Foliar applied titanium dioxide nanoparticles (TiO2 NPs) modulate growth, physiological and phytochemical traits in Melissa officinalis L. under various light intensities

Lemon balm (Melissa officinalis L.) is a valuable medicinal plant belonging to the mint family Lamiaceae with different uses in the food, cosmetic-hygienic, and pharmaceutical industries. Therefore, it is crucial to identify novel factors affecting growth, yield, and phytochemical compounds of this plant. In the current study, titanium dioxide (TiO2) nanoparticles (NPs) were foliarly applied at different concentrations (0, 50, and 100 mg L−1) following various light intensities (full (control), 75% and 50% sunlight) on lemon balm in a factorial experiment based on randomized complete block design (RCBD) with three replications (n = 3) for two continues years. IAnalytical techniques, SEM and EDX analyses, confirmed the uptake of TiO2 NPs through aerial parts of plant. Output of shoot dry weight, chlorophyll a (Chl-a) content, and total Chl and relative water content (RWC) were at their highest levels on plants grown at 50% light intensity upon foliar application of TiO2 NPs (100 mg L−1) in both years. High shading (50% of light intensity) without TiO2 NPs application markedly reduced EO content (by 60%), when compared to full solar radiation and exposure to TiO2 NPs (at 50 and 100 mg L−1) during two experimental years. Neral and geranial were the two main components of EO, with 30.7% and 45.05% in the first year of experiment, followed by 30.24% and 43.03% in full light intensity under 100 mg L−1 TiO2 NPs applications in the second year, respectively. Thus, a relatively shady environment and 100 mg L−1 TiO2 NPs foliar spray are recommended to attain a higher yield in lemon balm plants. Key findings of this study revealed the beneficial effects of high light intensity with TiO2 NPs foliar spray to improve the quality and quantity output of EO rich in neral and geranial constituents.