Metal Complexation in Xylem Fluid

Abstract

Theoretical considerations of metal complex formation in aqueous solutions were used to develop a computer program (CHELATE) to cakulate all equilibrium species (free metal ions, metal complexes, etc.) in any user-defined system, such as xylem fluid. Mass-balance equations were established to describe each free metal ion and each free ligand concentration as a function of solution pH, total metal or total ligand, hydrogen-association constants, and the stability constants of known metal complexes. A default data base can be altered by the user to define any desired system covered by the stored equilibrium data. The program can currently handle nine metal ions, 35 ligands, and 500 complex species. The validity of the program was confirmed by using experimental test systems in which free-metal ion activity measurements were made with ion-selective electrodes. Program CHELATE was used to calculate the distribution of six metals in 0-to 1-hour exudate from soybean (Glycine max L. Meff.) and tomato (Lycopersicon escuentum Mill.) plants grown in normal and Zn-phytotoxic nutrient solutions. The results indicated that Fe is bound by citric acid, and Cu is bound by several amino acids in the normal-Zn exudate. Most of the Cu in soybean exudate is bound to asparagine and histidine. In tomato, Cu is bound to histidine, glutamine, and asparagine. Zinc, Mn, Ca, and Mg are bound primarily by citric acid and malic acid in both species; the per cent bound for these metals is lower than that for Fe and Cu. Zinc phytotoxicity caused equilibrium concentration shifts and resulted in the formation of several adtional metal complexes not found in the normal-Zn exudate. Work reported in a companion paper (24) showed that xylem ' This work represents cooperative research from the fluid contains many natural complexing compounds and indicates that metal ions should form metal complexes in vivo in the xylem sap. There is in vitro electrophoretic evidence for stable metal complex formation in xylem fluid (2, 20, 23) but no quantitative estimates of the equilibrium distribution of metals, ligands, metal complexes, and other solutes. In a solution containing several metals and ligands, simultaneous competing equilibria distribute the metals among the ligands, creating numerous complexes. Such a system can be described both qualitatively and quantitatively; however, quantification of all equilibrium species in a multimetal, multiligand system is far more difficult than listing the theoretically permissible (qualitative) species. Although many aqueous species cannot be measured, equilibrium chemistry can be used to estimate metal binding in aqueous solutions. The degree to which hydrated metal ions associate with various ligands depends primarily on the relative metal and ligand concentrations , solution pH, and the pertinent empirically determined stability constants. These variables have been used to determine equilibrium distributions of metals among ligands in several diverse natural aqueous solutions (6, 9, 21). Most of these systems are...

Effects of Iron Deficiency on the Composition of the Leaf Apoplastic Fluid and Xylem Sap in Sugar Beet. Implications for Iron and Carbon Transport

“…Iron-citrate complexes were the major predicted iron chemical species in xylem sap and apoplastic fluid of sugar beet. This agrees with previous data obtained from tomato and soybean stem exudates (White et al, 1981a(White et al, , 1981b and supports that citrate plays a major role in long distance iron transport, as proposed by Tiffin (1966). Our data, however, indicate that iron deficiency causes significant changes in the chemical speciation of iron-citrate in sugar beet.…”

Nutritional deficiency in citrus with symptoms of citrus variegated chlorosis disease

“…In the xylem, there is a strong correlation between Fe and citric acid in exudate from numerous plants (Tiffin, 1970). The studies of White et al (1981) also confirms that Fecitrate is the chemical form of Fe in normal xylem sap, therefore iron is not available for endophytic bacteria, which will suffer from iron deficiency for cell growth. Under such conditions, the bacterial production of siderophores is induced.…”

Low-molecular weight organic acids and peptides involved in the long-distance transport of trace metals

“…It contains a mixture of carboxylic acids (2-9 mM) and amino acids (1-3 mM) (White et al 1981;Lopez-Millan et al 2000, 2001. This information came from the studies on the xylem sap from Beta vulgaris, Pyrus communis L, Glycine max, Lycopersicon esculentum.…”

“…This environment favors complexation of metals with the carboxylic acids rather than amino acids. Characterizations of xylem and even computational analyses of metal speciation have been ongoing for several decades, because xylem sap collection is relatively easy (White et al 1981;Mullins et al 1986). Both approachesexperimental and in silico simulations-for many years suggested a great role of LMWOA as ligands of various metals in xylem sap.…”