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

Abstract 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.