Mobilization of Cu and Zn by root exudates of dicotyledonous plants in resin-buffered solutions and in soil

Abstract

It has been frequently suggested that root exudates play a role in trace metal mobilization and uptake by plants, but there is little in vivo evidence. We studied root exudation of dicotyledonous plants in relation to mobilization and uptake of Cu and Zn in nutrient solutions and in a calcareous soil at varying Cu and Zn supply. Spinach (Spinacia oleracea L.) and tomato (Lycopersicon esculentum L.) were grown on resin-buffered nutrient solutions at varying free ion activities of Cu (pCu 13.0-10.4) and Zn (pZn 10.1-6.6). The Cu and Zn concentrations in the nutrient solution increased with time, except in plant-free controls, indicating that the plant roots released organic ligands that mobilized Cu and Zn from the resin. At same pCu, soluble Cu increased more at low Zn supply, as long as Zn deficiency effects on growth were small. Zinc deficiency was observed in most treatment solutions with pZn ≥ 9.3, but not in nutrient solutions of a smaller volume/plant ratio in which higher Zn concentrations were observed at same pZn. Root exudates of Zn-deficient plants showed higher specific UV absorbance (SUVA, an indicator of aromaticity and metal affinity) than those of non-deficient plants. Measurement of the metal diffusion flux with the DGT technique showed that the Cu and Zn complexes in the nutrient solutions were highly labile. Diffusive transport (through the unstirred layer surrounding the roots) of the free ion only could not explain the observed plant uptake of Cu and of Zn at low Zn 2+ activity. The Cu and Zn uptake by the plants was well explained if it was assumed that the complexes with root exudates contributed 0.4% (Cu) or 20% (Zn) relative to the free ion. In the soil experiment, metal concentrations and organic C concentrations were larger in the solution of planted soils than in unplanted controls. The SUVA of the soil solution after plant growth was higher for unamended soils, on which the plants were Zn-deficient, than for Znamended soils. In conclusion, root exudates of dicotyledonous plants are able to mobilize Cu and Zn, and plants appear to respond to Zn deficiency by exuding root exudates with higher metal affinity.

Rhizosphere processes and management in plant-assisted bioremediation (phytoremediation) of soils

“…arsenate) for binding sites (Jones et al 2004;Fitz and Wenzel 2002;Wenzel et al 2003a). The mobilising effect of root exudates has been demonstrated in vitro (Mench and Martin 1991) and recently in resin buffered nutrient solutions and soil experiments (Degryse et al 2007;Shenker et al 2001). However, increased mobility was not necessarily associated with increased uptake in plants (Shenker et al 2001).…”

Dynamics of plant metal uptake and metal changes in whole soil and soil particle fractions during repeated phytoextraction

“…Soil properties such as soil metal concentration, pH and soil components will control soil metal bioavailability (Kirkham 2006). Plants can decrease metal availability because of plant removal of metals from the soil and can also enhance metal availability by the production of root exudates (Degryse et al 2008). In the present study, metal ) in the contaminated soils after nine consecutive crops of S. plumbizincicola "Unplanted" and "Planted" denote soil without phytoextraction and with nine crops of phytoextraction respectively; S1, lightly polluted soil; S2 and S3 intermediate polluted soils; S4, highly polluted soil.…”

“…There are two possible explanations. Firstly, the rhizosphere activity of the hyperaccumulator may increase metal availability by root exudates (Degryse et al 2008;). In the case of low metal concentrations in the soil solution and relative large metal potential pool size C and D) in different size soil fractions before and after repeated phytoextraction.…”

“…S1, lightly polluted soil; S4, highly polluted soil; "Planted", soil with nine crops of phytoextraction; "Unplanted", soil without phytoextraction (e.g. a soil with high pH and high metal concentrations), metal released from the potential pool will be much higher with the root exudates than the metal released without these materials (Degryse et al 2008). In other words, the rhizosphere activity may make a major contribution to metal uptake.…”

Bioavailability of zinc and phosphorus in calcareous soils as affected by citrate exudation

“…Neither of these studies, however, proved Zn mobilization from soil by citrate. Degryse et al (2008) showed that carboxylates exuded from roots of spinach (Spinacia oleracea L.) and tomato (Solanum lycopersicum L.) were able to mobilize Cu and Zn from a calcareous soil. On the other hand Gao et al (2009) showed that malate concentrations observed in the rhizosphere of rice had a negligible effect on the concentration of Zn in soil solution of a low Zn soil.…”