Role of calcium as a possible regulator of growth and nitrate nitrogen metabolism in apple dwarf rootstock seedlings

• Published in: Scientia horticulturae 2021-01, Vol.276, p.109740, Article 109740
• Main Authors: Xing, Yue, Zhu, Zhan-Ling, Wang, Fen, Zhang, Xin, Li, Bing-Yu, Liu, Zhao-Xia, Wu, Xiao-Xian, Ge, Shun-Feng, Jiang, Yuan-Mao
• Format: Article
• Language: English
• Subjects: Apple seedlings; Calcium; Growth; Nitrate metabolism; NUE
• ISSN: 0304-4238; 1879-1018
• DOI: 10.1016/j.scienta.2020.109740

•Ca treatments enhanced Ca concentrations in apple seedlings.•Seedlings with 6 mM Ca2+ showed higher biomass and stronger root system.•Exogenous application of Ca affected nitrogen absorption and assimilation.•The optimum Ca treatments showed higher 15NUE.•Ca deficiency had greater negative effect than those of excessive Ca supply. As a crucial element for plants, calcium (Ca) is involved in both nitrogen (N) absorption and assimilation. Plants tend to exhibit lower nitrogen use efficiency (NUE) under Ca-deficient conditions. Improving NUE in apple production can reduce the negative effects of the excessive use of N fertilizer. However, the role of Ca in the regulation of nitrate uptake and reduction remains unclear. Herein, we investigated the growth response and nitrate (NO3−) metabolism of apple dwarf rootstock seedlings (M9T337) for Ca2+ concentrations of 0, 2, 4, 6, 8, 10, and 12 mM Ca2+ using 15N isotope labeling tracer, non-invasive micro-testing technology, and quantitative real-time polymerase chain reaction (qrt-PCR). Results showed that Ca accumulation rate in the plant tissue increased with increasing Ca supply levels; however, the plant biomass, photosynthesis, root activity, and 15NUE peaked for 6 mM of Ca2+ supply. Further investigation revealed that for 6 mM of Ca2+ supply, nitrate reductase (NR) activities were relatively high, the transcription of nitrate transporter (MdNRT1.1; MdNRT2.1) up-regulated and NO3− maximum influx rate in the roots occurred. Furthermore, seedlings treated with 6 mM Ca2+ had a significantly lower NO3- concentration in leaves and roots, a higher NH4+ concentration in leaves, and a higher amino acid compounds concentration. Moreover, both Ca deficiency and excessive Ca inhibit N absorption and utilization, and the adverse effects of Ca deficiency on seedling growth and N metabolism were greater than those associated with excessive Ca2+ supply. Conclusively, the results of this study indicate that appropriate Ca2+ supply (6 mM) was optimal as it increased NUE by enhanced photosynthesis, N metabolizing enzyme activities, NO3− uptake and transport.