Effects of nitrogen application rates on tuber yield, quality and plant physiological activities of Helianthus tuberosus L.

Objectives The study was designed to investigate the effects of nitrogen (N) application rates on tuber yield, quality and plant physiological activities of Helianthus tuberosus L. and identify the key indicators based on the partial least square model (PLS).

Methods Field experiments were conducted separately in Yuanyang County in 2019 and Fangcheng County in 2020, Henan Province, China. The tested N application rates were 0, 60, 120, 180, 240, and 300 kg/hm² in 2019, and 0, 90, 180, 270, and 360 kg/hm² in 2020. At the early, middle, and late vegetative and flowering periods, plant samples were collected for the chemical analysis of plant N content, leaf chlorogenic acid content, leaf SPAD value, soluble sugar, soluble protein, lignin and cellulose content. Meanwhile, the tuber yield, inulin and reducing sugar content were analyzed at maturing stage. The PLS model was employed to quantitatively describe the relationship among the above-mentioned indices and identify the main controlling factors of Helianthus tuberosus L. tuber yield inulin, and reducing sugar.

Results 1) The tuber yield of Helianthus tuberosus L. increased with increasing N application rates in “linear + platforms trends”. The optimal N application rate was 232 and 270 kg/hm² in 2019 and 2020. 2) The inulin and reducing sugar content in Helianthus tuberosus L. tubers stopped increasing when the N application rate reached 300 and 270 kg/hm². Compared with the N0 treatment, N application increased inulin and reducing sugar content by 19.4% and 47.3% on average in 2 years, respectively. 3) The plant N accumulation, leaf chlorogenic acid content, leaf SPAD value, soluble sugar, soluble protein and cellulose content increased with increasing N application rates except for the leaf lignin content. 4) The PLS models between the indicators and the tuber yield, inulin and reducing sugar content showed a good correlation, with R² values of 0.648, 0.618, and 0.629, and the relative percentage deviation (RPD) values of 1.676, 1.638, and 1.659, respectively. Moreover, the variable importance for projection (VIP) values calculated with the PLS models developed showed that the main controlling factors of tuber yield, reducing sugar and inulin content of Helianthus tuberosus L. were the leaf SPAD value and the leaf chlorogenic acid content, respectively.

Conclusions N fertilizer application could improve tuber yield and quality and enhance the physiological and biochemical indicators of Helianthus tuberosus L. The appropriate N application rate for Helianthus tuberosus L. for improved growth, tuber yield, and quality in the experimental area was N 230–270 kg/hm².