Comparative analysis of chemically and green synthesized titanium dioxide nanoparticles for the regulation of photosynthesis in Lactuca sativa L.

Green synthesized titanium dioxide nanoparticles (g-TiO2 NPs) have aroused widespread interest in agriculture. Nevertheless, whether they are safer than chemically synthesized TiO2 NPs (c-TiO2 NPs) remains to be demonstrated. Herein, the photosynthetic response of Lactuca sativa L. was evaluated by foliar spraying of 10, 100, 250, and 500 mg L-1 c-TiO2 NPs or g-TiO2 NPs. Results indicated that NPs interfered with nutrient accumulation and the cellular redox state, and all treatments displayed inhibition of growth except the 10 mg L-1 g-TiO2 NP group. Photosynthetic parameters and FTIR analysis revealed that NP stress on photosynthesis was also manifested by lower photosystem activity (500 mg L-1) and disrupted Calvin cycle metabolism, but carbohydrates and proteins were more sensitive to c-TiO2 NPs and g-TiO2 NPs, respectively. Notably, both NPs promoted photosynthetic electron transfer (<= 250 mg L-1), thus alleviating detrimental effects due to suppressed ATPase and Rubisco activity, thylakoid lysis, and chloroplast autophagy. Differently, chloroplasts were more responsive to ultraviolet (non-directly utilizable) and visible light under c-TiO2 NP treatment, which facilitated the Hill reaction while posing a photo-oxidative risk to plants. In contrast, g-TiO2 NPs were less phytotoxic, as evidenced by higher NADPH, ATPase activity, chlorophyll, and photosynthetic efficiency, and less chloroplast damage, where 10 mg L-1 g-TiO2 NPs effectively activated the plant defense system and improved light capture and conversion. Collectively, g-TiO2 NPs showed lower phytotoxicity by modulating energy conversion processes in chloroplasts, which provided a cutting-edge research perspective for the application of nanopesticides.