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ABSTRACT
Two-week old seedlings of Pinus pinaster Ait. were grown in nutrient solutions at pH values ranging from 3.5 to 6.5. Acidity exerted a different influence on the growth of the main and lateral roots, thus changing the morphology of the whole root system. Roots grown at pH 3.5 had the shortest length, but the highest biomass and were markedly thickened. The total number of 2nd order lateral roots per plant was not significantly influenced by pH, whereas their length and the development of 3rd order roots were the highest at pH 3.5 and the lowest at pH 6.5. Mineral nutrition was affected by the pH of the culture solution. The P and Fe concentrations in roots were higher in the pH 3.5 and 4.5 solutions than in pH 5.5 and 6.5. In contrast, lower K, Ca, Mg and especially Mn contents were found in the roots grown at pH 3.5. However, needle dry weight was unaffected by low pH and neither chlorosis nor critical levels of mineral elements were observed. These results suggest that the compensatory growth of lateral roots allows P. pinaster seedlings to maintain an adequate nutritional status in the shoot even under highly acidic conditions.