Differences in grain nitrogen contents of high-yielding wheat cultivars and relation to NPK uptake and utilization in drylands

Objectives It is of great importance to clarify the variation of grain nitrogen (N) content and the related main agronomical and nutritional constraining factors for the variation. The research will provide theoretical supporting for improving wheat grain yield and nutritional quality through breeding and regulating fertilizer application.

Methods Field experiments were conducted in the typical dryland area of the Loess Plateau from 2013 to 2016, 123 wheat cultivars were collected from the main wheat production areas of China. No fertilization control and treatment of applying N 150 kg/hm²、P₂O₅ 100 kg/hm² were setup. The yields were investigated and the grain N, P and K contents were determined after harvest. Those with yields higher than average were defined as high-yielding cultivars, of them, the top ten and last ten cultivars in grain N content were chosen as high-N and low-N group respectively. The relationship of grain N contents with dry matter accumulation, yield components and the uptake and utilization of N, P and K were calculated.

Results A significant negative correlation was found between the grain yield and grain N content in all the tested wheat cultivars, and for every 1000 kg/hm² increase of grain yield, the grain N content was decreased by 1.1 g/kg. For the cultivars at the same high-yielding level, the grain N contents also showed significant differences. The average grain N content of the high-N group was 24.2 g/kg, 24.7% higher than that of the low-N group (19.4 g/kg). Significant differences between the high-N and low-N groups were also observed in the grain yield, biomass and yield components, and the high-N group exhibited higher responses to fertilization, with grain yield increased by 70.0%, biomass by 60.2%, harvest index by 9.8%, spike number by 51.6% and grain number per spike by 14.3%, respectively. The vegetative part N content of the high-N group showed a 150.0% increase after fertilization, while P and K contents decreased significantly. Under the condition of sufficient soil nutrient, vegetative part N contents of the high-N group was always higher than those of the low-N group, the P and K contents showed no significant difference between the two groups, and the increase of K content was greater than that of P content after fertilization. The aboveground part N and P uptake of the high-N group was higher than that of the low-N group, especially in grain, but grain K uptake of the high-N group was significantly lower than that of the low-N group whether fertilizer was applied or not. When fertilizer was applied, the aboveground part N, P and K uptakes of the high-N group were increased much more than those of the low-N group, and the corresponding increase of vegetative part was more than that of grain. No significant differences were found in the N and P harvest indexes between the two cultivar groups, while the K harvest index averaged over three years for the high-N group was significantly lower than that for the low-N group.

Conclusions Under conditions of sufficient nutrient supply in drylands, the differences of grain yield, spike number, grain number per spike and thousand grain weight were not significant between the high-N and low-N wheat cultivars at high-yielding levels, while the high-N cultivars were more sensitive to fertilization. The high-yielding cultivars showed significant differences in the grain N content, and the high-N cultivars were observed with high grain P but low grain K contents. After fertilization, the N contents of high-N cultivars were increased, but the P and K contents were decreased. The high-N cultivars possessed higher N, P and K uptakes, but not superior in the shoot N, P and K translocation to grains than the low-N cultivars. Therefore, more efforts should be made to enhance the N, P and K harvest indexes and their translocation to grains in future wheat breeding, and attention should be paid to optimize fertilizer input and promote the crop N, P and K uptake and utilization, to realize simultaneously the increase of grain N, P and K contents and yields.