A balance between calcium and nitrate promotes the growth of M9T337 apple rootstocks

Unbalanced Calcium (Ca) and nitrogen (N) supply have been a struggle for apple fruit quality and yield. Calcium ion (Ca2+) and nitrate (NO3−) are two major forms of Ca and N absorbed by most terrestrial plants. The study was for investigating the effect of Ca2+ addition (5 and 10 mM) under different NO3− conditions (0, 5, 10, 15, 20, 25 mM) on M9T337 rootstocks phenotypes and their morphological, physiological, and related-genes differences in nutrient solution hydroponics. It was revealed from the study that independent variables significantly affected the plants growth. Relatively high NO3− plus less Ca2+ disturbed by different degrees of burned-down curling of rootstock leaves and root growth arrest. And Ca addition could relieve symptom caused by low and high N. The interaction of Ca and N increase their uptake by activating nitrate reductase (NR), glutamine synthetase (GS), Ca2+-ATPase and inducing CAM content, then up-regulating MdNRT1.1, MdNRT2.1, MdCDPK, and down-regulating MdACA expressions. Moreover, under lower-N (10 mM), moderate Ca (5 mM) could ensure optimal growth and development. And under higher-N (20 mM), more Ca (10 mM) was the most optimal, which was significantly different from other Ca2+ level (P˂0.05). Ultimately, maintaining a proper balance between N (0.01966-0.0303 kg kg−1) and Ca (0.0091-0.0141 kg kg−1) in leaves is important for developing desired apple rootstocks. Collectively, our data revealed that lower-N plus optimal Ca and higher-N plus more Ca could ensure better growth and improve their uptake efficiency, which would provide a scientific basis for rational fertilization and scientific management in extensive apple orchards.