Modeling leaf color dynamic in rice plant based on spad value

Modeling leaf color dynamics in rice (Oryza sativa L.) is an important task for realizing virtual and digital plant growth. This study was undertaken to analyze the dynamics of leaf color changes at different leaf positions on main stems and tillers under different growing conditions, and to build a dynamic simulation model on leaf color changes in rice plant in relation to growing degree-days (GDD). Time-course observations were made on leaf color (in SPAD value) changes at different leaf positions of stem and tillers under different nitrogen rates and water regimes with four rice cultivars. Leaf color changes in SPAD could be described in three phases. The first phase during leaf elongation period was based on the exponential relationship of leaf color to cumulative GDD; the second phase during leaf function period was represented with a relative stable SPAD; the third phase during leaf senescence period was described in a quadratic equation between SPAD and GDD. In addition, the effects of nitrogen and water conditions on leaf color were quantified through the effectiveness values of leaf nitrogen concentration and water content in relation to SPAD. Then, the RGB (red, green, and blue) values were further predicted from the changing SPAD during leaf development. The model was validated with the independent field experiment data involving different rice cultivars and nitrogen rates. The average root mean square errors (RMSE) between the simulated and observed SPAD values at different leaf positions were 2.58, 3.69 and 3.82, respectively, for three leaf color phases on main stem; 4.65, 4.39, 3.51 and 4.25, respectively, for four individual tillers; and 2.98 and 3.25, respectively, for SPAD and R, G values in rice. The results indicate that the present model has a good performance in predicting leaf color changes at different leaf positions in rice under various growth conditions, and thus lays a foundation for further constructing digital and visual rice growth sys- - tem.