Foliar magnesium chloride application enhances rice yield by improving inferior-grain filling and starch-synthesis-related enzyme activities with limited transcriptional changes

Context Magnesium (Mg) fertilization can improve crop productivity, but the physiological basis of foliar Mg effects on rice grain filling remains insufficiently understood, particularly for inferior grains (IG), which are a major limitation to yield formation in large-panicle cultivars. Objective This study investigated the effects of foliar MgCl₂·6H₂O application on rice yield and grain filling, with particular emphasis on the physiological and molecular responses of IG. Materials and methods A two-year field experiment (2023–2024) was conducted in the Erhai Lake Basin using a randomized complete block design with four treatments: a distilled-water control and foliar application of 1.0%, 2.0%, or 3.0% (w/v) MgCl 2 ·6 H 2 O. Grain yield and yield components were determined at maturity. Starch-synthesis-related enzyme activities were measured in superior grains (SG) and IG during grain filling, and developing grains collected 18 days after the final foliar application were used for metabolomic and transcriptomic analyses. Results Foliar MgCl₂·6H₂O application increased grain yield by 8.18%–11.77% and was accompanied by higher seed-setting rate (SSR) and thousand-grain weight (TGW), with the grain-weight response occurring predominantly in IG. In IG, the treatment was associated with changes in starch and sucrose metabolism, the tricarboxylic acid cycle, and amino acid metabolism, together with higher activities of key starch-synthesis-related enzymes during grain filling. By contrast, comparatively few differentially expressed genes were detected, and the transcript abundance of most major starch-biosynthesis genes remained unchanged. Conclusion Under the present field conditions, foliar MgCl₂·6H₂O application improved rice yield primarily by alleviating the grain-filling limitation of IG. This response was expressed more strongly at the metabolic and enzymatic levels than at the transcript level, although the underlying uptake and biochemical mechanisms remain unresolved.