Root exudation, organic acids, and element distribution in roots of Norwayspruce seedlings treated with aluminum in hydroponics

Seedlings of Norway spruce (Picea abies [L.] Karst.), which had been grown under sterile conditions for three months, were treated for one week in a h ydroculture system with either 500 muM AlCl3 or 750 muM CaCl2 solutions at pH 4. Organic acids were determined in hot-water extracts of ground root ti ssue. Oxalate (3.3-6.6 mu mol (g root dry weight)(-1)) was most abundant. M alate, citrate, formate, acetate, and lactate concentrations ranged between 1-2 mu mol (g root dry weight)(-1). Organic substances and phosphate found in the treatment solutions at the end of the experimental period were cons idered to be root exudates. Total root exudation within a 2-day period rang ed from 20-40 mu mol C (g root weight)(-1). In root exudates, organic acids , and total carbohydrates, total amino acids, and total phenolic substances were quantified. Citrate and malate, although present in hot-water extract s of root tissue, were not detected in root exudates. Phosphate was release d from Ca-treated plants. In Al treatments, there was indication of Al phos phate precipitation at the root surface. Oxalate and phenolics present in t he exudates of Norway spruce seedlings are ligands that can form stable com plexes with Al. However, concentrations of these substances in the treatmen t solutions were at micromolar levels. Their importance for the protection of the sensitive root apex under natural conditions is discussed.