Sweet corn (Zea mays var. saccharata) is widely recognized for its dual role as a nutritious staple food source and a popular vegetable (Singh et al., 2014). In recent years, its cultivation has expanded across China, primarily driven by growing consumer demand and its economic profitability for farmers (Xue et al., 2023). Given China’s limited arable land resources, improving sweet corn yield per unit area is essential. While increased planting density represents a crucial strategy for boosting corn yield, this practice often negatively impacts grains per ear and grain weight (Dhaliwal and Williams, 2019; Ye et al., 2023a, 2023b). This phenomenon can be attributed to two interdependent mechanisms. First, high-density planting accelerates leaf senescence, diminishing photosynthetic capacity and disrupting photosynthate allocation patterns (Lan et al., 2024). Second, restricted root elongation and reduced root-to-shoot ratios impair nutrient acquisition efficiency, intensifying intra-plant nutrient competition under dense planting (Shao et al., 2024). Consequently, developing optimal fertilization strategies to enhance grains per ear and grain weight under high planting density sweet corn cultivation remains a critical research priority.

Magnesium (Mg) is an essential nutrient that affects yield and quality formation but is often overlooked in crop cultivation (Cakmak and Kirkby, 2008). Mg enhances crop growth by improving photosynthesis (via chlorophyll stabilization), carbohydrate partitioning, and nitrogen (N) use efficiency through modulation of nitrate transporters (Farhat et al., 2016; Peng et al., 2020). It also increases resilience to abiotic stress by regulating reactive oxygen species homeostasis and activating antioxidant enzymes (Tian et al., 2021). Mg deficiency is prevalent in tropical and subtropical acidic soils, where high rainfall accelerates leaching, and crop harvesting removes soil Mg reserves (Ishfaq et al., 2022). Additionally, Chinese farmers commonly apply excessive amounts of N and potassium (K) fertilizers while neglecting Mg fertilizers. This unbalanced fertilization practice, coupled with soil Mg deficiency, exacerbates the antagonistic interaction between Mg²⁺ and other competing cations, further limiting Mg availability (Fageria, 2001; Gransee and Führs, 2013; Wang et al., 2020). Southeast China, a major sweet corn cultivation region, faces significant issues of soil Mg deficiency and imbalanced fertilization management (Ishfaq et al., 2022). As a result, Mg deficiency is a critical limiting factor for increasing sweet corn yields in Southeast China.

Mg fertilizer application serves as a critical agronomic practice for mitigating Mg deficiency and increasing yield (Wang et al., 2020). For instance, soil-applied Mg fertilizer significantly increases field corn yield and Mg accumulation in northeast China (Zhang et al., 2020). Meanwhile, soil application of Mg fertilizer has little effect on increasing yield under non-adverse conditions (Grzebisz, 2013). In contrast, foliar Mg fertilization offers distinct advantages over soil application. This method enables rapid nutrient uptake by evading soil constraints such as cation antagonism and restricted root proliferation, directly delivering Mg through foliar tissues (Grzebisz, 2013; Xie et al., 2021; Niu et al., 2021). Moreover, foliar spraying ensures timely delivery of Mg during critical growth stages, boosting photosynthesis, enhancing field corn resistance to stress, and ultimately improving yield (Rodrigues et al., 2021). Nevertheless, previous studies indicate proper foliar Mg fertilizer rates vary with crop types and agroecological regions (Kibria et al., 2021). Consequently, establishing quantifiable relationships between foliar Mg dosage and sweet corn yield remains imperative. However, research on sweet corn in acidic soils under field conditions in China remains limited.

