Utilization of olive husks as plant growing substrates: phytotoxicity and plant biochemical responses

The substitution of peat in growing substrates used in plant nurseries with organic materials is gaining much attention due to environmental and economic advantages. In the present research, thirteen substrates were obtained by using different doses of two types of composted or non-composted olive-mill husks and their toxicity was assessed by a seed germination bioassay using Lepidium sativum L and compared with a conventional substrate used as a control. Based on these results, six substrates were selected and their effect on Festuca and Italian ryegrass was evaluated and data compared with the same control. In particular, length of shoots and roots, fresh weight, and the activity of glutathione S-transferase (GST), glutathione peroxidase (GPx) and triosephosphate isomerase (TPI), were determined. Generally, the substrates did not cause lethal symptoms of stress, and it was ascertained that GST were responsive to phenols content, GPx to copper and nickel, and TPI to salinity. The results show the importance of composting in reducing phytotoxicity as evidenced by significant differences in GI and enzyme activities. Moreover the adoption of agricultural soil as a constituent of growing substrates is beneficial in contrasting the phytotoxic effects due to the presence of compounds contained in the soluble organic matter fraction, such as phenols.