2006;Pearse et al, 2006); this increases P mobility and uptake (Hinsinger, 2001;Yang et al, 2013). Nevertheless, to the best of our knowledge, there are no studies comparing wheat and pea's P uptake and root traits under the same field experiments, which would allow us to elucidate possible differences between these crops in relation to this subject.…”

Physiological and molecular alterations in plants exposed to high [CO2] under phosphorus stress

“…For example, increased production and exudation of organic acids and increased carbon allocation to mycorrhizal symbionts are well-known plant responses to P limitation (Neumann and Romheld, 1999;Pandey et al, 2005a;Pearse et al, 2006;Pandey et al, 2013;Pandey et al, 2014) that may reduce plant growth even at constant photosynthesis (as summarised in Fig. 1 B).…”

Genetic approaches to enhancing phosphorus-use efficiency (PUE) in crops: challenges and directions

“…Studies have shown that compared with wheat, chickpea, field pea (Pisum sativum L.), and lupin (Lupinus spp.) have the ability to acidify the rhizosphere by releasing organic acids, which has been associated with relatively better growth at low available P (Pearse et al 2006). Faba bean (Vicia faba L.) has shown a variable response to P but it has been considered the grain legume that is most responsive to P when grown on neutral-acid soils of Western Australia (Bolland et al 1999).…”