Behaviour of different horticultural species in phytotoxicity bioassays of bark substrates

The use of forestry wastes (barks, sawdust, wood chips) as potting media components can involve serious problems of toxicity that must be evaluated. Evaluation methods may consist of analytic techniques to detect and quantify phytotoxic molecules, or rapid bioassays, with low technical requirements, using species sensitive to the toxic elements. These bioassays should be able to predict plant behaviour in real growing conditions. In this study we used as substrate cork oak bark (Quercus suber L.). This material, like other forestry wastes, contains phytotoxic phenolic compounds which can be washed out with hot water. The phenolic content of cork (C) and washed cork (WC) was measured, and two germination bioassays were carried out with eight horticultural species. In the first, aqueous extracts of cork (C) and washed cork (WC) obtained with water at 15°C and 70°C were used, and the second was done directly on the substrates. In addition, a seedling growing test was carried out with the same species. We concluded that tomato and lettuce are the most sensitive species, and the direct test on the substrate is recommended owing to its simplicity and close correspondence to growing conditions.