Simulating nitrogen effects on fruit branch, fruit site, square and boll formation and abscission in cotton

Nitrogen, the most important crop nutrient, has a regulatory role in crop growth. In order to support nutrient management in main morphogenetic processes of cotton, a simple model was presented to simulate the effects of nitrogen on fruit branch, fruit site, square and boll formation and abscission of cotton. A nitrogen test was carried out with six nitrogen application levels to study the quantitative relationship between nitrogen concentration in subtending leaf of lower fruit branches (NCSLL) and morphological index at flowering and boll-setting stage in cotton. Cumulative thermal and solar radiative effectiveness (TSE) integrating independent thermal and solar radiative effectiveness and their interaction was introduced as the driving variable. Results showed that the changes of NCSLL at flowering and boll-setting stage followed the equation: y=axb. The average NCSLL was approximately equal to the NCSLL of some day after anthesis and the ratio of the total number of days at flowering and boll-setting stage was stable. The dynamics of the number of fruit branches, fruit nodes, big bolls with TSE were generally best described by logistic curves, while the changes of the number of squares and young bolls followed quadratic curves. At the same time, the relationships between the model parameters of morphological indixes and the average NCSLL at flowering and boll-setting stage in cotton also followed quadratic curves. The model was validated using a data from an independent experiment with four N application rates and root mean squared error (RMSE) was used for assessing the model performance. Validation of the model resulted in RMSE values of 1.1, 2.7, 2.4, 1.6, 1.4 and 3.5% per plant, respectively for the number of fruit branches, fruit nodes, squares, young bolls, big bolls, and the abscission rate. This indicated that the simulated and observed values were inosculated well. Under different planting density conditions, the simulated and observed values were also agreed well. The study can provide references for fertilization management of cotton field.