The Role of Plant-Derived Essential Oils in Eco-Friendly Crop Protection Strategies Under Drought and Salt Stress

Essential oils (EOs) are volatile, aromatic, and hydrophobic extracts of plant origin, known for their complex chemical composition, which often includes over 300 natural molecules with low molecular weights. They are extracted from various plant organs through physical-mechanical processes or dry distillation, and their qualitative composition and quantity change depending on the species, cultivar, and environmental growth conditions. They play a key role in plants' response to abiotic stresses, such as drought and salinity, whose effects are intensified by climate change. Several studies showed that drought and salinity can increase or decrease EO production, depending not only on the plant species but also on the severity of the stress; in fact, in many crops an enhancement of EO synthesis was often observed under mild stress, whereas moderate or severe stress reduced production. For a few years, EOs have been considered important biostimulants and bioprotectors, capable of replacing chemical pesticides in sustainable agriculture. Consequently, seed pre-treatments (e.g., seed priming or seed coating) with EOs may represent promising tools to improve germination, stress tolerance, and crop productivity under stress conditions. Nevertheless, the high costs of extraction of EOs and the little evidence collected from field experiments still limit their use in agronomic applications. The aim of this review was to gather the most important information, published over the last ten years, concerning the effects of drought and salinity on the production of EOs and their use as biostimulants. This review critically examines the available literature, highlighting a positive perspective towards the adoption of natural approaches to reduce the environmental impact of agricultural production. Current results indicate encouraging progress in the application of EOs as biostimulants; however, further studies are needed to verify their effectiveness in real agronomic environments.