Responses Of Reactive Oxygen Species And Methylglyoxal Metabolisms To Magnesium- Deficiency In The Roots,Upper And Lower Leaves Of Citrus Sinensis

In China,Mg-deficiency is one of the most prevalent physiological disrders leading to the decline of crop yield and quality,and it is becoming an unfavorable factor restricting the sustainable development of Citrus industry.In Mg-starved plants,Mg remobilization from the old leaves to the young leaves was elevated,thus providing Mg for the normal growth and development of young leaves.Therefore,the Mg-deficiency-induced alterations of photosynthesis,chlorophyll?Chl?a fluorecence,and reactive oxygen species?ROS?and methylglyoxal?MG?metablisms should be greater in the lower leaves than those in the upper leaves.Evidence shows that Mg-deficient effects on anatomy,gas exchange,and carbohydrate,amino acid,protein,organic acid and phenolic metabolisms are different between roots and leaves.Thus,the responses of ROS and MG metabolisms to Mg-deficiency should be different between roots and leaves.So far,such data are very limited.Seedlings of‘Xuegan'?Citrus sinensis?were supplied every two days with nutrient solution at a concentration of 0?Mg-deficiency?or 2 mM?Mg-sufficency,control?Mg?NO₃?₂ for 16 weeks by sand culture method.Therefore,the Mg-deficiency-induced alterations of growth,gas exchange and Chl a fluorescence in the lower?quarter height,¹1-week-old?and upper?three quarter height, 11-week-old?leaves,and Mg,superoxide anion and H₂O₂ production rates,electrolyte leakage,malondialdehyde?MDA?,MG,antioxidant metabolites,and enzymes involved in ROS and MG detoxification in the roots,lower and upper leaves were investigated.The specific objectives were to corroborate the hypothesis that the Mg-deficiency-induced alterations of ROS and MG metabolisms were greater in the lower leaves than those in the upper leaves,and different between the leaves and roots.The main results are as follows:1.In addition to inhibiting growth,Mg-deficiency led to typical Mg-deficient symptoms in the lower leaves?leaf chlorosis?,but not in the upper leaves.2.Mg level was higher in the Mg-deficient upper leaves than that in the Mg-deficient lower leaves.This might be responsible for the Mg-deficiency-induced larger alterations of all the measured parameters in the lower leaves than those in the upper leaves,but they showed similar change patterns between the Mg-deficient lower and upper leaves.Accordingly,Mg-deficiency increased greatly their differences between the lower and upper leaves.3.Although the Mg-deficiency-induced decrease of Mg level was greater in the leaves than that in the roots,most of parameters involved in ROS and MG metabolisms had similar variation trends and degrees between the Mg-deficient lower leaves and roots.Further analysis showed that several parameters?namely glutathione S-transferase,sulfite reductase,ascorbate and dehydroascorbate?displayed the opposite variation trends between the Mg-deficient lower leaves and roots.Obviously,some differences existed in the Mg-deficiency-induced alterations of ROS and MG metabolisms between the lower leaves and roots.4.Although the activities of most antioxidant and sulfur metabolism-related enzymes and glyoxalase I and the level of reduced glutathione?GSH?in the Mg-deficient leaves and roots and the level of ascorbate in the leaves were kept in higher levels,the levels of malonaldehyde and MG and/or electrolyte leakage were increased in the Mg-deficient lower and upper leaves and roots,especially in the Mg-deficient lower leaves and roots.Obviously,the ROS and MG detoxification systems as a whole did not provide sufficient detoxification capacity to prevent the Mg-deficiency-induced production and accumulation of ROS and MG,thus leading to lipid peroxidation and the loss of plasma membrane integrity,especially in the lower leaves and roots.