Plant Physiology 67:292-300 (1981)
© 1981 American Society of Plant Biologists
Articles
Metal Complexation in Xylem Fluid 1
I. CHEMICAL COMPOSITION OF TOMATO AND SOYBEAN STEM EXUDATE
Michael C. White2
A. Morris Decker
Rufus L. Chaney
Maryland Environmental Service, Annapolis, Maryland 21401,
Department of Agronomy, University of Maryland, College Park, Maryland 20742,
Biological Waste Management and Organic Resources Laboratory, Agricultural Environmental Quality Institute, Beltsville, Maryland 20705,
United States Department of Agriculture, Science and Education Administration-Agricultural Research, Beltsville, Maryland 20705
Xylem fluid was analyzed for numerous solutes to characterize chemically the sap as a medium for forming and transporting metal complexes. The stem exudate was collected hourly for 8 hours from topped 31-day-old soybean (Glycine max L. Merr.) and 46-day-old tomato (Lycopersicon esculentum Mill.) plants grown in normal (0.5 micromolar) and Za-phytotoxic nutrient solutions. Soybean plants were grown in the normal and high-Zn solutions for 24 days; tomato plants were grown for 32 days. The exudate was analyzed for seven organic acids, 22 amino acids, eight inorganic solutes, apparent ionic strength, and pH. Significant changes in many solutes occurred over the 8-hour sampling period. These fluctuations depended on plant species, individual solute, and Zn treatment, and demonstrated that extrapolation of xylem-fluid analyses to whole-plant xylem sap is valid only for sap samples collected shortly after topping a plant. Exudate pH decreased over the 8-hour period for both species; exudate ionic strength increased for tomato and decreased for soybean. At the normal-Zn treatment (0 to 1 hour), the highest acid micromolar concentrations in soybean exudate were: asparagine, 2,583; citric, 1,706; malic, 890; and malonic, 264. Under the same conditions, the highest acid micromolar concentrations in tomato exudate were: maleic, 1,206; malic, 628; glutamine, 522; citric, 301; and asparagine, 242. Cysteine and methionine were above detection limits only in soybean exudate. Zinc phytotoxicity caused significant changes in many solutes. The analyses reported here provide a comprehensive data base for further studies on metal-complex equilibria in xylem fluid.
2 Present address: Enviro Control, Inc., One Central Plaza, 11300 Rockville Pike, Rockville, MD 20852.
1 Cooperative research from the Department of Agronomy (University of Maryland, College Park, MD), the Biological Waste Management and Organic Resources Laboratory (Agricultural Environmental Quality Institute, United States Department of Agricultural, Science and Education Administration-Agricultural Research, Beltsville, MD), and the Maryland Environmental Service (Annapolis, MD). This is also part of a dissertation submitted by M. C. W. in partial fulfillment of Ph.D. degree requirements.
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