Added Cerium Relieves Photosynthetic Inhibition Of Maize Seedlings Caused By Exposure To Manganese Deficiency

Since rare earths resources are rich in China, rare earth elements （REEs） have beenused large-scaled in agriculture since1970s. Recently, researches about REEsapplication for agricultures in China have become a hot subject for scientific study.Lanthanum （La）, cerium （Ce）, and neodymium （Nd） significantly promoted theabsorption and utilization of light energy in plants, as well as the qualities and quantitiesof crops. However, the complex of photosynthesis, which has not been fully completedyet, makes it hard to study details of itself or the biological effects and mechanisms ofREEs to photosynthesis of plants. Moreover, as a result of mineral distrubitions,climatic conditons and abuse of fertilizers, corps might be faced to various stresses, oneof which is manganese （Mn） deficient stress. For example, approximately30%ofcutivatied soils are Mn deficient. As a microelement, Mn plays several important rolesin physiological metabolisms, especially directly involed in photosynthesis. Accordingto many researches, Mn deficiency restricted plant growth, photosynthetic ability andstructure of thylaoid membrane. Thus, the academic dissertation focused on the transferand transform of light energy in photosynthesis, and conducted series of experiments tostudy effects of added Ce on alleviating photosynthesis stressed by Mn deficiency, asscientific evidences of REEs utilization in agiculture.Three main aspects are involved:（1） Maize seedlings were cultivated in the Hoagland’s solution. They weresubjected to Mn deficiency and to20μmol·L^-1CeCl3solution administered in theMn-deficient Hoagland’s media. After30days, the changes of seedlings morphologyand their effects of photosynthetic abilities and chlorophyll contents were observed. Theresults showed that Mn deficient seedlings showed tipical symptoms: undersized plants, chlorosis between veins of young leaves, chlorophyll contents decreased, as well as theutilization of light energy, and rates of oxygen evolution and photophosphorylation.However, added Ce³⁺alleviated the inhibition of photochemical reactions. Moreover,with Dual-PAM, added Ce³⁺was proved to be effective in accelerating the electrontransport of two photosystems, especially PSâ…¡; the photoprotection ability wasimproved significantly, suggesting that Ce³⁺promoted light absorption, and excitationenergy distribution to PSâ…¡.（2） Added Ce³⁺improved the capablity CO2assimilation of Mn deficient maizeseedlings. Mn deficiency was found to inhibit activities of key enzymes, includingRubisco and Rubisco activase, and added Ce³⁺prevented the inhibition. And thepossible mechanisms may involve two aspects: firstly, with the RT-PCR and Real-timePCR analysis, mRNA expression of rca, rbsL, rbsS were promoted significantly, thusmaking the protein levels of Rubisco and Rubisco activases increased. Secondly, as anATPase, the activation of Rubisco catalysed by Rubisco activase is closely associatedwith ATP, the main product of photosynthesis. Added Ce³⁺was proved to be able toimprove activities of Mg2+-ATPase and Ca2+-ATPase, therefore it could increase CO2assimilation activities in the enzyme level.（3） Nitrogen （N） assimilation is anther basic reaction for plant growth except forphotosynthesis. Effects of Mn deficiency and added Ce to N assimilation of seedlingswere explored. It was been found that activities of some enzymes, including nitratereductase （NR）, glutamate dehydrogenase （GDH）, glutamine synthetase （GS）,glutamic-oxaloacetic transaminase （GOT）, were inhibited, as well as the synthesis of Nassimilation products. However, added Ce³⁺decreased the inhibition of N assimilationof Mn deficient seedlings, and levels of chlorophyll and total soluble protein wereincreased significantly.