Nutrient uptake fundamentally governs corn yield, with optimized fertilization strategies enhancing both nutrient uptake and yield potential (Ye et al., 2023a; Bindraban et al., 2015). While elucidating Mg uptake is critical for rational Mg fertilization, substantial genotypic variation in Mg absorption persists among field corn genotypes despite documented yield and nutrient accumulation improvements through Mg supplementation (Ciampitti and Vyn, 2013; El-Dissoky et al., 2017; Zhang et al., 2020). This knowledge gap necessitates quantitative characterization of Mg uptake patterns in sweet corn to inform precision Mg management. In addition, grain weight is one of the most crucial yield components, and grain carbohydrates play important roles in grain weight formation (Paponov et al., 2005). Recent evidence indicates that dense planting maintains grain carbohydrate concentration but reduces grain weight in sweet corn (Ye et al., 2023b). Although Mg application improves grain weight in wheat and field corn (Zhang et al., 2021a; Ceylan et al., 2016), its effects on grain weight and carbohydrates in sweet corn still require further research.

Mg deficiency is common for sweet corn cultivation in Southeast China, and dense planting deteriorates sweet corn yield and quality traits. However, little information focuses on whether foliar Mg spraying can alleviate the impact of high density on sweet corn. We assumed that Mg application can increase the yield of sweet corn under high planting by improving nutrient absorption and sugar synthesis. Therefore, this study aims to investigate the effect of foliar spraying Mg on fresh ear yield, Mg accumulation and allocation, grain filling, and carbohydrate concentration of sweet corn under field conditions in Southeast China, providing a basis for the scientific application of Mg fertilizer.

Abiotic stress, such as dense planting, has significantly contributed to yield loss in corn, as it reduces the number of grains per ear and grain weight (Ye et al., 2023b). This stress leads to nutritional competition, grain abortion, and lodging, all of which impede yield potential (Sher et al., 2017; Shah et al., 2021). Strategic nutrient management, particularly Mg supplementation, becomes critical under high planting conditions, given Mg’s central role in enzyme activation, energy transfer, and nutrient metabolism (Tian et al., 2021). The urgency for Mg optimization is heightened in southern China’s acidic soils, where 55% of arable lands exhibit Mg deficiency-a key constraint for high-yield cropping systems (Ishfaq et al., 2022). In field corn production in Brazil, foliar application of only 0.5–1 kg ha^-1 Mg has been shown to significantly increase both Mg concentration and grain yield (Altarugio et al., 2017; Rodrigues et al., 2021). Nevertheless, foliar applying 3.5 kg ha^-1 Mg significantly increases organ Mg concentration and yield in wheat (Kibria et al., 2021). These inconsistent reports indicate that the effects of foliar spraying Mg depend on crop type and environmental condition, so it is necessary and important to note the effect of reasonable foliar Mg on sweet corn in Southeast China. In this study, foliar spraying 8.78 kg ha^-1 Mg significantly increased sweet corn fresh ear yield, grains per ear, and grain weight. Szulc et al. (2011) also found that foliar application of 6 kg ha^-1 Mg significantly field corn yield and grain weight. This can be attributed to several factors: first, Mg enhances the photosynthetic efficiency by optimizing photosystem activity and improving carbon fixation rates. Additionally, it plays a critical role in activating enzymes involved in starch synthesis, thereby enhancing carbohydrate metabolism and contributing to yield formation (Cakmak and Kirkby, 2008; Ceylan et al., 2016; Ba et al., 2020). Second, Mg improves abiotic stress tolerance to by boosting antioxidant enzyme activity and delaying leaf senescence, particularly under high-density planting conditions (Rodrigues et al., 2021). It also improves cell membrane thermostability and pollen viability, both of which are vital for maintaining cellular integrity and reproductive success, leading to increased grain weight and yield (Raza et al., 2023). Third, Mg promotes root growth and upregulates the nitrate transporter genes expression, thereby increasing N uptake, assimilation, and use efficiency, which further contributes to increased yield (Peng et al., 2020; Lyu et al., 2023). In this study, sweet corn fresh ear yield was significantly positively correlated with harvest index but not biomass, indicating that the ability to allocate biomass to the ear plays a more significant role in yield formation than total biomass accumulation. This finding is consistent with previous research of Djaman et al. (2013). Notably, grains per ear, grain weight, and abortion rate exhibited season-specific responses, likely influenced by temperature and precipitation during grain-filling. Further multi-year trials under varying environmental conditions are necessary to confirm these observations and gain a deeper understanding of the underlying mechanisms.

