AUTHOR=Tian Ge , Qin Hanhan , Liu Chunling , Xing Yue , Feng Ziquan , Xu Xinxiang , Liu Jingquan , Lyu Mengxue , Jiang Han , Zhu Zhanling , Jiang Yuanmao , Ge Shunfeng 

TITLE=Magnesium improved fruit quality by regulating photosynthetic nitrogen use efficiency, carbon–nitrogen metabolism, and anthocyanin biosynthesis in ‘Red Fuji’ apple

JOURNAL=Frontiers in Plant Science

VOLUME=14

YEAR=2023

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2023.1136179

DOI=10.3389/fpls.2023.1136179

ISSN=1664-462X

ABSTRACT=<sec><title>Introduction</title><p>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.</p></sec><sec><title>Methods</title><p>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.</p></sec><sec><title>Results</title><p>The results showed that the application of Mg significantly increased the <sup>15</sup>NUE and increased the allocation rate of <sup>15</sup>N in the leaves whereas the <sup>15</sup>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 (<inline-formula><mml:math display="inline" id="im1" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msubsup><mml:mrow><mml:mtext>NO</mml:mtext></mml:mrow><mml:mn>2</mml:mn><mml:mo>−</mml:mo></mml:msubsup></mml:mrow></mml:math></inline-formula>, <inline-formula><mml:math display="inline" id="im2" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msubsup><mml:mrow><mml:mtext>NH</mml:mtext></mml:mrow><mml:mn>4</mml:mn><mml:mo>+</mml:mo></mml:msubsup></mml:mrow></mml:math></inline-formula>, and free amino acid) was significantly decreased. Furthermore, an appropriate rate of Mg significantly promoted the net photosynthetic rate (P<sub>n</sub>) 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 (<italic>MdSOT1</italic>, <italic>MdSOT3</italic>, <italic>MdSUT1</italic>, and <italic>MdSUT4</italic>) in fruit stalk and fruit fresh; <sup>13</sup>C isotope tracer technology also showed that Mg significantly increased the <sup>13</sup>C allocation in the fruits. Mg also significantly increased the expression of anthocyanin biosynthesis genes (<italic>MdCHS</italic> and <italic>MdF3H</italic>) and transcription factors (<italic>MdMYB1</italic> and <italic>MdbZIP44</italic>) and the content of anthocyanin in apple peel.</p></sec><sec><title>Conclusion</title><p>The comprehensive analysis showed that the appropriate application of Mg (150 kg/ha) promoted PNUE, C–N metabolism, and anthocyanin biosynthesis in apple trees.</p></sec>