Quantifying and mapping citrate exudation in soil-grown root systems

<p>The determination of citrate exuded from soil-grown roots is very challenging due to its rapid microbial degradation and mineralization, sorption to the solid soil phase and ongoing release of organic molecules from organic matter breakdown. For this reason, our knowledge about citrate release is mainly based on experiments carried out in hydroponics. Results obtained in hydroponics cannot directly be transferred to soil-plant systems, as hydroponics represents an artificial environment. This study aimed to develop a localization and quantification technique for citrate exuded from soil-grown plant roots, based on diffusive gradients in thin film (DGT). Polyacrylamide gels containing precipitated zirconium hydroxide (ZrOH) were applied to the rhizosphere of soil grown plants, on which citrate is efficiently immobilized, thereby creating a zero sink to sample the citrate exuded from the roots. Citrate was eluted with 1 mL 0.5 mol L<sup>-1</sup> NaOH from the ZrOH gel and quantified by ion chromatography. ZrOH gel discs were able to bind the citrate contained in 10 mL of 2.77 mg citrate L<sup>-1</sup> solutions within a 4h uptake period. Elution efficiency was ~89%. ZrOH gel capacity at pH 8 was 200 µg per gel disc and 299 µg per gel disc at pH 4, which is sufficient to act as a zero sink for citrate released from plant roots. As a first exemplary method application, we grew white lupin plants in rhizotrons using a highly phosphorus deficient soil. ZrOH gel sheets were applied for 26 h onto cluster roots for citrate sampling following established DGT protocols. Gels were cut afterwards into 5×5 and 5×2 mm slices for obtaining a citrate exudation map. In both cases we were able to localize and quantify up to 7.89 µg citrate on individual gel slices, as well as to identify longitudinal and lateral citrate gradients around the cluster roots. Moreover, the characterization of ZrOH gels showed its suitability for citrate sampling in terms uptake kinetics and capacity. These results demonstrate that the developed method is suitable for citrate sampling and localization in a non-destructive way from soil-grown plant roots. As it is applicable to soil grown-roots and provides unprecedented spatial resolution, this sampling technique advances the experimental possibilities for researching root exudates considerably. Using suitable binding materials, this approach is also applicable to other carboxylates such as malate or oxalate and other compound classes such as phenolics, flavonolos etc. Furthermore, this technique can be combined with complementary imaging methods for mapping e.g. nutrients, contaminants, pH or enzyme activity distributions.</p>

Quantifying citrate-enhanced phosphate root uptake using microdialysis

Aims Organic acid exudation by plant roots is thought to promote phosphate (P) solubilisation and bioavailability in soils with poorly available nutrients. Here we describe a new combined experimental (microdialysis) and modelling approach to quantify citrate-enhanced P desorption and its importance for root P uptake. Methods To mimic the rhizosphere, microdialysis probes were placed in soil and perfused with citrate solutions (0.1, 1.0 and 10 mM) and the amount of P recovered from soil used to quantify rhizosphere P availability. Parameters in a mathematical model describing probe P uptake, citrate exudation, P movement and citrate-enhanced desorption were fit to the experimental data. These parameters were used in a model of a root which exuded citrate and absorbed P. The importance of soil citrate-P mobilisation for root P uptake was then quantified using this model. Results A plant needs to exude citrate at a rate of 0.73 μmol cm−1 of root h−1 to see a significant increase in P absorption. Microdialysis probes with citrate in the perfusate were shown to absorb similar quantities of P to an exuding root. Conclusion A single root exuding citrate at a typical rate (4.3 × 10−5 μmol m−1 of root h−1) did not contribute significantly to P uptake. Microdialysis probes show promise for measuring rhizosphere processes when calibration experiments and mathematical modelling are used to decouple microdialysis and rhizosphere mechanisms.

Citrate Root Exudation under Zn and P Deficiency

Zinc and phosphorus are essential nutrients with low bioavailability in calcareous soils. Some plants exude organicacids to increase the solubility of these two nutrients. The objective of this study was to examine citrate exudation rates of different lupin (Feodora and Energy) and rapeseed (Dunkeld, Yickadee and Rainbow) cultivars under deficiencies of Zn and P. The plants were cultivated into three different nutrient solutions (complete, -Zn, and -P) with pH around 7. Under Zn deficiency, rapeseed cultivars lost about 80% of its shoot fresh weight, but the roots did not exude any organic acids such as citrate, malate or oxalate. Both lupin and rapeseed cultivars exuded citrate onlyunder phosphorus deficiency. The exudation rates of Feodora and Energy were 3.89 μmol g-1 RDW h-1 and 3.45 μmol g-1 RDW h-1, respectively, while that of Dunkeld was 15.1 μmol g-1 RDW h-1. The results indicated that lupin and rapeseed lost their production under Zn deficiency but they did not exude organic acid, while under P deficiency both plants exuded citrate.Keywords: Citrate; deficiency; exudation rate; lupin; phosphorus; rapeseed; Zn[How to Cite: Siane BAE. 2012. Citrate Root Exudation under Zn and P Deficiency. J Trop Soils, 17 (3) : 219-225. doi: 10.5400/jts.2012.17.3.219][Permalink/DOI: www.dx.doi.org/10.5400/jts.2012.17.3.219]