## 1 Introduction  
Magnesium is one of the essential nutrient elements for normal plant growth and development and numerous key functions in plants [1]. It is characterized as a macronutrient, since it is required in relatively high concentrations (0.15-0.35% Mg dry weight; d.w.) in order to achieve maximum growth of different plant organs [2]. Magnesium is a structural element of the chlorophylll molecule; it activates more than 300 enzymes and contributes to the stabilization of certain subcellular structures (organelles), such as ribosomes [3, 4]. Moreover, it is found to play an important role in plant photosynthesis, carbohydrate transport,nucleic acid and protein synthesis, and also in the generation of reactive oxygen species (ROS) [5; 6; 7; 8; 9]. Since Mg is extremely mobile within the plant and its remobilization occurs from older leaves to younger ones, acute Mg deficiency causes interveinal chlorosis of older and fully developed leaves, due to the crucial functions of Mg in chlorophyll and protein production [2; 10]. Mg deficiency is especially noted in seeded _Citrus_ cultivars, because of the translocation of Mg to the seeds [11]. Some specific symptoms of Mg deficiency in _Citrus_ species are yellow blotches along the midrib on mature leaves, which spreads until the pattern covers most of the leaf, apart from a characteristic delta-shaped dark green area at the base [11]. Several studies have noted the connection between Mg nutrition and plant growth in a variety of higher plants during the past few decades [5; 12; 13; 14; 15; 16]. Worldwide, Mg deficiency is frequently observed in many agricultural crops, including citrus. Interestingly, according to an investigation, up to 83% of the soils from 152 citrus orchards in a province of China were rated as low in Mg [17]. Mg nutrition affects crop productivity and quality [4; 15; 16]; therefore, Mg increased the quality in oranges and improved the flavor, soluble solids, sugar, and vitamin C content in grapefruit, in comparison with fruit from Mg-deficient trees [18]. Mg deficiency affects the uptake of nutrients and their concentrations in stems, leaves, and roots [19; 20]. Under Mg deficiency conditions, there is a reduction in photosynthesis and protein synthesis, resulting in disorder of the normal function and structure of plant cells, tissues, and organs [2; 17; 21; 22]. Numerous higher plants, including _Citrus_, have been reported to present decreased leaf CO\({}_{2}\) absorption in response to Mg shortage [17; 22]. Considering the antagonistic effects of an unbalanced supply of cationic nutrients (K, NH\({}_{4}\), Ca, Mg), these deficiencies in the plant's physiology may also simply be connected to nutritional imbalances, such as K, Ca, and Mn imbalances, in plants [1]. Some of the most significant evergreen fruit trees are those belonging to the _Citrus_ genus. Magnesium shortage is a common physiological condition that affects citrus plants and results in a decrease in the rate of photosynthetic activity, yellowing, and senescence of the leaves [17; 23], all of which have a negative impact on crop yield and quality [7; 20; 24; 25; 26; 27; 28; 29; 30]. Due to soil acidification [31] and producers' overuse of N, P, and K chemical fertilizers [27; 32], Mg shortage is now a common occurrence in orchards. Other competing cations, including Ca\({}^{2+}\), H\({}^{+}\), NH\({}_{4}\)\({}^{+}\), and Al\({}^{3+}\), also had a similar antagonistic effect [7]. There is insufficient information on the differential responses of various _Citrus_ genotypes when exposed to Mg deficiency, and very few reports on citrus scion-rootstock combinations under Mg-poor conditions [33]. On the other hand, extra Mg supply enhances citrus trees' biochemical and physiological responses to alleviate the harmful effects of unfavorable environmental conditions, such as excessive heat and irradiation [34].