Effect of Short and Long Term Irrigation with Treated Wastewater on Chemical Composition and Herbicidal Activity of Eucalyptus camaldulensis Dehn. Essential Oils

Context

The agricultural system is exposed to several threats that can influence crop yields and sustainability such as diseases and pests especially weeds. Weeds take benefit of the advantageous state that occur in agriculture structure and present a good adaptability to different environment. Indiscriminate use of synthetic herbicides can cause serious ecological and environmental problems such as the degradation of soil resistance in weeds and amplify environmental pollution and health dangers

Objective

The aim of this work was  to evaluate the effect of treated wastewater short and long-term irrigation on essential oil composition from E. camaldulensis Dehn in order to detect allelopathic impacts on seed germination and seedling growth of Amaranthus hybridus L, Chenopodium album L, Echinochloa crus-galli (L) Beauv and Lolium perenne L as important weeds on many crops.

Results

We used the conventional hydrodistillation (HD) extraction process which is considered a reference method for the extraction of essential oils. The results showed that treated wastewater irrigation was beneficial on the yield of E. camaldulensis that showed variation between 1.06% and 1.58% for young pot crops and between 1.23% and 2.95% for plot planting. Chromatogram analysis of E. camaldulensis makes it possible to identify 40 compounds representing 90% to 100% of the total composition. These compounds are distributed over several chemical classes and in the majority of them are monoterpenes (oxygenates and hydrocarbons).
For E.camaldulensis pot cultures (6 months), EO were rich in essentially oxygenated monoterpenes with similar proportions (54.85% and 58.55%) with the dominance of 1,8-cineole (51.77% and 55.02%). The same observations were illustrated after 12 months of irrigation. Both types of culture produce EO rich in substantially oxygenated monoterpenes with similar proportions (57.78% and 58.25%) with the dominance of 1,8-cineole (50.01% and 52.26%). OE of E. camaldulensis in plot showed a different chemical composition depending on the type of water. An increase in oxygenated monoterpenes was recorded following TME irrigation from 25.53% to 60.24% with an increase of the major 1,8-cineole compound from 26.64% to 55.47%. P-cymene (32.78%) was the predominant compound in well water-irrigated versus US-irrigated plantations with the dominance of 1,8-cineole (55.47%).
In the end, the effect of water quality on the herbicidal activity of essential oils and aqueous extracts of E. camaldulensis was investigated by considering the germination rate and growth of: Amaranthus hybridus L, Chenopodium album Echinochloa crus-galli and Lolium perenne L.
Water quality influenced herbicide activity at several levels: the test species, the effective dose and the level of intervention (germination rate or sprouted seed growth). The herbicidal activity of EO from young pot crops of E. camaldulensis and EA plot planting was dependent on water quality for some tested seeds. However, irrespective of the irrigation water, the herbicide activity of EO in the field plantations was identical for all species tested. In general, essential oils have revealed a higher phytotoxicity than aqueous extracts. The major constituents of the essential oils of Eucalyptus camaldulensis were 1,8-cineole and p-cymene.

Recommendations

The use of these essential oils and aqueous extracts of E. camaldulensis is likely to be a powerful adjunct to current weed management practices, reduce production costs and the potential for environmental pollution and reduce some of the social constraints associated with intensive manual weeding.

Impacts and weaknesses

Impacts: The use of these essential oils and aqueous extracts of E. camaldulensis is likely to be a powerful adjunct to current weed control practices, reduce production costs and the potential for environmental pollution and reduce some of the social constraints associated with intensive manual weeding.
Weaknesses: Risks related to the agricultural reuse of treated wastewater /Health risks / Environmental risks

Future developments

Although E. camaldulensis in this study have shown a high herbicidal potential, some aspects need to be deepened before being marketed as bio-herbicide: the evolution of the performance and composition of HE throughout the year, with different climates, within the same species and with the age of the plant, the volatility of the HE and its components, its lipophilicity, the difficulties of absorption by the plant, the effectiveness the conditions of ground (of tests in vivo) and their phytotoxicity to other invasive plants. However, it is a viable option to replace synthetic herbicides mainly in organic or ecological agriculture. During this work, we studied a real case of valorization of TMEs for irrigation of aromatic crops (Eucalyptus) for the production of bio-herbicides.
It is therefore clear that irrigation by EUTs (abiotic stress factor) influences the growth and production of secondary metabolites in higher plants. Thus, molecular understanding of the stress response will be useful in plant breeding with improved adaptation and efficiency