Nutrient uptake exhibits a significant positive correlation with crop yield, predominantly regulated by fertilization strategies (Setiyono et al., 2010). In this study, foliar spraying Mg application increased N, P, K, Ca, and Mg accumulation of sweet corn, aligning with prior reports observations of Mg-induced nutrient enrichment in field corn (Zhang et al., 2021a; El-Dissoky et al., 2017). It may be due to the contribution of Mg to chlorophyll formation and photosynthesis, producing more dry matter, thereby driving more N uptake (Peng et al., 2010; Ahmed et al., 2020). Additionally, Mg can increase root length and dry weight, acquiring more nutrients (Kong et al., 2020). In particular, foliar Mg fertilizer significantly increased Mg and N concentration in the organs of sweet corn but did not affect P, K, and Ca concentrations in this study. Similar results are demonstrated under field corn in the Mediterranean region (Ortas, 2018). However, Ahmed et al. (2020) highlighted that Mg supplementation significantly decreased K concentration, although improving Mg and N concentrations of field corn in calcareous soils. These discrepancies suggest that foliar Mg circumvents root-level Mg-K/Ca antagonism and soil Mg-P precipitation (Grzebisz, 2013; Xie et al., 2021), highlighting its precision in dense planting systems. Moreover, the synergistic Mg-N relationship is mechanistically driven by two factors: (1) Mg activation of Mg²⁺-ATPase and glutamine synthetase, which catalyzes NH₄ ⁺ assimilation (Jezek et al., 2015a); and (2) Mg-induced upregulation of N transporter genes, enhancing N uptake and utilization efficiency (Peng et al., 2020). Unlike soil application, which supports a long-term nutrient supply, foliar fertilization can only provide a limited quantity of nutrients and is sensitive to environmental factors (Ssemugenze et al., 2025). However, it offers a rapid method for nutrient delivery, especially during critical growth stages, by bypassing soil limitations such as low nutrient availability and ion antagonism (Niu et al., 2021). Therefore, combining foliar and soil application of Mg fertilization may ensure immediate and long-term availability, ultimately improving corn growth and yield.

Understanding Mg’s dynamic accumulation and distribution characteristics is an essential foundation for the scientific application of Mg fertilizer (Chen et al., 2016). Mg accumulation of sweet corn was an average of 16.9 kg ha^-1 and 21.1% of total Mg absorbed after silking in the current study. Similar results were reported in field corn: a total accumulation of approximately 33 kg ha^-1 Mg, with 23%-30% accumulating after silking (Ciampitti and Vyn, 2013). However, Zhang et al. (2020) documented that the total Mg accumulation was over 120 kg ha^-1,and 44.2% of Mg accumulation was obtained at the reproductive stage of field corn in Northeast China. This may be due to differences in variety, cultivation practices, and environmental conditions. In addition, in this study, sweet corn Mg remobilization from vegetative organs was < 5% of total Mg accumulation, and contribution of Mg remobilization to ear was < 10%. This is inconsistent with the results showing that Mg’s remobilization efficiency from stalk was 23%, while the leaf Mg remobilization efficiency was -36% in field corn (Chen et al., 2016). However, Mg remobilization from the stalk happens 30 days after silking in field corn (Chen et al., 2016), and sweet corn ears are harvested 27 days after silking. This indicates that the Mg requirement of the ear in sweet corn primarily relies on absorption after silking, rather than on translocation from vegetative organs. The limited Mg transfer to the ear may be attributed to restricted sink capacity, which hinders Mg mobilization from the stems and leaves to the ear (Szczepaniak et al., 2016). Furthermore, genetic differences among cultivars significantly influence Mg transport efficiency, with low Mg transport efficiency in sweet corn limiting the extent of Mg translocation (Kovacevic et al., 2004). However, the approach used to study Mg remobilization in our research has certain limitations. Future work should employ isotope tracing techniques to accurately quantify Mg remobilization and its contribution to the ear. Furthermore, the Mg uptake rate from V12 to the silking stage was the biggest during different growing stages in sweet corn, which agreed with the previous study’s finding (Zhang et al., 2020). Therefore, V12 - silking are the critical stages for Mg fertilizer application. These results provide a clear understanding of Mg uptake and remobilization dynamics in sweet corn, particularly during the post-silking period. These findings suggest that foliar application of Mg (total 8.78 kg Mg ha^-1) during the mid-to-late growth stages is an effective strategy to meet Mg demand and optimize grain filling. This provides a practical and scientific foundation to improve Mg fertilization efficiency in sweet corn cultivation.

