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-year-old Newhall navel orange plants grafted onto trifoliate orange [Poncirus trifoliate(L.)] rootstock.The plants were grown in sand ∶perlite(1 ∶1,v/v) medium and irrigated with deionized water for 24 days to stimulate new root growth.Plants were then irrigated every other day for two months with half-strength Hoagland′s nutrient solution containing three Mg concentrations: 0 mmol/L(Mg-deficiency,DM),0.1 mmol/L(low-Mg,LM) or 1 mmol/L(control) MgSO4.After Mg deficient treatment for two months,a portable chlorophyll meter(SPAD-502) was used to determine the relative amount of leaf total chlorophyll content at different foliar ages.Simultaneously,using a Pulse-Amplitude-Modulation(PAM-2500) fluorometer,the maximum quantum efficiency of photochemistry in photosystem II in the dark(Fv/Fm) was calculated for the three treatments at different foliar ages.In addition,the rapid light response curves of the leaves were determined in situ with a PAM-2500 fluorometer to obtain the chlorophyll fluorescence characteristic parameters,including the effective quantum yield of photosystem II(ΔFv/Fm),photochemical quenching(qP),non-photochemical quenching(qN) and relative electron transport rate(rETR).Results from recent studies showed that in low-Mg and Mg-deficient treatments,SPAD readings and Fv/Fm in last year′s autumn flush leaves increased remarkably with the rising of leaf position,whereas those in this year′s spring flush leaves were not statistically significant among leaf positions.SPAD readings,Fv/Fm and light response ability(ΔFv/Fm,qP,rETR) decreased as the levels of Mg reduced,but qN increased.For chlorophyll fluorescence characteristics of last year′s autumn flush leaves,the Mg-deficient treatment was remarkably different compared with the control,whereas there was no significant difference between the low-Mg treatment and the control.For chlorophyll fluorescence characteristics of this year′s spring flush leaves,no significant differences were observed among the low-Mg,Mg-deficiency and control treatments.These findings suggested that Mg deficiency stress caused obvious decrement of photosynthetic capacity in last year′s autumn flush leaves,and reduction of photosynthetic capacity in this year′s spring flush leaves.In addition,short-term Mg deficiency would have a greater adverse effect on last year′s autumn flush leaves depending on the degree of Mg deficiency stress.Therefore,under high irradiance in summer,Mg deficiency would make last year′s autumn flush leaves of Newhall navel orange sensitive to photoinhibition.Severe Mg deficiency could even cause light-induced damage leading to the occurrence of premature senility(e.g.chlorosis).