Silicon amelioration and aluminium toxicity in hydroponically grown Picea abies seedlings: root growth and modelling data.

M. Ryder¹, F. Gerard², D.E. Evans¹, and M.J. Hodson¹.

Aluminium (Al) has no known role in higher plants, and is generally identified as a toxic element; phytotoxicity is a major problem in acidic soils and Al³⁺ is regarded as the main toxic species. It has been shown that silica can ameliorate Al toxicity for a range of plant taxa grown hydroponically.

Three week old Picea abies seedlings were transferred to a Ca(NO₃)₂ based, continuous flow hydroponic culture system at 20°C. Seedlings were treated with 100µM Al in combination with 1000 or 2000µM silicon (Si) for 7 days. Solution pH was adjusted to 4.00, 4.25, 4.50, 4.75, or 5.00.

It was found that at pH 4.00 Si did not ameliorate the toxic effect of Al on root growth, whereas increasing amelioration was found as the pH increased. Complete amelioration of Al toxicity was recorded at pH 5.00, and this was observed for both 1000 and 2000µM Si. An equilibrium speciation model (PHREEQC), in conjunction with a current thermodynamic database was used to predict the behaviour of Al and Si in growth solutions at the experimental temperature, elemental concentrations, and pH values. The Al³⁺ content was higher for runs where formation of hydroxyaluminosilicates (HAS) was not permitted, than where the possibility of HAS formation was included. Solutions were modelled to include the possibility of HAS formation, and the concentration of Al³⁺ decreased from about 80% of Al bearing species at pH 4.00, to approx. 18% of Al at pH 5.00. There was an increase in HAS content, showing an inverse relationship with Al³⁺.

Root growth data pointed to a decrease in phytotoxic Al³⁺ with increasing pH. Modelling data showed a similar trend predicting that at higher pH more Al was immobilised as HAS. The growth and modelling results were particularly interesting because previous work on angiosperms does not indicate such a close match between theoretical Al activity and experimental root growth data.

Michelle Ryder
Tel. 01865 483267
E-mail: mryder@brookes.ac.uk

1. School of Biological and Molecular Sciences. Oxford Brookes University, Headington Campus, Gipsy Lane, Oxford, OX3 0BP, UK.
2. INRA - Centre de Recherches Forestières. Unité Biogéochimie des Ecosystèmes Forestiers, Forêt d'Amance, 54280, Champenoux, France.