Effect Of Cerium On Photosynthesis Of Maize Seedlings Under Magnesium Deficiency

Since China is rich in rare earths resources, which accounts for 41.36% of the known world wide reserves, and how to scientifically and rationally utilize these resources has become a hot subject for scientist's study. It has been proved that rare earths could promote plant growth and development, increase crop yield and promote products quality, because of the increment of photosynthesis and enhancement of adversity resistance. Rare earths have been used large-scaled in agriculture in China since 20th century 70 years, which had acquired benefits. However, mineral element resources are not evenly distributed, e. g. soils in the southern China are deficient in magnesium (Mg). Mg is an essential macronutrient, and plays a key role in living organisms. We wonder if Lanthanoids (Ln) could substitute for Mg under Mg -deficient environment in plants. So this study focuses on the effects of Ln on plant photosynthesis to clarify the mechanism why improvement of Ln on plant photosynthesis, and provide a scientific basis for scientific and reasonable using in agriculture.Four main aspects are involved:(1) Maize plants were cultivated in Hoagland's solution. They were subjected to Mg deficiency and to cerium (Ce) administered in the Mg-deficient Hoagland's media. After 3 weeks, the changes of seedlings morphology were observed; Chlorophyll contents and photosynthetic activities were assayed. The results showed that Ce can prevents inhibition of synthesis of photosynthetic pigment, improves light energy absorption and conversion, oxygen evolution and the activity of photo-phosphorelation and its coupling factor Ca²⁺-ATPase under magnesium-deficient conditions. These results suggest that Ce could partly substitute magnesium, improving photosynthesis and plant growth.(2) Chlorophyll (Chl) fluorescence now is widespread used in photosynthetic research, because of the non-invasive and highly sensitive measured. However, the mechanism of Ce improves the photosynthesis of plants under Mg deficiency is poorly understood. The main aim of the study was to determine effects of Ce on Chl fluorescence of maize under Mg deficiency more accurately. Maize seedlings were cultivated in Hoagland's solution. They were subjected to Mg deficiency without or with Ce administrated in the Mg-deficient Hoagland's media, and then optimum quantum (Fv/Fm), effective quantum yield of PSII photochemistry (Y(II)), effective quantum yield of PSI photochemistry (Y(I)), relative rates of photosynthetic electron transport of PSII (ETR(II)) and PSI (ETR(I)) were examined. The results showed that the effects of Mg starvation on plants were multilateral: the Chl and carotenoid levels, Fv/Fm, Y(II), Y(I), ETR(II) and ETR(I) were depressed, which are the characteristics of the damages of PSII and PSI reaction centers. Mg-deficient maize leaves were highly photosensitive caused by a reduction of Chl contents, leading to a lack of photoprotection and photochemistry. However, the appropriate amount Ce addition relieved the inhibition of pigment synthesis and the oxygen evolution rate of chloroplasts, partly resumed the quantum yield of Fv/Fm, Y(II), Y(I), ETR(II), ETR(I) and improved the growth of maize plants caused by exposure to Mg deficiency.(3) Ln was demonstrated to improve Chl formation and the growth of plants. But the mechanism of the fact that Ln promotes the Chl biosynthesis of plants is poorly understood. The main aim of the study was to determine Ln effects in Chl formation of maize under Mg deficiency. Maize plants were cultivated in Hoagland's solution. They were subjected to Mg deficiency and to cerium administered in the Mg-deficient Hoagland's media, and then contents of various Chl precursors and gene expressions of key enzymes of Chl biosynthesis were examined. The decrease of Chl contents in maize leaves caused by Mg deficiency suggested an inhibition of Chl synthesis that was inhibited by a reduction of the precursors as measured by analyzing the contents ofδ-aminolevulinic acid, porphobilinogen, uroporphyrinogenⅢ, Mg-protoporphyrinⅨand protochlorophyll, as well as the expression levels of magnesium chelatase, magnesium-protoporphyrinⅨmethyltransferase and chlorophyll synthase, and Mg deficiency significantly inhibited the transformation from coproporphyrinogenⅢor protoporphyrinⅨto Chl. However, cerium addition significantly relieved the inhibition of the Chl biosynthesis in maize caused by Mg deficiency and increased Chl content, and promoted a serious of transformation fromδ-aminolevulinic acid to Chl, and maize growth under Mg deficiency. It implied that cerium might partly substitute for the role of Mg.(4) It was well known that in plants, carbon (C) and nitrogen (N) assimilation were competing for energy, but there was"cross-talking"between them. Maize plants were cultivated in Hoagland's solution. They were subjected to Mg deficiency and to cerium administered in the Mg-deficient Hoagland's media. After 3 weeks, we measured Chl a fluorescence and the activities of nitrate reductase (NR), sucrose-phosphate synthase (SPS), phosphoenolpyruvate carboxylase (PEPCase) in metabolic checkpoints coordinating primary nitrogen and carbon assimilation in leaves of maize. The results showed that Mg deficiency significantly inhibited plant growth, the activities of NR, SPS, and PEPCase, the synthesis of Chl and protein, and except for the contents of nitrate and carbohydrate products. Mg deprivation in maize also significantly decreased the oxygen evolution, electron transport, efficiency of photochemical energy conversion by photosystem II (PSII). However, Ce3+ addition could promote nitrogen and carbon assimilations increase and PSII activities, improving maize growth under Mg deficiency. Taken together, Mg deficiency inhibited the assimilations of both nitrogen and carbon, and the PSII activity, and Ce3+ addition to Mg2+-deficiency media in the maize plants could substitute Mg2+ and promote plant growth.