Objectives Effects of reduced nitrogen application on growth and medicinal quality of Isatis indigotica Fort., was explored for providing some basis for optimizing the application of nitrogen fertilizer in cultivation.

Methods A field experiment was carried out using the I. indigotica cultivars from Shanxi (SX) and Gansu (GS) as tested materials. Taking the current N application rate of 675 kg/hm² as base, nitrogen application levels 0, 169, 338 and 675 kg/hm² were designed and recorded as CK, 1/4N, 1/2N, N, respectively. The growth indicators, nutrient contents, active ingredient contents and the effective economic yields (Isatidis Folium and Isatidis Radix) were investigated.

Results Both the fresh and dry biomass of Shanxi showed an increasing trend with the increase of nitrogen level, and reached a maximum at the normal nitrogen treatment, in which the fresh and dry weight of the plants were respectively 68.4 g, 15.3 g; While the fresh and dry biomass of GS showed a trend of increasing first and then decreasing with the increase of nitrogen levels, the maximum were obtained at the 1/2N treatment, which were 78.5 g and 19.7 g, respectively. The application level of nitrogen was positively correlated with the root length, plant height, and main root diameter of I. indigotica. There was no significant difference in the drying rates among the treatments. The CK had the highest leaf soluble sugar contents and the carbon-nitrogen ratios, and the N treatment had the highest free amino acid contents, and there was no significant difference between Shanxi and Gansu. The contents of indigo, indirubin and total flavonoids in leaves and (R, S)-epigoitrin in the roots of both Shanxi and Gansu were the highest under the CK treatment, and had decreasing trends with the increase of nitrogen application levels. Gansu was more sensitive to nitrogen levels. The nitrogen application level significantly affected the effective economic yield of I. Folium and Radix. The effective economic yield of I. Folium under 1/4N and 1/2N treatments were significantly higher than that under the N treatment. The effective economic yield of I. Radix of Shanxi was positively correlated with nitrogen level, with the maximum of 3.31 mg/g under N treatment, and that of Gansu was under the 1/4N treatment (2.94 mg/g).

Conclusions Current N application rate of 675 kg/hm² has a certain adverse effect on the growth and shape quality of I. indigotica, but beneficial to the accumulation of active ingredients. Reduced nitrogen application rate is helpful to improve the effective economic yield.AS the response difference of I. indigotica. to nitrogen in Shanxi and Gansu fields, the application level of nitrogen should be controlled at 169−338 kg/hm², according to the actual situation of the field.