Plant and Soil 248: 187–197, 2003. © 2003 Kluwer Academic Publishers. Printed in the Netherlands.
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Chickpea and white lupin rhizosphere carboxylates vary with soil properties and enhance phosphorus uptake Erik J. Veneklaas1,3 , Jason Stevens1,2 , Gregory R. Cawthray1 , Stephen Turner1 , Alasdair M. Grigg1 & Hans Lambers1 1 School
of Plant Biology, The University of Western Australia, 35 Stirling Highway, Crawley WA 6009, Australia. address: School of Earth and Geographical Sciences, The University of Western Australia, 35 Stirling Highway, Crawley WA 6009, Australia. 3 Corresponding author∗ 2 Present
Received 26 February 2002. Accepted in revised form 18 July 2002
Key words: carboxylates, chickpea, phosphorus, rhizosphere, root exudates, root morphology, wheat, white lupin
Abstract Chickpea and white lupin roots are able to exude large amounts of carboxylates, but the resulting concentrations in the rhizosphere vary widely. We grew chickpea in pots in eleven different Western Australian soils, all with low phosphorus concentrations. While final plant mass varied more than two-fold and phosphorus content almost five-fold, there were only minor changes in root morphological traits that potentially enhance phosphorus uptake (e.g., the proportion of plant mass allocated to roots, or the length of roots per unit root mass). In contrast, the concentration of carboxylates (mainly malonate, citrate and malate, extracted using a 0.2 mM CaCl2 solution) varied ten-fold (averaging 2.3 µmol g−1 dry rhizosphere soil, approximately equivalent to a soil solution concentration of 23 mM). Plant phosphorus uptake was positively correlated with the concentration of carboxylates in the rhizosphere, and it was consistently higher in soils with a smaller capacity to sorb phosphorus. Phosphorus content was not correlated with bicarbonate-extractable phosphorus or any other single soil trait. These results suggest that exuded carboxylates increased the availability of phosphorus to the plant, however, the factors that affected root exudation rates are not known. When grown in the same six soils, three commonly used Western Australian chickpea cultivars had very similar rhizosphere carboxylate concentrations (extracted using a 0.2 mM CaCl2 solution), suggesting that there is little genetic variation for this trait in chickpea. Variation in the concentration of carboxylates in the rhizosphere of white lupin did not parallel that of chickpea across the six soils. However, in both species the proportion of citrate decreased and that of malate increased at lower soil pH. We conclude that patterns of variation in root exudates need to be understood to optimise the use of this trait in enhancing crop phosphorus uptake. Introduction The roots of certain crop species, including white lupin (Lupinus albus L.) and chickpea (Cicer arietinum L.) exude large amounts of low-molecularweight organic anions (carboxylates) which enhance the availability of soil phosphorus to the plant (Ae et al., 1990; Dinkelaker et al., 1989; Gardner et al., 1983; Gerke et al., 1994; Hocking et al., 1997; Ohwaki ∗ FAX No: +61-8-93801108.
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and Hirata, 1992). Organic anions can complex metal cations, which bind phosphates (Al3+ , Fe3+ , Ca2+ ), or displace phosphate from the soil matrix (Jones, 1998). Where carboxylate exudation is associated with proton extrusion, the lower pH may itself contribute to greater P availability, if the soil pH is relatively high. However, cotransport of protons is certainly not associated with organic anion release in all species in all circumstances (Roelofs et al., 2001, Ryan et al., 2001). The effectiveness of carboxylates depends on their number of carboxyl groups and molecular structure. Tricarboxylates (citrate) are generally more effective
188 than dicarboxylates (e.g., malate, malonate) due to stronger ligand binding. Soil properties also have large effects on the effectivity of carboxylates. For example, the stability of organic anion – metal complexes depends strongly on pH (Jones, 1998), and there is large variation in the mechanisms and strength with which phosphate is held in the soil. Root exudate patterns may need to be different on different soils for optimal effects on P availability. It is known that species differ in the carboxylates they exude, e.g., predominantly citrate in white lupin, malonate in chickpea, malate in wheat, a wide range of carboxylates in Proteaceae species (Ryan et al., 2001; Roelofs et al., 2001), but it is not clear if that affects the suitability of these species for certain soils or, more specifically, their ability to mobilise soil P. Knowledge about variation in exudation in response to soil P status is virtually limited to observations that lupin root exudation