Unravelling mechanisms of CaO nanoparticle-induced drought tolerance in Brassica napus: an analysis of metabolite and nutrient profiling

Nanotechnology has been widely used in agriculture to improve plant growth and stress tolerance. The exogenous application of calcium nanoparticles (CaO NPs) can improve plant tolerance to drought stress. However, the underlying physiological molecular mechanisms are still unclear. Herein, 100 mg L-1 of CaO NPs were applied to rapeseed plants when grown under the conditions of 0-15% w/v PEG-6000 solution. Drought stress reduced the rapeseed growth, CO2 assimilation rate, stomatal conductance, and photosynthetic pigments. The application of 100 mg L-1 CaO NPs improved the growth of rapeseed plants under drought conditions (shoot dry weight, 77%; root dry weight, 69%). Growth improvement due to CaO NPs was positively associated with the photosynthetic rate, quantum yield of photosystem II and quantity of photosynthetic pigments. The net photosynthetic rate (Pn), stomatal conductance (Gs), internal CO2 (Ci), and transpiration rate (Tr) increased by 65%, 85%, 69%, and 67%, respectively. The increases in quantum yield of photosystem II and photosynthetic pigments due to CaO NPs were 85% and 53%, respectively. A positive association between the growth and each of the gas exchange attributes, PSII activity and photosynthetic pigments indicated that CaO NPs improved the photosynthetic rate by reducing stomatal, as well as non-stomatal, limiting factors. The CaO NP treatment also improved the uptake of mineral nutrients under drought stress, including calcium by 82%, and potassium, phosphorous, magnesium, manganese, and boron by 78%, 89%, 72%, 80% and 73%, respectively. Furthermore, the application of CaO NPs caused a greater accumulation of 28 metabolites and reduced the accumulation of 18 metabolites that are mainly related to N-metabolism and amino acid biosynthesis, such as cysteine/homocysteine, lysine, tryptophan, alanine, glutamate, and proline, compared to droughted plants. The application of CaO NPs under drought conditions induced the up-regulation of upstream genes such as CHS, CHI, F3 ' H, and F3H, early development genes such as PAL, C4H, 4CL1, 4CL5, DFR, and ANS, and late development genes such as UGT78D2, UGT79B1, MT, PAP1, and PAP2 in plants involved in flavonoid biosynthesis expression. The findings of this study suggest that CaO NPs improved the photosynthetic capacity through modulating the stomatal conductance, photosystem II activity, accumulation of nutrients, and reprogramming of both primary and secondary metabolic pathways such as N-metabolism, hormonal and flavonoid biosynthesis for regulating rapeseed growth under drought stress.