The Relationship Between The Unique Physico-chemical Property Of Rare Earth Elements And Photochemical Reaction In Photosynthesis

Rare Earth Elements(REEs)belongs to theШB family in the periodic table of elements. And the most typical elements wisedly used in agriculture among them are Lanthaum(La),Cerium(Ce) and Neodymium (Nd) which have different 4f electron characteristic and alternation valence. And the electron of 4f shell can result in a series of light, magnetism and other catalysis effects.It was proved that a low concentration of REEs could improve plant growth and enhances the crop yield, however, the reason why REEs have such physiological effect still remain unclear, particularly, the effects of REEs on the photosynthesis, including the linkage between the physiological effects and the 4f electron characteristic and alternation valence of REEs, the effects of REEs on photosystemâ…¡(PSâ…¡)and light-harvesting complex (LHCâ…¡), etc. In the dissertation, we studied effects of La, Ce and Nd on the photochemical reaction of photosynthesis in spinach and arabidopsis. Five main aspects are involved.1. Chloroplasts were isolated from spinach treated by LaCl3, CeCl3 and NdCl3. Because of owning 4f electron characteristics and alternation valence, Ce³⁺ treatment presented the highest enhancement in light absorption, energy transfer from LHCâ…¡to PSâ…¡, excitation energy distribution from PSâ… to PSâ…¡, and fluorescence quantum yield around 680 nm. Compared with Ce³⁺ treatment, Nd³⁺ treatment resulted in relatively lower enhancement in these physiological indexes, for Nd³⁺ didn't have alternation valence. La treatment presented the lowest enhancement, as La³⁺ didn't have either 4 f electron or alternation valence. The increase in activities of whole chain electron transport, PSâ…¡DCPIP photoreduction and oxygen evolution of chloroplast was of the following order: Ce³⁺>Nd³⁺>La³⁺>control. However, the photoreduction activities of spinach PSâ… almost didn't change with La³⁺, Ce³⁺, or Nd³⁺ treatments. The results suggested that 4f electron characteristics and alternation valence of REEs had a close relation with photosynthesis improvement.2. Due to 4f electron characteristics and alternation valence, cerium involved in an oxidation–reduction reaction in plant, closely relating to photosynthesis. We studied the effects of Ce³⁺ on energy transfer and oxygen evolution in photosystemâ…¡(PSâ…¡) isolated from spinach. The results showed that Ce³⁺ could suitably change PSâ…¡microenvironment, increase the absorbance of visible light, improve the energy transfer among amino acids within PSâ…¡protein-pigment complex, and accelerate energy transport from tyrosine residue to chlorophyll a. In summary, the photochemical activity of PSâ…¡(fluorescence quantum yield) and its oxygen evolving rate were enhanced by Ce³⁺.3. The mechanism of absorbing and transferring light energy in arabidopsis chloroplasts with REEs treatments was studied. The results showed that REEs could induce Lhcâ…¡b of arabidopsis expressed largely and significantly increase LHCâ…¡content on the thylakoid membranes and the trimer of LHCâ…¡. Meanwhile, spectroscopy assays indicated that the absorption intensity of LHCâ…¡was obviously increased in red and blue region, fluorescence quantum yield near 685 nm of LHCâ…¡was enhanced, oxygen evolution rate of PSâ…¡was greatly improved. It suggested that REEs 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. Among three rare earths, the result of Ce³⁺ treatment was the best, and then Nd³⁺, La³⁺ treatment is not as good as other two elements. The reason is Ce, Nd have 4f electron characteristic, Ce even has alternation valence and La has neither 4f electron nor alternation valence.4. Linolenic acid has great effects on the structure and function of chloroplast. The function of Ce³⁺ on the improvement of chloroplast photoreduction activity and oxygen evolution damaged by linoleic acid in spinach were studied. Results showed that linoleic acid in various concentrations could inhibit the whole chain electron transport rate and the photoreduction activity of the two photosystems significantly, especially to the photoreduction activity and oxygen evolution of PSâ…¡reducing side and oxidative side. Adding Ce³⁺ to the linoleic acid treated chloroplast could greatly reduce the inhibition linoleic acid exerted on the electron transport of chloroplast, photoreduction activity and oxygen evolution, which indicated that Ce³⁺ had the ability to relieve the inhibition of photochemical reaction of chloroplast caused by linoleic acid to some extent.