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Fluid Journal : Fluid Journal 2008-2009
uptake of micronutrients by the plant due to complexation of the free metal cation (M2+). Thus, the efficiency of chelates such as EDTA and DTPA in terms of improving crop nutrition is compromised by the poor ability of the complexed forms of micronutrient to be absorbed by plant roots. A better way? In this article we shall examine new potential applications for two new chelates--polyethylenimine (PEI) and rhamnolipid--to improve crop micronutrient nutrition through exploitation of different physical and chemical behaviors of the chelates. Ko/w values for the metals complexed by rhamnolipid were high. Normally, metallic cations are hydrophilic and do not partition to the octanol phase (and hence have very low Ko/w values). High Ko/w values for micronutrient cations found with rhamnolipid indicated that the chelate had formed a lipophilic complex with the cation, a property likely to assist in uptake by plant roots. PEI forms cationic complexes with micronutrients, so Ko/w values were low for this chelate (Figure 1). Shoot concentration. Canola plants were grown under Zn deficient conditions. Therefore, on Streaky Bay soil, shoot Zn concentrations were below the published critical tissue concentrations for Zn of 7-8 mg Zn/kg dry matter (DM). Canola plants grown on Birchip soil had Zn concentrations at or above the critical Zn concentration; treatment with rhamnolipid and PEI increased shoot Zn concentration above the critical levels (Figure 2). Rhamnolipid also significantly increased concentration (P<0.05) of Zn in canola shoots grown on the highly calcareous Streaky Bay soil. EDTA did not significantly (P>0.05) increase Zn uptake from either soil, compared to the ZnSO4 control even though EDTA substantially increased the solution concentrations of Zn in both soils (data not shown). Root concentration. Examination of canola roots exposed to ZnSO4, Zn-EDTA, and Zn-rhamnolipid revealed a significantly different pattern of accumulation, and a different speciation of Zn within the plants (Figure 3). The lowest Zn x-ray fluorescence signal was obtained from the Zn-EDTA treated roots (Figure 3, top), probably due to a reduction in Zn absorption by roots due to low solution Zn2+ activities in the presence of EDTA. The Zn signal was higher in ZnSO4-treated roots and highest in Zn-rhamnolipid roots. EXAFS data (taken from XAS spots labeled in Figure 3) suggested that Figure 1. Octanol/water partition coefficients (Kw/o values) for Cu, Mn, and Zn with sulphate, EDTA, rhamnolipid and PEI), Stacey, 2007. Zn was predominantly in the form of Zn-phytate-like compounds in Zn- free, ZnSO4, and Zn-EDTA treated roots with 70-87 percent of total root Zn present as Zn-phytate-like compounds in these treatments. Zn- EDTA complexes were not detected inside root cross sections, consistent with published literature that showed Zn-EDTA complexes are not readily absorbed by intact roots via active or passive uptake pathways. In roots treated with Zn-rhamnolipid, EXAFS suggested that 55.3 and 87.6 percent of Zn was probably Figure 2. Uptake and translocation of Zn to canola shoots. 0 2 4 6 8 ZnSO EDTA Rhamnolipid PEI Zn uptake (μg Zn/g shoot) . 8 10 12 Birchip Streaky Bay 4 0 5 10 Rhamnolipid EDTA PEI Ko/w 15 20 25 Zn Cu Mn SO 42-
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