Comparative Physiological Analysis Of Two Citrus Scion Cultivars (Citrus Sinensis Cvs.Newhall And Langfeng) Grafted On Poncirus Trifoliata Under Magnesium Deficient Condition

Citrus is one of the most important fruits in China even the whole world.Magnesium（Mg）is indispensable for plant growth and development,while Mg deficiency is frequently observed in many citrus-producing areas with acid soil in Southern China.Chemical fertilizers are usually used to address this problem,but it is known neither economical nor environmental friendly.Therefore,the negative impact of Mg deficiency on plant growth and development as well as on fruit yield and quality has largely limited the sustainable development of citrus industry in Southern China.Selecting or breeding cultivars or rootstocks tolerant to Mg deficiency is known an effective strategy to alleviate the Mg deficiency symptom.‘Langfeng’（LF,Citrus sinensis（L.）Osbeck）,a spontaneous bud mutant of Washington navel orange,showed strong tolerant to Mg deficiency in acid soil field condition,whereas the Newhall navel orange（NHE,Citrus sinensis（L.）Osbeck）is sensitive to Mg deficiency in the same field condition.In this study,the LF and NHE grafted on Trifoliate orange（TO,Poncirus trifoliata（L.）Raf.）rootstock grown in greenhouse were used to investigate the physiological difference between these two scion-rootstock combinations（LF/TO and NHE/TO）under Mg-deficient condition.The main results of the present study are as follows:Effect of Mg-deficiency on plant nutrient status and plant biomassThe Mg²⁺concentrations in all plant parts（leaf,stem and root）of LF/TO and NHE/TO generally were significantly decreased upon Mg deficiency treatment（0 m M MgSO₄）compared with control（1m M MgSO₄）,and significantly higher concentrations of Mg²⁺were observed in LF/TO as compared to NHE/TO regardless of the treatments.By contrast,under the Mg deficiency treatment,the concentrations of Ca,K,Zn,and Mn were increased in all the plant parts of NHE/TO whereas decreased in LF/TO,leading to a lower concentration of these mineral elements in LF/TO than NHE/TO.Mg deficiency treatment significantly decreased the root and shoot growth of NHE/TO,but no effect was observed on LF/TO.Overall,compared to NHE/TO,the LF/TO plants exhibited higher dry weight in various plant parts under both control and Mg-deficient conditions.Effect of Mg-deficiency on chlorophyll,leaf gas exchange,and fluorescence parametersUnder Mg deficiency treatment,LF/TO plants showed higher concentrations of Chl a,Chl b,Chl a+b,Chl a/b,and carotenoid（Car）contents in upper and lower leaves as compared to NHE/TO plants.Overall,higher concentrations of Chlorophyll parameters were observed in the upper leaves and lower leaves of LF/TO plants as compared to NHE/TO plants.At 0 m M MgSO₄ treatment,the higher levels of CO₂ assimilation,transpiration rate,stomatal conductance,water use efficiency were observed in upper and lower leaves of LF/TO plants as compared to NHE/TO plants,while higher levels of intercellular CO₂ concentration and Ci/Ca were observed in the upper and lower leaves of NHE/TO.The F_v/F_m,F_v/F₀ ratios,the fraction of OEC,PI_tot.abs levels were significantly higher in the upper and lower leaves of LF/TO as compared to NHE/TO under both control and Mg-deficient conditions,while the F₀/F_m ratio,DI₀/RC,and DI₀/ABS levels were higher in NHE/TO as compared to LF/TO under Mg-deficient conditions.Effect of Mg-deficiency on non-structural carbohydrates,antioxidant activity and relative gene expression of Mg²⁺transportersAt 0 m M MgSO₄treatment,it was observed that leaf non-structural carbohydrates levels were significantly higher in the upper and lower leaves of NHE/TO as compared to LF/TO.However,under Mg-deficient treatment,roots non-structural carbohydrates were significantly higher in the LF/TO plants compared with the NHE/TO.The antioxidant activity（SOD,POD,MDA,H₂O₂,APX,GPX,GR,MG,MDHAR,DHAR）of NHE/TO plants parts（leaves and roots）were significantly higher in NHE/TO plants as compared to LF/TO plants under 0 m M MgSO₄treatment.In this study relative expression of eight Mg²⁺transporter genes（Ptr MGT1,Ptr MG2,Ptr MGT3,Ptr MGT4,Ptr MGT5 Ptr MGT6,Ptr MGT7 and Ptr MGT8）was observed investigated.In upper leaves,LF/TO plants showed higher relative genes expression levels of Ptr MGT3,Ptr MG4,Ptr MGT7,and Ptr MGT8 under Mg-deficient conditions,while in lower leaves,Ptr MGT1,Ptr MG2,Ptr MGT3,Ptr MGT4,Ptr MGT6,Ptr MGT7 and Ptr MGT8showed higher relative gene expression levels under the same treatment as compared to NHE/TO plants.In roots,at 0 m M MgSO₄ the Ptr MGT1,Ptr MG2,Ptr MGT3,Ptr MGT4,Ptr MGT5 Ptr MGT6,Ptr MGT7 and Ptr MGT8 showed significantly higher expression levels in LF/TO plants as compared to NHE/TO plants.In conclusion,the present study reports that LF plants grafted onto TO rootstock performed better growth than the NHE plants grafted onto TO rootstock under Mg deficient conditions.LF/TO plants exhibited better performance possibly by promoting Mg uptake from soil and/or improving the distribution of mineral nutrients among different plant parts,thus increased CO₂ assimilation and photosynthesis,optimized carbohydrate distribution,and increased plant biomass,leading to a phenotype that tolerant to Mg deficiency.