Effects of ion pairing with calcium and magnesium on selenate availability to higher plants

Abstract

AbstractThe effects of solution speciation on the bioavailability of trace metals are well documented, but the role of speciation in the bioavailability 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 representative monocotyledonous and dicotyledonous higher plants. Formation constants for the CaSO and CaSeO ion pairs were experimentally determined using a Ca2+ ion‐selective electrode. The published value of 102,3 for CaSO formation was confirmed, but the value of 1027 for CaSeO was found to be in error; a value of 102,0 is proposed here as the correct formation constant. Five solution culture experiments were conducted using alfalfa (Medicago sativa L.) or tall wheatgrass (Elytrigia pontica [Podp.] Holub) with treatments consisting of NaSeO4 levels in combination with various levels of MgCl2 or CaCl2. Both shoot Se concentrations and whole‐plant Se contents were highly correlated with the free SeO2−4 activity but were poorly correlated with the sum of the free ion plus Ca and Mg ion pair species. Thus, we have shown, for the first time, that the free ion model of trace metal bioavailability is also valid for oxyanions that form complexes with Ca and Mg in saline media but that this conclusion hinges critically on the accuracy of the pertinent formation constants.