Grain filling is a process of grain accumulation and storing photosynthetic products, a critical stage affecting yield (Li et al., 2020). Grain filling rate has a significant positive correlation with yield and grain dry weight (Ye et al., 2023b), similar results were shown in this study. Dense planting usually reduces grain filling rate (Ye et al., 2023b); however, foliar spraying Mg significantly sweet corn leaf SPAD and grain filling rate in the early stage of grain filling under close planting in this study. One explanation is that Mg application can delay leaf senescence and produce more photosynthetic products during grain filling, subsequently transferring more photosynthetic products to the grain, increasing the grain filling rate (Jezek et al., 2015b; Ning et al., 2018; Hauer-Jákli and Tränkner, 2019). While our findings suggest that improving the grain filling rate under high-density planting is critical for yield enhancement, the underlying physiological and molecular mechanisms remain unclear and require further investigation.

Grain carbohydrates, including sucrose, fructose, glucose, and starch, are vital components that determine grain weight and yield. Hence, understanding the dynamic changes of carbohydrate components is an important way to improve crop yield and quality (Ye et al., 2023b; Paponov et al., 2005). Sucrose, fructose, and starch concentration gradually increased as the grain-filling progresses, which is agreed with the previous literature (Paponov et al., 2005). However, glucose concentration was first increased and then decreased in this study. It may be due to glucose transfer to another form of carbohydrates during sweet corn grain filling. In the current investigation, Mg significantly increased grain carbohydrates (sucrose, fructose, and starch) concentration. Ba et al. (2020) also stated that foliar application of Mg fertilizer enhanced grain carbohydrate concentration in wheat. This may be attributed to Mg’s role in facilitating long-distance carbohydrate transport (Jiao et al., 2023), enhancing the activity of key enzymes and gene expression involved in carbohydrate metabolism (Ba et al., 2020; He et al., 2024a, 2024b). Mg also promotes the synthesis of abscisic acid (ABA) and the activity of catalase (CAT), which are known to regulate grain carbohydrate accumulation (He et al., 2024a). These mechanisms highlight how Mg influences carbohydrate transport and metabolism, ultimately contributing to improved carbohydrate availability for grain development. Carbohydrate metabolism plays a critical role in determining yield and grain quality in sweet corn. However, the mechanisms that foliar Mg regulates carbohydrate dynamics, particularly its effects on key enzymes and transcriptional networks involved in starch and sucrose biosynthesis, remain unclear. Future studies should focus on elucidating how Mg regulates carbohydrate metabolism both at the biochemical and transcriptional levels, with a particular emphasis on rate-limiting enzymes and associated gene pathways involved in carbohydrate accumulation (Tahir et al., 2020; Shen et al., 2022). Although conventional planting density treatments (45,000 plants ha⁻¹) were not included in the current study, future investigations should implement multi-density field trials to mechanistically assess how planting density regulates the Mg uptake and translocation process.

Foliar spraying of 4% Mg significantly increased sweet corn fresh ear yield, grains per ear, and grain weight under 60,000 plants ha⁻¹ over three seasons in 2021 and 2022. It also markedly increased dry matter, N, K, Ca, and Mg accumulation, as well as the concentrations of sucrose, fructose, and glucose in the grains during the spring and autumn growing seasons of 2022. Therefore, foliar spraying of 4% MgSO₄·7H₂O three times (total 8.78 kg Mg ha^-1) can increase yield, nutrient uptake, and grain sugar concentrations, making it a recommended method for Mg application in sweet corn. This finding establishes a scientifically optimized Mg fertilization protocol for intensive sweet corn cultivation, demonstrating that targeted foliar supplementation can mitigate nutrient competition in high-density cropping.