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Responses of nitrogen-fixing and nitrate-supplied alfalfa (Medicago sativa L.) to iron chelates in an alkaline hydroponic medium

G.V. JOHNSON and G.J. YOUNGBLOOD
Plant and Soil
Vol. 130, No. 1/2, Selected papers from the FIFTH INTERNATIONAL SYMPOSIUM ON IRON NUTRITION AND INTERACTIONS IN PLANTS (January 1991), pp. 219-224
Published by: Springer
Stable URL: http://www.jstor.org/stable/42937306
Page Count: 6
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Responses of nitrogen-fixing and nitrate-supplied alfalfa (Medicago sativa L.) to iron chelates in an alkaline hydroponic medium
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Abstract

Ferric ethylenediamine di-(o-hydroxyphenylacetate) (FeEDDHA) and ferric hydroxyethylethylenediaminetriacetic acid (FeHEDTA) were evaluated as Fe sources for hydroponic growth of alfalfa (Medicago sativa L., cv. Mesilla), either dependent on N₂ fixation or supplied with NO₃. The hydroponic medium was maintained at pH 7.5 by addition of CaCO₃. Nitrogen-fixing cultures were inoculated with Rhizobium meliloti 102 F51 and grown in medium without added nitrogen. After five to seven weeks of growth under greenhouse conditions, plants were harvested. Nitrogen fixation was measured by the acetylene reduction method. When FeEDDHA was supplied, growth of alfalfa, whether dependent on N₂ fixation or supplied with NO₃, was severely limited at concentrations typically used in hydroponic medium (10 or 20 μM). Maximum yield of NO₃-supplied alfalfa was obtained at 100 μM while maximum yield of N₂-fixing alfalfa was obtained in the range of 33 to 200 μM FeEDDHA. Nodule fresh weights and N₂ fixation rates increased with FeEDDHA concentration up to 33 μM and remained essentially constant up to 200 μM. With FeHEDTA, maximum yields of both NO₃-grown and N₂-fixing alfalfa were obtained at 10 μM. Growth of NO₃-supplied plants was inhibited at 200 μM FeHEDTA while growth of N₂-fixing plants was inhibited at 100 μM FeHEDTA. The numbers of nodules per plant increased between 3.3 and 10 μM FeHEDTA; however, inhibition of nodule formation occurred at a concentration of 33 μM or higher. Nodule weights per plant and N₂ fixation rates were depressed at 3.3 μM as well as at 100 μM FeHEDTA. The results suggest that alfalfa dependent on N₂ fixation is more sensitive to limited Fe availability than alfalfa supplied with NO₃.

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