rates are highest at low P availability (Keerthisinghe et al., 1998). However, Lambers et al. (2002) found that Banksia grandis, an Australian Proteaceae species, modified its exudation pattern when grown on sand containing either poorly soluble Al-phosphate or Fe-phosphate as the only P source, and grew equally well on either P source. Studies of Al3+ resistance have shown clearly that plants respond directly to the presence of Al3+ by releasing organic anions, and that genotypes of wheat, maize and soybean vary enormously in this trait (Ryan et al., 2001). If such genetic variation were associated with the ability to respond to P deficiency by producing the appropriate root exudates to enhance P availability, this could be exploited to improve the efficiency of P fertiliser use. The present paper aims at assessing soil-induced variation in rhizosphere carboxylates of chickpea and lupin, and correlating this with P uptake. Chickpea is a promising and rapidly expanding new crop in Western Australia (Siddique and Sykes, 1997), where it is grown in rotation with wheat. Western Australian soils generally have very low levels of P, and much P fertiliser is ineffective due to high P sorption capacities (Bolland et al., 1994; Brennan et al., 1994). Although iron oxides are often present in high concentrations, the sorption of P is most closely associated with aluminium (oxides and organic complexes; Gilkes and Hughes, 1994). We report the results of two pot experiments, in which we determined plant growth and P uptake as well as rhizosphere chemistry. In the first experiment, focusing on soil-induced variation in root exudates, we grew one chickpea cultivar on
11 soils, covering a range of Western Australian soil types considered suitable for chickpea. In the second experiment, we used six of these soils but grew three chickpea cultivars as well as white lupin and wheat, to establish inherent and soil-induced variation in root exudates amongst cultivars and species.
Material and methods Experiment 1 Chickpea (Cicer arietinum L.) cultivar ‘Sona’ was grown in a pot experiment in a greenhouse in 11 different soils. Sowing date was the 12th of August 2000. Experiment 2 Three cultivars of chickpea, and one cultivar of white lupin (Lupinus albus L.) and wheat (Triticum aestivum L.) were grown in pots in the greenhouse in six different soils. The chickpea cultivars were ‘Sona’, ‘Tyson’ (both desi type, small-seeded, chickpea) and ‘Kaniva’ (kabuli type, large-seeded, chickpea). The white lupin cultivar was ‘Kiev Mutant’, and the wheat cultivar was ‘Westonia’. Sowing date was the 31st of August 2000. Soils The soils used in the experiments were sampled at 11 locations in the medium-rainfall area (mean rainfall about 400 mm) of southwestern Australia. Topsoil was collected from unfertilised land, and passed through a 4 mm sieve. The soils were loamy to clayey, and all were low in bicarbonate-extractable P (10 and plants took up larger amounts of P than expected; it was also rather exceptional in having the highest pH and extractable Ca, suggesting that P may be held by a different mechanism. The positive effect of carboxylates on P availability is supported by results of soil extractions. The amounts of P extracted by 0.5 mM solutions of malate, malonate and citrate were (averaged for 11 soils, and
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Figure 4. Relationship between the total amount of phosphorus (mg P per two plants in pot) and the concentration of carboxylates (µmol total carboxylates per g root dry mass, extracted using a 0.2 mM CaCl2 solution) in the rhizosphere of chickpea cultivar ‘Sona’ grown in different Western Australian soils. Soils represented by circles had a Phosphorus Retention Index (PRI) smaller than 10; soils represented by triangles had a PRI greater than 10. The exceptional case of the open triangle is discussed in the text. Curves fitted by eye.
soils, with mean values of 0.66 mm for the chickpeas, 0.66 mm for lupin non-cluster roots, 0.39 mm for lupin cluster roots, and 0.22 mm for wheat roots. Cluster roots represented 21–43% of the total root mass in white lupin, but this variation was not correlated with plant P status or any soil property. Amounts and composition of carboxylates in the rhizosphere showed large differences amongst species and soils (Figure 5). Wheat had low rhizosphere concentrations of CaCl2 -extractable carboxylates (mainly lactate and malate), whereas white lupin and chickpea had high concentrations. Malonate was the dominant carboxylate in chickpea root exudates, followed by citrate and malate. Citrate plus malate made up 95–98% of the total amount of carboxylates in lupin, but their relative contributions varied between 10 and 90% in different soils (Figure 5). Interestingly, the proportion of malate was higher in soils with low pH (pH
