EFFECTS OF ION-PAIRING WITH CALCIUM AND MAGNESIUM ON SELENATE AVAILABILITY TO HIGHER-PLANTS

The effects of solution speciation on the bioavailability of trace met als are well documented, but the role of speciation in the bioavailabi lity of oxyanionic trace elements that may form significant ion pairs with Ca and Mg in saline media has not been investigated. We assessed the effects of such ion pairing on the availability of selenate to rep resentative monocotyledonous and dicotyledonous higher plants. Formati on constants for the CaSO40 and CaSeO40 ion pairs were experimentally determined using a Ca2+ ion-selective electrode. The published value o f 10(2.3) for CaSO40 formation was confirmed, but the value of 10(2.7) for CaSeO40 was found to be in error; a value of 10(2.0) is proposed here as the correct formation constant. Five solution culture experime nts were conducted using alfalfa (Medicago sativa L.) or tall wheatgra ss (Elytrigia pontica [Podp.] Holub) with treatments consisting of NaS eO4 levels in combination with various levels of MgCl2 or CaCl2. Both shoot Se concentrations and whole-plant Se contents were highly correl ated with the free SeO42- activity but were poorly correlated with the sum of the free ion plus Ca and Mg ion pair species. Thus, we have sh own, for the first time, that the free ion model of trace metal bioava ilability is also valid for oxyanions that form complexes with Ca and Mg in saline media but that this conclusion hinges critically on the a ccuracy of the pertinent formation constants.