Spatial structure and relationships of nitrogen balance index and protein content of grain in winter wheat

Abstract: Wheat is one of three major food crops in China, not only it is a source of calories, but also an important source of protein. With the improvement of people's living standards, the demand for high-quality food products also rose rapidly. However, the number of high-quality wheat was far from being able to meet domestic demand with largely dependence on foreign imports. The development of large-scale production of high-quality wheat became the trend of China's wheat production. Grain protein content was an important quality character in wheat, which was closely related to nitrogen absorption, transport and assimilation. Many studies showed that the status of leaf nitrogen content had significant correlation with grain protein content, thus rapid nitrogen diagnosis had great significance for guiding crop production. Multiplex is the latest hand-held optical fluorescence sensor introduced to the non-destructive measurement of various parameter representatives of plant nitrogen status. This instrument generates fluorescence in the plant tissues using light sources (LED) generating four wavelengths: UV_A (375 nm, UV); blue (450 nm, B); green (530 nm, G); and red (630 nm, R). The present research focused on the winter wheat of the National Experimental Station for Precision Agriculture of China. Based on griding sampling, nitrogen balance index (NBI) was conducted with Multiplex 3 from jointing stage to middle and late filling stages. The spatial structure between winter wheat stress fluorescent parameter nitrogen balance index (NBI) and grain protein content were analyzed using geostatistical methods, and their semivariogram was developed. Correlation coefficient was chosen as the indicator to reflect the intimate level of linear correlation between nitrogen balance index (NBI) and grain protein content. The method of contour lines of indicator value used in this article was to analyze the relationships between grain protein content and combined nitrogen balance index (NBI) obtained from four developmental stages. This method had always been used for reference in spatial relationship between one variable and several variables or that between one variable and dynamic distributions of the other one. The outcome showed that the nitrogen balance index (NBI) and the data of winter wheat grain protein content were approximately normally distributed; Variables had good regionalized variable characteristics and spatial structures; Grain protein content was significantly correlated to NBI at each developmental stage. Among them, nitrogen balance index (NBI) at the middle and late filling stage was the most sensitive variable for monitoring grain protein content with correlation coefficient（r）of 0.828; Large enough nitrogen balance index (NBI) existing for a long time was beneficial to forming higher grain protein content. Correspondingly, large enough nitrogen balance index (NBI) existing for a short time often led to forming lower grain protein content; The spatial distribution of combined nitrogen balance index (CNBI) and grain protein content had similarity, so nitrogen balance index(NBI) could be used to predict grain protein content. These results provided a theoretical basis and technical approach for crop nitrogen management and grain protein content prediction.