Changes of biomass, Mg content, total N, free amino acid, and theanine content in tea plants under different Mg application levels. A) Phenotype of tea plants treated with Mg deficiency (−Mg) and Mg application (Mg1 and Mg2, full-strength nutrient solution). The images were digitally extracted for comparison. B) Biomass, C) Mg content, D) total N content, E) amino acid content, and F) theanine content in mature leaf (ML), root, young/new shoot (YS), and stem of tea plants treated with Mg deficiency (−Mg), Mg concentration in the full-strength nutrient solution (Mg1, 0.4 mmol/L), and 2 times the Mg concentration in the full-strength nutrient solution (Mg2). Individual images of plants were digitally extracted for comparison. Error bars show the mean values ± SD (n = 3). Statistical analysis was carried out with Duncan's multiple range test. Different letters indicate significant differences (P < 0.05). DW, dry weight; FW, fresh weight; ns, not significant.