1 Introduction In acid soils, weathering of Al-bearing minerals can lead to high concentrations of dissolved Al. Free Al 3+ ions are potentially toxic to plants as they can inhibit root growth and impair nutrient uptake (Delhaize and Ryan, 1995; Kinraide, 1991; Kochian, 1995). Plants that grow on acid soils, therefore, must have strategies to cope with these conditions. Norway spruce (Picea abies [L.] Karst.) is a common conifer in Northern and Central Europe, and is found extensively on acid soils. This tree is known to tolerate concentrations of up to 0.3 mM Al 3+ before growth is reduced (Göransson and Eldhuset, 1991). Lethal concentrations of monomeric Al are even 30 times higher (Göransson and Eldhuset, 1991). There is, however, little information on the mechanism of the Al resistance in this and other forest tree species. Aluminum resistance mechanisms can be divided into internal and external mechanisms (Kochian, 1995; Taylor, 1991). Internal tolerance mechanisms detoxify Al in the symplasm, whereas external mechanisms either exclude Al from sensitive sites in the root (Taylor, 1991) or, more generally, from root uptake. It is known that Al is mainly retained in the roots of Norway spruce and little is translocated to the shoot (Göransson and Eldhuset, 1991; Hentschel et al., 1993). Dahlgren et al. (1991) discussed co- precipitation of Al with oxalate and phosphate as possible retention mechanisms of Al in fine roots of a northwestern American conifer stand dominated by Abies amabilis (Dougl.) Forbes and Tsuga mertensiana (Bong.) Carr. In the outer root compartments of Al-treated 3-year-old J. Plant Nutr. Soil Sci. (2001), 164, 519±526 (2001)  WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001 1436-8730/01/0510-0519 $17.50+.50/0 Root exudation, organic acids, and element distribution in roots of Norway spruce seedlings treated with aluminum in hydroponics Alexander Heim 1* , Ivano Brunner 1 , Beat Frey 1 , Emmanuel Frossard 2 , and Jörg Luster 1 1 WSL Swiss Federal Research Institute, CH-8903 Birmensdorf, Switzerland 2 Institute of Plant Sciences, Swiss Federal Institute of Technology, Research Station Eschikon, CH-8315 Lindau, Switzerland Accepted 5 June 2001 Summary ± Zusammenfassung 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 hydroculture system with either 500 mM AlCl 3 or 750 mM CaCl 2 solutions at pH 4. Organic acids were determined in hot-water extracts of ground root tissue. Oxalate (3.3±6.6 mmol (g root dry weight) ±1 ) was most abundant. Malate, citrate, formate, acetate, and lactate concentrations ranged between 1±2 mmol (g root dry weight) ±1 . Organic substances and phosphate found in the treatment solutions at the end of the experimental period were considered to be root exudates. Total root exudation within a 2-day period ranged from 20±40 mmol 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 extracts of root tissue, were not detected in root exudates. Phosphate was released from Ca-treated plants. In Al treatments, there was indication of Al phosphate precipitation at the root surface. Oxalate and phenolics present in the exudates of Norway spruce seedlings are ligands that can form stable complexes with Al. However, concentrations of these substances in the treatment solutions were at micromolar levels. Their importance for the protection of the sensitive root apex under natural conditions is discussed. Key words: Al tolerance / hydroculture / organic acids / phosphate / Picea abies Wurzelausscheidungen, organische Säuren und Elementverteilung in Wurzeln aluminiumbehandelter Fichtensämlinge in Hydrokultur Fichtensämlinge (Picea abies [L.] Karst.), die drei Monate lang unter sterilen Bedingungen angezogen worden waren, wurden eine Woche lang in einem Hydrokultursystem mit Lösungen von 500 mM AlCl 3 oder 750 mM CaCl 2 bei pH 4 behandelt. In Heiûwasserextrakten gemahlener Wurzeln wurden organische Säuren gemessen. Dabei war Oxalat (3.3±6.6 mmol (g Wurzeltro- ckengewicht) ±1 ) am häufigsten, während die Konzentrationen an Malat, Citrat, Formiat, Acetat und Lactat zwischen 1±2 mmol (g Wurzeltrockengewicht) ±1 lagen. Organische Substanzen und Phos- phat, welche am Ende des Versuchsabschnitts in den Behand- lungslösungen gefunden wurden, wurden als Wurzelausscheidungen aufgefasst. Die gesamte Wurzelexsudation während zweier Tage betrug 20±40 mmol C (g Wurzeltrockengewicht) ±1 . In den Wurzel- exsudaten wurden organische Säuren, Gesamtkohlenhydrate, Gesamtaminosäuren und Gesamtphenole bestimmt. Citrat und Malat konnten trotz ihres Vorkommens in Heiûwasserextrakten des Wurzelmaterials nicht in den Exsudaten nachgewiesen werden. Die Ca-behandelten Pflanzen schieden Phosphat aus. In den Al- Behandlungen gab es dagegen Hinweise auf Aluminiumphosphat- Ausfällungen an der Wurzeloberfläche. Oxalat und phenolische Substanzen in den Exsudaten der Fichtensämlinge sind Liganden, welche stabile Komplexe mit Al bilden können. Die Konzentratio- nen dieser Liganden lagen jedoch nur im mikromolaren Bereich. Ihre Bedeutung für den Schutz der empfindlichen Wurzelspitzen unter natürlichen Bedingungen wird diskutiert. * Correspondence: Dr. A. Heim; E-mail: alexander.heim@wsl.ch 519