Magnesium improved fruit quality by regulating photosynthetic nitrogen use efficiency, carbon-nitrogen metabolism, and anthocyanin biosynthesis in 'Red Fuji' apple.

Introduction: Both nitrogen (N) and magnesium (Mg) play important roles in biochemical and physiological processes in plants. However, the application of excessive N and insufficient Mg may be the factor leading to low nitrogen utilization rate (NUE) and fruit quality degradation in apple production.
Methods: In this study, we analyzed the effects of different application rates of Mg (0, 50, 100, 150, 200 kg/ha) on the photosynthetic nitrogen use efficiency (PNUE), the accumulation and distribution of carbon (C), N metabolism, anthocyanin biosynthesis and fruit quality of the 'Red Fuji' apple in 2018 and 2019.
Results: The results showed that the application of Mg significantly increased the ¹⁵ NUE and increased the allocation rate of ¹⁵ N in the leaves whereas the ¹⁵ N allocation rate in the perennial organs and fruits was decreased. With the increase in Mg supply, the activities of N metabolism enzymes (NiR, GS, and GOGAT) were significantly promoted and the content of intermediate products in N metabolism ( NO 2 - , NH 4 + , and free amino acid) was significantly decreased. Furthermore, an appropriate rate of Mg significantly promoted the net photosynthetic rate (P _n ) and photosynthetic nitrogen use efficiency (PNUE), enhanced the enzyme activities of C metabolism (SS, SPS, S6PDH), and increased the contents of sorbitol and sucrose in leaves. In addition, Mg upregulated the gene expression of sugar transporters ( MdSOT1 , MdSOT3 , MdSUT1 , and MdSUT4 ) in fruit stalk and fruit fresh; ¹³ C isotope tracer technology also showed that Mg significantly increased the ¹³ C allocation in the fruits. Mg also significantly increased the expression of anthocyanin biosynthesis genes ( MdCHS and MdF3H ) and transcription factors ( MdMYB1 and MdbZIP44 ) and the content of anthocyanin in apple peel.
Conclusion: The comprehensive analysis showed that the appropriate application of Mg (150 kg/ha) promoted PNUE, C-N metabolism, and anthocyanin biosynthesis in apple trees.
Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
(Copyright © 2023 Tian, Qin, Liu, Xing, Feng, Xu, Liu, Lyu, Jiang, Zhu, Jiang and Ge.)