Magnesium alleviates growth inhibition under low potassium by enhancing photosynthesis and carbon-nitrogen metabolism in apple plants

Potassium (K) and magnesium (Mg) play analogous roles in regulating plant photosynthesis and carbon and nitrogen (C–N) metabolism. Based on this consensus, we hypothesize that appropriate Mg supplementation may alleviate growth inhibition under low K stress. We monitored morphological, physiological, and molecular changes in G935 apple plants under different K (0.1 and 6 mmol L⁻¹) and Mg supply (3 and 6 mmol L⁻¹) conditions. Low K stress caused changes in root and leaf structure, inhibited photosynthesis, and limited the root growth of the apple rootstock. Further study on Mg supplementation showed that it could promote the uptake of K⁺ and NO₃⁻ by upregulating the expression of K⁺ transporter proteins such as Arabidopsis K⁺ transporter 1 (MdAKT1), high-affinity K⁺ transporter 1 (MdHKT1), and potassium transporter 5 (MdPT5) and nitrate transporters such as nitrate transporter 1.1/1.2/2.1/2.4 (MdNRT 1.1/1.2/2.1/2.4). Mg promoted the translocation of ¹⁵N from roots to leaves and enhanced photosynthetic N utilization efficiency (PNUE) by increasing the proportion of photosynthetic N and alleviating photosynthetic restrictions. Furthermore, Mg supplementation improved the synthesis of photosynthates by enhancing the activities of sugar-metabolizing enzymes (Rubisco, SS, SPS, S6PDH). Mg also facilitated the transport of sucrose and sorbitol from leaves to roots by upregulating the expression of sucrose transporter 1.1/1.2/4.1/4.2 (MdSUT 1.1/1.2/4.1/4.2) and sorbitol transporter 1.1/1.2 (MdSOT 1.1/1.2). Overall, Mg effectively alleviated growth inhibition in apple rootstock plants under low K stress by facilitating the uptake of N and K uptake, optimizing nitrogen partitioning, enhancing nitrogen use efficiency (NUE) and PNUE, and promoting the photosynthate synthesis and translocation.