RESPONSES OF NITROGEN-FIXING AND NITRATE-SUPPLIED ALFALFA (MEDICAGO-SATIVA L) TO IRON CHELATES IN AN ALKALINE HYDROPONIC MEDIUM

Ferric ethylenediamine di-(o-hydroxyphenylacetate) (FeEDDHA) and ferric hydroxyethylethylene-diaminetriacetic 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-mu-M). Maximum yield of NO3-supplied alfalfa was obtained at 100-mu-M while maximum yield of N2-fixing alfalfa was obtained in the range of 33 to 200-mu-M FeEDDHA. Nodule fresh weights and N2 fixation rates increased with FeEDDHA concentration up to 33-mu-M and remained essentially constant up to 200-mu-M. With FeHEDTA, maximum yields of both NO3-grown and N2-fixing alfalfa were obtained at 10-mu-M. Growth of NO3-supplied plants was inhibited at 200-mu-M FeHEDTA while growth of N2-fixing plants was inhibited at 100-mu-M FeHEDTA. The numbers of nodules per plant increased between 3.3 and 10-mu-M FeHEDTA; however, inhibition of nodule formation occurred at a concentration of 33-mu-M or higher. Nodule weights per plant and N2 fixation rates were depressed at 3.3-mu-M as well as at 100-mu-M FeHEDTA. The results suggest that alfalfa dependent on N2 fixation is more sensitive to limited Fe availability than alfalfa supplied with NO3.