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Magnesium (Mg) , an essential nutritional element for citrus growth and development, plays an important role in maintaining chloroplast structure, activating a variety of enzymes and improving crop yield and quality. In recent years, with the increased rate of NPK fertilizer application and the decreased rate of manure application, many Newhall navel orange orchards in Gannan in Jiangxi Province, the south of Hunan Province and the north of Guangxi Province have suffered leaf chlorosis due to Mg deficiency. This has become one of the important factors in the South region impeding the yield and quality of citrus fruit. The purpose of the present study is to explore the influence of Mg deficiency stress on chlorophyll synthesis and the fluorescence characteristics of Newhall navel orange (Citrus sinensis Osbeck) at different foliar ages. The experimental materials were one鄄year鄄old Newhall navel orange plants grafted onto trifoliate orange [ Poncirus trifoliate (L.) ] rootstock. The plants were grown in sand 颐perlite (1颐1, v / v) medium and irrigated with deionized water for days to stimulate new root growth. Plants were then irrigated every other day for two months with half鄄strength Hoagland忆 s nutrient solution containing three Mg concentrations: 0 mmol / L (Mg鄄deficiency, DM) , 0. 1 mmol / L (low鄄Mg, LM) or 1 mmol / L (control) MgSO 4. After Mg deficient treatment for two months, a portable chlorophyll meter (SPAD鄄502) was used to determine the relative amount of leaf total chlorophyll content at different foliar ages. Simultaneously, using a Pulse鄄 Amplitude鄄Modulation (PAM鄄2500) fluorometer, the maximum quantum efficiency of photochemistry in photosystem II in

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