To investigate the effect of magnesium(Mg) deficiency and excess on chlorophyll content and chlorophyll fluorescence characteristics of leaves,2-year-old trees of Newhall navel orange(Citrus sinensis Osbeck) grafted on trifoliate orange(Poncirus trifoliata(L.) Raf.) rootstocks were transferred to pots containing a mixture of quartz sand-perlite(1∶1,v/v) and irrigated with modified Hoagland-Aron nutrient solution containing three Mg concentrations:0.1 mmol/L(Mgdeficiency),1 mmol/L(control) or 10 mmol/L MgSO4(Mg-excess).The relative chlorophyll content and chlorophyll fluorescence characteristics of leaves on different shoots were measured regularly in the three treatments.A portable chlorophyll meter(SPAD-502) was used to test the relative chlorophyll content of leaves.Simultaneously,the chlorophyll fluorescence characteristics of leaves were determined in situ by a pulse-amplitude modulated(PAM-2500) fluorometer.Results showed that Mg deficiency led to a more significant decline in chlorophyll content of 2-year-old autumn leaves thandid Mg excess,whereas the latter caused a greater decline of 1-year-old leaves than did the former.In the 4thmonth after Mg stress treatment,SPAD readings of leaves on 2-year-old autumn,1-year-old spring,1-year-old summer and 1-year-old late summer shoots in the Mg-deficient group decreased by 12.3%(P0.05),10.5%(P0.05),4.0%(P0.05) and 4.0%(P0.05),respectively,compared with the control.Those in the Mg-excess group were 107.3%(P0.05),95.5%(P 0.05),88.9%(P0.05) and 92.5%(P 0.05) of the control group,respectively.Moreover,both Mg-deficiency and excess reduced maximum quantum efficiency(Fv/Fm) and relative electron transport rate(rETR) of leaves on different shoots,with the impact of the former greater than that of the latter.In the 4thmonth,compared with the control,Fv/Fmof 2-year-old autumn,1-year-old spring,1-year-old summer and 1year-old late summer leaves in the Mg-deficient group decreased by 13.9%(P 0.05),12.6%(P 0.05),2.9%(P 0.05) and 2.0%(P 0.05),respectively.Correspondingly,there was a decline of 0.5%(P 0.05),2.2%(P 0.05),3.4%(P 0.05) and 1.5%(P 0.05),respectively,in the Mg-excess group.Further investigation showed that the rETRmaxof these leaves decreased by 35.7%(P 0.05),56.2%(P 0.05),32.6%(P 0.05) and 15.2%(P 0.05) in the Mg-deficient group,respectively,which corresponded to 110.1%(P0.05),68.8%(P0.05),87.2%(P0.05) and 84.5%(P0.05) of the control in the Mgexcess group.In addition,non-photochemical quenching(NPQ) of all leaves in the Mg-deficient group increased in the first 3 months and then declined dramatically in the 4thmonth,although this was not as marked as in the Mg-excess group.In the 4thmonth,the maximum NPQ of 2-year-old autumn,1-year-old spring,1-year-old summer and 1-year-old late summer leaves in the Mg-deficient group was lower than those in control,with a drop of 52.8%(P0.05),26.6%(P 0.05),19.8%(P0.05) and 0.6%(P 0.05),respectively.In the Mg-excess group,the peak value of NPQ of those leaves was 94.9%(P0.05),110.3%(P 0.05),101.3%(P 0.05) and 104.5%(P 0.05) of the control group,respectively.Therefore,Mg-deficient plants exposed to high light intensity in summer would be subject to a reduction in photosynthetic capacity more easily than would Mg-excess plants.This indicated that Mg-deficient leaves would be susceptible to photoinhibition and photooxidation.