The effects of nitrogen and water stresses on the nitrogen-to-protein conversion factor of winter wheat

An accurate estimation of grain protein concentration (GPC) based on the nitrogen-to-protein conversion factor (Fp) and grain nitrogen concentration is crucial for scientific research and human nutrition. The aim of this study was to evaluate the effects of nitrogen and water stresses on Fp. Field experiments consisting of 16 treatments that included the coupled stresses of nitrogen and water were conducted over 2 wheat growing seasons from 2014 to 2016. The true GPC was determined based on the total amino acid concentration, and the wheat grain nitrogen concentration was determined using the Kjeldahl method. Water stress was estimated based on the soil volumetric water content, and nitrogen stress was estimated based on the crop nitrogen concentration. Water stress, nitrogen stress, and coupled water and nitrogen stresses were used to fit dynamic Fp (Fp'), which was different from common Fp value of 5.70. The results indicated that stresses resulted in the decline of Fp. The nitrogen concentration of total amino acid had a positive relationship with the minimum of the two stresses (SMIN). The observed Fp' ranged from 5.14 to 5.86, and a certain significant negative linear relationship existed between SMIN and Fp' (P < 0.001). The relative root mean square error (RRMSE) values of Fp' and GPC based on an Fp' were 0.024 and 0.072, respectively, while the RRMSE of GPC estimated based on an Fp value of 5.70 was 0.098. GPC estimated using a variable Fp value that accounted for coupled water and nitrogen stresses was more precise than GPC estimates based on Fp value of 5.70. The new general linear model provides an improved method to calculate GPC more accurately in different environment.