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Abstract  The improvement of spinach growth is proved to relate to N2 fixation by nano-anatase TiO2 in this study. The results show that all spinach leaves kept green by nano-anatase TiO2 treatment and all old leaves of control turned yellow white under culture with N-deficient solution. And the fresh weight, dry weight, and contents of total nitrogen, $$ {\text{NH}}^{ + }_{4} $$, chlorophyll, and protein of spinach by nano-anatase TiO2 treatment presented obvious enhancement compared with control. Whereas the improvements of yield of spinach were not as good as nano-anatase TiO2 treatment under N-deficient condition, confirming that nano-anatase TiO2 on exposure to sunlight could chemisorb N2 directly or reduce N2 to NH3 in the spinach leaves, transforming into organic nitrogen and improving the growth of spinach. Bulk TiO2 effect, however, was not as significant as nano-anatase TiO2. A possible metabolism of the function of nano-anatase TiO2 reducing N2 to NH3 was discussed. [ABSTRACT FROM AUTHOR]

A correction to the article "Oxidative Stress Induced by Lead in Chloroplast of Spinach," by Xiao Wu, Hao Huang, and Xiaoqing Liu, is presented.

Seedlings of spinach were grown in Hoagland's medium containing 0, 20, 40, 60, 80, 100 microM PbCl2, respectively, for 4 weeks. Chloroplasts were assayed for overproduction of reactive oxygen species (ROS) such as superoxide radicals (O2(*-)) and hydrogen peoxide (H2O2) and of lipid peroxide (malonyldialdehyde) and for activities of the antioxidant enzymes such as superoxide dismutase, catalase, ascorbate peroxidase, and guaiacol peroxidase and glutathione content, oxygen-evolving rate, and chlorophyll content. Increase in both ROS and lipid peroxide content and reduction in photosynthesis and activities of the antioxidant defense system indicated that spinach chloroplast underwent a stress condition due to an oxidative attack. Seedling growth cultivated in containing Pb2+ media was significantly inhibited. The results imply that spinach chloroplast was not able to tolerate the oxidative stress induced by Pb2+ due to having no effective antioxidant defense mechanism.

Linolenic acid is an inhibitor of electron transport in chloroplasts of higher plants. It has obvious effects on the structure and function of chloroplasts. In the present paper, we investigated the nano-anatase relieving the inhibition of photoreduction activity and oxygen evolution caused by linolenic acid in spinach chloroplasts. The results showed that linolenic acid in various concentrations could obviously reduce the whole chain electron transport and the photoreduction activity of two photosystems, especially on the oxidative reside and reduce reside of photosystem II (PS II). After adding nano-anatase to chloroplasts treated by linolenic acid, the whole chain electron transport rate, the photoreduction activity of two photosystems, and the oxygen evolution rate were increased significantly, indicating that nano-anatase could obviously decrease the inhibition of linolenic acid on the electron transport, photoreduction activity, and oxygen evolution of spinach chloroplasts.

Lead (Pb(2+)) is a well-known highly toxic element. The mechanisms of the Pb(2+) toxicity are not well understood for nitrogen metabolism of higher plants. In this paper, we studied the effects of various concentrations of PbCl(2) on the nitrogen metabolism of growing spinach. The experimental results showed that Pb(2+) treatments significantly decreased the nitrate nitrogen (NO(-)(3)-N) absorption and inhibited the activities of nitrate reductase, glutamate dehydrogenase, glutamine synthase, and glutamic-pyruvic transaminase of spinach, and inhibited the synthesis of organic nitrogen compounds such as protein and chlorophyll. However, Pb(2+) treatments increased the accumulation of ammonium nitrogen NH(+)(4)-N)in spinach cell. It implied that Pb(2+) could inhibit inorganic nitrogen to be translated into organic nitrogen in spinach, thus led to the reduction in spinach growth.

In the article, we report that effects of nano-anatase on the spectral characteristics and content of light-harvesting complex II (LHCII) on the thylakoid membranes of spinach were investigated. The results showed that nano-anatase treatment could increase LHCII content on the thylakoid membranes of spinach and the trimer of LHCII; nano-anatase could enter the spinach chloroplasts and bind to PSII. Meanwhile, spectroscopy assays indicated that the absorption intensity of LHCII from nano-anatase-treated spinach was obviously increased in the red and the blue region, fluorescence quantum yield near 685 nm of LHCII was enhanced, the fluorescence excitation intensity near 440 and 480 nm of LHCII significantly rose and F 480/F 440 ratio was reduced. Oxygen evolution rate of PSII was greatly improved. Together, nano-anatase promoted energy transferring from chlorophyll (chl) b and carotenoid to chl a, and nano-anatase TiO2 was photosensitized by chl of LHCII, which led to enhance the efficiency of absorbing, transferring, and converting light energy.

Being a proven photocatalyst, nano-anatase is capable of undergoing electron transfer reactions under light. In previous studies we had proven that nano-anatase improved photosynthesis and greatly promoted spinach growth. The mechanisms by which nano-anatase promotes energy transfer and the conversion efficiency of the process are still not clearly understood. In the present paper, we report the results obtained with the photosystem II (PSII) isolated from spinach and treated by nano-anatase TiO2 and studied the effect of nano-anatase TiO2 on energy transfer in PSII by spectroscopy and on oxygen evolution. The results showed that nano-anatase TiO2 treatment at a suitable concentration could significantly change PSII microenvironment and increase absorbance for visible light, improve energy transfer among amino acids within PSII protein complex, and accelerate energy transport from tyrosine residue to chlorophyll a. The photochemical activity of PSII (fluorescence quantum yield) and its oxygen-evolving rate were enhanced by nano-anatase TiO2. This is viewed as evidence that nano-anatase TiO2 can promote energy transfer and oxygen evolution in PSII of spinach.

The effects of nano-anatase TiO2 on light absorption, distribution, and conversion, and photoreduction activities of spinach chloroplast were studied by spectroscopy. Several effects of nano-anatase TiO2 were observed: (1) the absorption peak intensity of the chloroplast was obviously increased in red and blue region, the ratio of the Soret band and Q band was higher than that of the control; (2) the great enhancement of fluorescence quantum yield near 680 nm of the chloroplast was observed, the quantum yield under excitation wavelength of 480 nm was higher than the excitation wavelength of 440 nm; (3) the excitation peak intensity near 440 and 480 nm of the chloroplast significantly rose under emission wavelength of 680 nm, and F 480 / F 440 ratio was reduced; (4) when emission wavelength was at 720 nm, the excitation peaks near 650 and 680 nm were obviously raised, and F 650 / F 680 ratio rose; (5) the rate of whole chain electron transport, photochemical activities of PSII DCPIP photoreduction and oxygen evolution were greatly improved, but the photoreduction activities of PSI were a little changed. Together, the studies of the experiments showed that nano-anatase TiO2 could increase absorption of light on spinach chloroplast and promote excitation energy to be absorbed by LHCII and transferred to PSII and improve excitation energy from PSI to be transferred to PSII, thus, promote the conversion from light energy to electron energy and accelerate electron transport, water photolysis, and oxygen evolution.