Mechanism Of Nano-TiO₂ On Photochemical Reaction And Nitrogen Metabolism Of Spinach

Nanomaterials (NMs) and nanotechnology have been widely applied in world.Recently, nanocompound fertilizer has also been applied in agriculture. To date, fewstudies have focused on the effects and mechanisms of NMs on plant growth. Having aphotocatalyzed charactreistcs, nano-TiO₂ under light could cause an oxidation-reductionreaction, and produce superoxide ion radical and hydroxide. These reactive oxygenspecies can go after organic substances and be effective antimicrobial agents. In ourrecent researches, nano-TiO₂ treatment could markedly promote the vigor of aged seedsand chlorophyll formation of spinach; particularly, the ribulosebisphosphate carboxylaseactivity and photosynthesis were obviously increased, and the growth and developmentof spinach was promoted. It is well known that photosynthesis is consistent withphotocatalyzed chemical reaction and enzyme-catalyzed chemical reaction. As aphotocatalyzed reaction, we speculated that photosynthetic rate enhancement bynano-TiO₂ catalysis might be closely related to photochemical reaction activity such asthe absorption of light energy, transforming light energy into electron energy, electrontransport rate, oxygen evolution rate and photophosphorylation efficiency, and so forth.Based on these results, we studied to study the effects of nano-TiO₂ (rutile or anatase) onphotosynthesis and nitrogen metabolism of spinach, as a theoretical basis and technicalapproach for the agricultural applications of NMs. Four main aspects are involved.1) The effects of nano-TiO₂ on the photochemical reaction of chloroplasts ofspinach were studied. The results showed that when spinach was treated with 0.25%nano-TiO₂, the Hill reaction, such as the reduction rate of FeCy, and the rate of evolutionoxygen of chloroplasts was accelerated and noncyclic photophosphorylation (nc-PSP)activity of chloroplasts was higher than cyclic photophosphorylation (c-PSP) activity, the chloroplast coupling was improved and activities of Mg²⁺-ATPase and chloroplastcoupling factorâ… (CF₁)-ATPase on the thylakoid membranes were obviously activated. Itsuggested that photosynthesis promoted by nano-TiO₂ was closely related to activation ofphotochemical reaction of chloroplasts of spinach.2) The effects of nano-TiO₂ on the chloroplast aging of spinach under light werestudied. The results showed that when the chloroplasts were illuminated for 1, 5, and 10min with 500μmol/cm²/min light intensity, respectively, the evolution oxygen rate wasrapidly increased; when the chloroplasts were treated for 20, 30, and 40 min with 500μmol/cm²/min light intensity, respectively, the evolution oxygen rate was graduallydecreased. While spinach was treated with 0.25% nano-TiO₂, the rate of evolutionoxygen of chloroplasts in different illumination times (1, 5, 10, 20, 30, and 40 min) washigher than that of control, and when the illumination time was over 10 min, thereduction of the evolution oxygen rate was lower than that of control. It suggested thatnano-TiO₂ treatment could protect chloroplasts from aging for long-time illumination.The mechanism researches indicated that nano-TiO₂ treatment could significantlyincrease the activities of superoxide dismutase (SOD), catalase (CAT), and peroxidase(POD), decrease accumulation of reactive oxygen free radicals and the level ofmalondialdehyde (MDA), and keep stability of membrane structure of chloroplast underlight.3) The effects of nano-anatase TiO₂ on the nitrogen metabolism of growing spinachwere studied by treating them with nano-anatase TiO₂. The results showed thatnano-anatase TiO₂ treatment could obviously increase the activities of nitrate reductase,glutamate dehydrogenase, glutamine synthase, and glutamic-pyruvic transaminaseduring the growing stage. Nano-anatase TiO₂ treatment could also promote spinach toabsorb nitrate, accelerate inorganic nitrogen (such as NO₃^--N and NH₄⁺-N) to betranslated into organic nitrogen (such as protein and chlorophyll), and enhance the freshweight and dry weights.4) The improvement of spinach growth is proved to relate to N₂ fixation by nano-anatase YiO₂ in this study. The results showed that all spinach leaves kept green bynano-anatase TiO₂ treatment and all old leaves of control turned yellow white underculture with N-deficient solution. And the fresh weight, dry weight and contents of totalnitrogen, NH₄⁺, chlorophyll and protein of spinach by nano-anatase TiO₂ treatmentpresented obvious enhancement compared with control. While the improvements of yieldof spinach were not as good as nano-anatase YiO₂ treatment under N-deficient condition,confirming that nano-anatase TiO₂ on exposure to sunlight could chemisorb N₂ directlyor reduce N₂ to NH₃ in the spinach leaves, transforming into organic nitrogen andimproving the growth of spinach. Bulk YiO₂ effect, however, was not as significant asnano-anatase TiO₂. A possible metabolism of the function of nano-anatase YiO₂ reducingN₂ to NH₃ was discussed.