Table 1. Nitrogen source of medium used in this study.

	Nitrogen source (mM)
Medium	KNO3	KNO2	(NH4)2SO4	Glutamine
M1	―	―	3.5	―
M2	―	2	3.5	―
M3	―	―	3.5	20
M4	―	2	3.5	20
N6	28	―	3.5	―

Nitrate was deprived from the N6 medium and 3.5 mM (NH4)₂SO₄ was used as the nitrogen source in M1, M2, M3, M4 medium. 2 mM KNO₂ or 20 mM glutamine was added in these medium if necessary. The other components composed of N6 basal medium (deprive of nitrogen source), 20 mM KCl, 800 mg/L casein hydrolysate, 600 mg/L proline, 2.0 mg/L 2,4-D, 3% sucrose, and 0.3% phytagel at pH 5.8. ‘―’ indicates the component was not added. The standard N6 medium was used as a control in this experiment.