ORIGINAL ARTICLE J. Oleksyn P. Karolewski M. J. Giertych A. Werner M. G. Tjoelker P.B. Reich Received: 5 December 1994 / Accepted: 24 May 1995 AbstractmOne-year-old Scots pine (Pinus sylvestris L.) seedlings were grown for 9 weeks in nutrient solutions containing 0, 0.5, 1, 2 and 4 mM aluminum nitrate (Al(NO3)3) at pH 4.2. Nine weeks exposure to Al signifi- cantly reduced total plant, shoot and root mass and caused a linear decline in proportional allocation of biomass to roots. Relative growth rate of roots declined to as low as zero. Aluminum treatment decreased calcium and magnesium uptake and increased Al content in roots and needles. After 3 weeks of exposure a 10 – 60% increase in total phenols in roots and a 20 – 40% increase in o-diphenols in roots and needles were noted. Roots affected by Al showed degener- ation of meristematic cells, fewer cell divisions, deforma- tion in cell walls and higher lignification and suberization. The majority of root apices were structurally similar to dormant roots, and a premature senescence of the entire root system was observed. Net photosynthetic rate after 6 weeks of treatment was negatively correlated with needle Al content and Al/Ca ratio (r – 0.9, P 0.1). The results suggest that Scots pine may be more susceptible to Al than was expected based on previous experiments. Key wordsmScots pine Aluminum Pollution Photosynthesis Roots Air pollution is an important factor contributing to forest decline in industrialized regions. Trees can be affected directly by various toxic gases or indirectly by soil- mediated effects. Acidic deposition of sulfur and nitrogen compounds can have a marked impact on many types of forest soils, due to their low buffering capacity (Ulrich 1983; Matzner and Ulrich 1984). In acidic soils, indirect negative effects on forest trees may result from dissolution of Al at low pH resulting in increasing toxicity of this element (McCormick and Steiner 1978; Keltjens and van Loenen 1989; Reich et al. 1994). The main mechanisms involved in aluminum toxicity (Boudot et al. 1994) include (1) competition between Al 3+ , Ca 2+ and Mg 2+ for the uptake sites of root meristems, leading to decreases in Ca and Mg content, (2) inhibition of meristematic cell division resulting in low root growth, (3) alteration of the root membrane structure and function, and (4) decline of nutrient and water uptake by roots. To date most studies of aluminum toxicity in plants have focused on effects on root structure and function. However, the accumulation of Al by foliage can cause physiological disturbances. Foliar Al/Ca ratios in Picea rubens were related to differences in respiration among sites of contrast- ing pollution deposition in North America (McLaughlin et al. 1990, 1991). At a heavily polluted site in Poland, we found excessive soil Al and low Mg availability, leading to low needle Mg, high Al, high Al/Ca ratios, reduced photo- synthetic capacity, and increased respiration in Scots pine foliage (Reich et al. 1994). Very little is known about the effect of toxic metals on phenols in plants. In studies conducted with such toxic gases as SO2, HF and O3, it was shown that the level of phenols in plants is associated with the injuries inflicted by these gases (Yee-Meiler 1977; Jordan at al. 1991; Zobel and Nighswander 1991; Giertych and Karolewski 1993). The level of phenols in plants can contribute to their sensitivity to biotic factors (Balsberg Pa ˚hlsson 1992) and is linked to plant growth (DeKock and Vaughan 1975; Esterbauer et al. Trees (1996) 10: 135 – 144  Springer-Verlag 1996 J. Oleksyn ( ) P. Karolewski M. J. Giertych A. Werner Polish Academy of Sciences, Institute of Dendrology, Parkowa 5, PL-62-035 Ko ´rnik, Poland M. G. Tjoelker P.B. Reich University of Minnesota, Department of Forest Resources, 115 Green Hall, 1530 N. Cleveland Avenue, St. Paul, MN 55108, USA