The Effects Of Nano-TiO₂ On The Photochemical Reaction Of Spinach

With a photo—catalyzed characteristic, nano-anatase-TiO₂ under light could cause an oxidation–reduction reaction. Our studies had proved that nano-anatase-TiO₂ could promote photosynthesis and improve greatly spinach growth, the light absorbance and the transformation from light energy to electron energy and to active chemical energy,but also promote carbon dioxide (CO2) assimilation and nitrogen metabolism of spinach. However, the mechanism of promoting the light absorbance, transfer and the transformation from light energy to electron energy and to active chemical energy by nano-anatase-TiO₂ remains largely unclear. So we study the effects of nano-anatase-TiO₂ on the photochemical reaction of spinach. Five main aspects are involved.1. The effects of nano-rutile-TiO₂ and bulk TiO₂ on the germination and growth of naturally aged spinach seeds were studied by measuring the germination rate and vigor indexes of aged spinach seeds. An increase of these factors was observed at 0.025–0.4% nano-rutile-TiO₂ treatment. During the growth stage, the plant dry weight was increased, as was the chlorophyll formation, the ribulosebisphosphate carboxylase/oxygenase activity, and the photosynthetic rate. The best results were found at 0.25% nano-rutile-TiO₂ treatment. The effects of bulk-TiO₂ are not significant. It is shown that the physiological effects are related to the nanometer-size particles, but the mechanism by which nano-rutile -TiO₂ improves the growth of spinach seeds still needs further study.2. The mechanism of nano-anatase-TiO₂ on promoting conversion from light energy to electron energy and from electron energy to active chemistry energy remains largely studied. The results showed that the electron transfer, oxygen evolution and photophosphorylation of chloroplast from nano-anatase-TiO₂ treated sipnach were greatly increased under visible light and ultraviolet light illumination, respectiviley. It was demonstrated not only that nano-anatase-TiO₂ could greatly improve whole chain electron transport, photoreuction activity of PSâ…¡, O2-evolving and photophosphorylation activity of spinach chloroplasts under visible light, but also enriched-energy electron from nano-anatase-TiO₂ which entered chloroplast under ultraviolet light was transfered in photosynthetic electron transport chain, made NADP+ be reduced into NADPH, and coupled to photophosphorylation, then made electron energy be transformed to ATP. And nano-anatase h+ which photogenerated electron-holes captured a electron from water, which accelerated water photolysis and O2 evolution.3. The effects of nano-anatase-TiO₂ on the spectral characteristics and content of light-harvesting complexâ…¡(LHCâ…¡) on the thylakoid membranes of spinach were investigated. The results showed that nano-anatase-TiO₂ treatment could increase LHCâ…¡content on the thylakoid membranes of spinach and the trimer of LHCâ…¡; nano-anatase-TiO₂ could enter the spinach chloroplasts and bind to PSâ…¡. Meanwhile, spectroscopy assays indicated that the absorption intensity of LHCâ…¡from nano-anatase-TiO₂ treated spinach was obviously increased in red and blue region, fluorescence quantum yield near 685 nm of LHCâ…¡was enhanced, the fluorescence excitation intensity nearby 440 and 480 nm of LHCâ…¡significantly rose and F480/F440 ratio was reduced. Oxygen evolution rate of PSâ…¡was greatly improved. Together, nano-anatase-TiO₂ promoted energy transferring from chlorophyll b and carotenoid to chlorophyll a, and nano-anatase-TiO₂ was photosensitized by chlorophyll of LHCâ…¡, thus led to enhance the efficiency of absorbing, transferring and converting light energy. 4. The mechanism of absorbing and transferring light energy in arabidopsis chloroplasts with nano-anatase-TiO₂ treatment was studied. The results showed that nano-anatase- TiO₂ treatment could induce Lhcâ…¡b of arabidopsis expressed largely and significantly increase the LHCâ…¡content on the thylakoid membranes of chloroplasts, then promoted the absorption and transfer of light energy in chloroplasts and regulated excitation energy distribution largely from PSâ… to PSâ…¡, thus led to enhance the efficiency of converting light energy and O2 evolution.5. The effects of nano-anatase-TiO₂ on the antioxidant stress in spinach chloroplasts under UV-B radiation were investigated. The results showed that nano-anatase-TiO₂ treatment could significantly decrease accumulation of superoxide radicals (O2·–), hydrogen peoxide (H2O2) and malonyldialdehyde (MDA) content, and increase activities of superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), guaiacol peroxidase (GPX), and elevate O2-evolving rate in spinach chloroplasts under UV-B radiation. Together, nano-anatase-TiO₂ could decrease the oxidative stress to spinach chloroplast caused by UV-B radiation.