Phytotoxicity of aluminium-fluoride complexes and their uptake from solution culture by Avena sativa and Lycopersicon esculentum

Avena sativa (oats) and Lycopersicon esculentum (tomatoes) were grown in dilute nutrient solutions supplemented with Al only, F only or a combination of both. In solutions containing Al and F, shoot growth was limited when predicted Al-r (Al3+, AlOH2+ and Al(OH)(2)(+)) activities were < 0.1 mu M, activities three orders of magnitude lower than the critical value determined with Al only. The data suggest that at the activities used in these experiments, Al-r is most toxic, AlF2+ and AlF2+ are toxic to a lesser extent, and AlF3, AlF4- and F- are least toxic. Fluoride concentrations in shoots grown in solutions correlated best with positively charged AlFx3-x species (i.e. AlF2+ AlF2+) and the molar ratio of F:AI in most plants shoots was about 3:2. However, when activities of positively charged AIF(x)(3-x) species were low (< 50 mu M) and the activity of AlF3 species high (500 mu M the molar ratio of F:Al in plant shoots suggested AlF3 was taken up. These findings are discussed in relation to plant uptake and toxicity. Measured concentrations of Al-r and F- in solutions containing Al and F were compared with concentrations predicted by a computer model (GEOCHEM-PC). The method for measuring F- concentrations, using a noncomplexing buffer and F ion-selective electrode, gave good agreement with predicted F-concentrations. The 8-hydroxyquinoline method for measuring Al-r concentrations did not agree with predicted concentrations, highlighting the limitations of this method when measuring Al-r in the presence of F in a multi-ligand system with high concentrations of F and Al.