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The use of Treated Waste Water (TWW) as an alternative to limit water use in agriculture practices in the context of global warming is of particular importance, especially in countries where water resources are limited. In this study, we assessed the effect of TWW irrigation on two Eucalyptus species (E. camaldulensis and E. occidentalis) in their early stages of growth (up to one year after seedling and transplantation) in pot experiment and compared to Well-Water (WW) irrigation. Changes in plant growth traits/ characteristics including plant height, stem diameter, number of leaves and Leaf Mass per Area (LMA) were analysed as well as the modification of root and leaves secondary metabolic profiles and total phenolic contents. Soil physico-chemical parameters were also measured before and after the experiment.

Our results showed that both species do not have the same response to TWW irrigation: while most growth recorded parameters were most affected in E. camaldulensis after one year of growth with TWW, E. occidentalis had its leaf number significantly increased with TWW after seven months. Leaves were lighter and smaller than the ones obtained with WW irrigation after four months but the reduction of weight and area was proportional in such a way that LMA was not affected and this effect was transitory. Such effect was not observed with E. camaldulensis.

Concerning root and leaf secondary metabolite profiles, changes in both plant part extract compositions were observed according to irrigation type, though these were weak compared to plant species identity. Common metabolic patterns following TWW irrigation could be showed in both species like for example the increase of quercetin glucosinnapate in leaf extracts or the decrease of two peduncalagin isomers detected as major compounds in the same extracts. Though some differences were also observed, particularly, E. occidentalis had more compounds decreased in its roots than E. camaldulensis when irrigated with TWW while in leaves the tendency was inverted. Total phenolic content was decreased in both species and for both plant parts, but this diminution was more pronounced in E. occidentalis roots. We also noticed modifications in soil parameters after TWW exposure and some were dependant on the species of Eucalyptus like for example soil organic matter content which increased with TWW in the case of E. occidentalis, whereas it decreased for E. camaldulensis. All these results let us hypothesize that E. occidentalis possess better adaptation capabilities to TWW irrigation than E. camaldulensis and that these might be related to changes in root metabolism.