Submitted to: Journal of Plant Nutrition
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 20, 2005
Publication Date: March 15, 2006
Citation: Lucena, J.J., Chaney, R.L. 2006. Synthetic iron chelates as substrates of root ferric chelate reductase in green stressed cucumber plants. Journal of Plant Nutrition. 29:423-436.
Interpretive Summary: Use of iron-chelates to prevent iron-deficiency chlorosis in crops grown on highly calcareous soils is commercially important. Identification of new chelating agents which may reduce the cost, or increase the efficacy of using iron-chelates has been difficult because chelate manufacturers usually measure the ability of the compound to keep Fe soluble in calcareous soils, but do not ordinarily assess how well the potential new product is used by plants. Plants must reduce ferric to ferrous in the chelate so that ferrous can be released for absorption by root cells. The present test evaluated chelates which have been used, or are being considered as replacements for Fe-EDDHA, a chelate which has been used commercially for 50 years. Our test of reduction and uptake of iron from different chelates used iron-stressed cucumber plants which were still green by using chelator-buffered Fe in the growth. Different chelates had varied levels of reduction and uptake. Some of the newer very strong iron chelates served as effective substrates, and Fe was readily absorbed by the cucumber roots and translocated to xylem exudate. This information is now available along with the chemical characteristics of the compounds for use by industry in selecting chelating agents for manufacture which are both effective in keeping iron soluble in the soil, and in providing Fe for roots.
While the chemistry of chelates in soils is fairly well understood, the role of the Fe3+-chelates as substrates for roots has received less attention. This work evaluated the efficacy of Fe-chelates to supply iron to mildly chlorotic plants. Fe-chelate reductase activity (FCR) and Fe in xylem sap were used as measures of that efficacy. A series of experiments were conducted with cucumber (Cucumis sativus L. cv. Burpless) seedlings in aerated hydroponics with pH 7.5 buffered nutrient solution. In the first experiment plants were supplied with different levels of Fe-EDTA (0, 5, 20 and 100 uM) in a 100 uM EDTA excess micronutrient buffered solution (pretreatment solutions T0, T1, T2 and T3 respectively). FCR was determined using BPDS and Fe-EDTA or Fe-EDDHA as substrates at pH 6 or 7.5. Then plants were placed in the same solution of the FRC assay but without the Fe2+-trapping agent and decapitated in order to collect the sap for two hours. Fe concentration was determined in the sap. These parameters were correlated. The more Fe-stressed the plants were, the more rapidly they reduced. When BPDS was used as trapping agent, Fe-EDDHA reduction was measurable at pH 7.5 but at lower rate than with Fe-EDTA. In a second experiment plants were grown as before with 5 uM Fe and FCR assay and xylem sap collection made with Fe-EDTA, Fe-EDDHA, racemic Fe-EDDHA, meso Fe-EDDHA, Fe-EDDHMA, Fe-EDDHSA, Fe-PDDHA and Fe-HBED. In general, reduction and Fe uptake were higher for the less stable chelates (lower ferric chelate stability constant). In a third experiment plants were grown with 5 uM Fe as the different chelates noted above for 17 days. Nutritional indexes followed the same trend than FCR in the second experiment, except for Fe-EDTA.