Drought is one of the main limitations impacting the growth, yield, and phytochemical properties of medicinal plants. The present study examined how foliar application of nano- silicon (n-Si) can impact drought tolerance, physiological responses, and levels of bioactive compounds (bioactives) in the medicinal herb, Lallemantia iberica. Plants were grown in a greenhouse under either well-watered (100% field capacity) or moderate drought stress (50% field capacity) conditions. Drought-stressed plants received a spraying of n-Si (at a concentration of 100 mg L-1) or bulk silicon at a similar concentration, every three days. Compared to control (well-watered) plants, biomass accumulation and photosynthetic efficiency were significantly decreased, along with seed yield and essential oil yields in drought-stressed plants. Application of nano-silicon in drought conditions improved plant water condition, chlorophyll levels, antioxidant enzyme activity, and yield-related traits compared with untreated drought-stressed plants. On the contrary, the responses produced by bulk silicon were much weaker and inconsistent. The improvement in the beneficial effects of nano-silicon resulted from better regulation of water-use efficiency, oxidative stress, and the number of secondary metabolites produced, demonstrating that there were physiological changes instead of isolated responses by individual traits. Results indicate that the beneficial effects of n-Si can lessen the impact of drought stress through the enhancement of physiological stability and the production of phytochemicals from L. iberica in a controlled environment.
