Advances in magnesium nutritional status and its mechanisms of physiological and molecule in citrus

Abstract: Citrus is a globally important cash crop and leading fruit crop, and has become the largest fruit production and the largest juice drink in the world. In the past decades, the planting area, productivity, and consumption of Citrus have grown strongly. China is one of the countries with the most abundant citrus germplasm resources, and the annual productivity of citrus has leaped to the first place in the world. In 2017, the planting area and fruit yield of citrus in 19 provinces of China were 2.7×106 hm² and3.8×107 t, respectively, and the export volume and foreign exchange of citrus are reached 5.6×105 t and8.4×108 USD (China Statistical Yearbook 2018). Thus, the stable and sustainable development of the citrus industry is not only related to the balance of international trade between income and expenditure,but also closely related to the poverty alleviation in citrus production counties and the income of growers in citrus growers in China. Magnesium(Mg), a secondary essential mineral nutrient, is a component of various structural substances in plants, and plays a critical role in numerous physiological and biochemical processes of plant growth and development. Meanwhile, Mg is also an essential nutrient for human health. Mg deficiency in citrus not only significantly affects plant growth and fruit quality formation, but also significantly relates to the effective acquisition of Mg nutrition in the human body with citrus fruit and beverages as a non-staple food or daily diet. In recent years, it has been neglected by researchers, agriculture practitioners and fertilizer industry members, who did not concern Mg deficiency in soils and plants, which make Mg has become a‘forgotten'nutrient element in intensive agricultural production. In the production of citrus, due to poor soil quality conditions, excessive application of chemical fertilizers and neglect of supplementation of organic manure fertilizers and trace elements such as Mg-contained fertilizer. Mg deficiency in soil, that consequently resulted in the decline of fruit yield and quality, which is a common phenomenon in the main citrus producing areas of China. It is one of the important factors restricting the sustainable development of citrus industry, and has become a hotspot issue of sustainable citrus production in China and even in the world. Therefore, it is of great theoretical and practical significance to comprehensively analyze the status of Mg nutrition in soil-plant systems of main citrus producing areas, to clarify the advance in physiological and molecular mechanisms of Mg nutrition in citrus plants, and to understand the integrated nutrient management including Mg nutrient in citrus production. In present paper, we mainly focused on two aspects, both the current situation of Mg nutrition in citrus production, and the research progress of physiology and molecule biology of Mg nutrition in citrus plants. The current status and the main causes of Mg nutrition in soil and plant in citrus production were systematically summarized. According to the classification standard of soil Mg content, the soil exchangeable-Mg content less than 80-120(150) mg/kg is deficient, however, the soil Mg content of citrus garden in different provinces exist obviously differences with a higher proportion of deficient situations, which navel orange orchard in southern Jiangxi province and honey pomelo orchard in Pinghe county of Fujian province are the most seriously deficient in Mg. Meanwhile, our also illustrates the typical symptoms of Mg deficiency in leaves of‘Guanximiyou'pomelo in Pinghe county, Fujian province, which pointed out that the Mg deficiency chlorosis of leaves mostly occurs in the old leaves or near-base lower leaves with fruit, indicates that there is an internal relationship between source(fruit) and sink(leaf) on Mg deficiency symptom based on the Mg mobility and reutilization in plants. At the same time, combining the related research in the last nearly 10 years with our researcher teams, mainly from plant growth and mineral element content, photosynthesis and carbohydrate transport, enzyme activity and antioxidant metabolism, organic acid metabolism, protein metabolism and magnesium transporter protein, and gene expression and microRNA transcription regulation and so on aspects were introduced the research progress of physiological and molecule mechanisms of Mg nutrition in citrus plants. Furthermore, the systematic revelation the physiological and molecular mechanisms of Mg nutrition by the multi-omics collaborative research methods, the characteristics of soil microbial community structures on Mg nutrition supply and its potential analysis and research fields of regulating Mg nutrition in citrus plants or production were discussed. Additionally, the research and application of integrated nutrient management, including development of Mg non-destructive diagnosis technology in plants, rational application of Mg-contained fertilizers, and nutrient resource management, for improving fruit yield and quality while increasing fertilizer use efficiency to achieve green development of citrus industry were also putting forward.