Influence of dose and extraction method of biostimulants on drought stress tolerance in Coleus amboinicus Lour. Plants

This study addresses the challenge of drought stress in ornamental plants by evaluating the drought response of Cuban oregano (Coleus amboinicus Lour.) and the potential of sea fennel (Crithmum maritimum L.) extracts as biostimulants to mitigate the effects of water deficit. Conducted in a greenhouse under two irrigation regimes, full irrigation (100% water replacement) and deficit irrigation (50%), the research applied foliar treatments of sea fennel extracts prepared via aqueous extraction (WE) and alcoholic extraction (AE) at various concentrations. Drought stress significantly reduced total dry biomass by 48%, leaf number by 33%, and leaf area by 57%, severely impacting both aboveground and root growth. Biostimulant treatments alleviated these negative effects, with alcoholic extracts at 2.5 mL L⁻¹ showing the greatest efficacy by increasing biomass by approximately 60% and restoring leaf area to levels comparable to fully irrigated plants. Aqueous extracts provided moderate improvements. Drought induced an increased root-to-shoot ratio, indicating adaptive biomass allocation, while SPAD values and gas exchange parameters (net photosynthesis and stomatal conductance) declined under stress but improved with biostimulant application, especially with AE treatments. Photosystem II efficiency (Fv/Fm) confirmed stress but showed partial recovery in AE-treated plants. Additionally, WE at 5 mL L⁻¹ enhanced anthocyanin and amino acid accumulation, suggesting metabolic adjustments to drought. Overall, alcoholic sea fennel extracts demonstrated superior potential as sustainable biostimulants to enhance drought tolerance in ornamental plants.