Crop models are vital for trait assessment and breeding, yet most lack mechanistic detail in carbon and nutrient partition and its effects on growth and yield. The generic organ arbitrator for biomass partitioning recently was developed, but it has NOT been widely validated apart from the initial testing. The study aimed to assess the organ arbitrator for simulating leaf and tiller numbers, leaf size and leaf area, biomass and nitrogen (N) partition into different organs, and the distributions of root length, root biomass and N in soil profile. We conducted the field experiment with two cultivars of winter wheat under three irrigation treatments in water-limited areas of the North China Plain from 2016 to 2018. The monitoring metrics were employed to assess the performance of APSIM next generation Wheat model (APSIM NG). The findings revealed that original APSIM NG overestimated tiller numbers, leaf area index (LAI), shoot and root biomass and nitrogen (N), but underestimated grain yield, with the Nash Sutcliffe Efficiency (NSE) ranging from -12.26–0.92. Modification to leaf, root growth parameters and temperature response curves of thermal time led to improved simulation of the development and growth of winter wheat. The simulation for individual leaf size was optimal (coefficient of determination (R²) = 0.95, NSE = 0.81). Similarly, the simulations for tiller density (except at the recovery stage) and for LAI from recovery to jointing also performed best, with Root Mean Square Error (RMSE) values of 302.91 tillers/m² and 1.83 m²/m², respectively. The biomass in above-ground, stems, grains of winter wheat under different water treatments, providing further confidence for the model to be used for trait evaluations (RMSE ranging from 38.04 to 123.85 g/m2, NSE ranging from 0.83 to 0.96). However, the modified model tended to overestimate partitioning to leaves and underestimate that to stems and spikes. In addition, the modifications also overestimated crop N uptake (with RMSE values of 11.09, 10.16, and 1.36 g/m² for the N content in above-ground biomass, leaves, and spikes, respectively). The simulation of root biomass and N and their distributions in the soil profile was good, except for the underestimation of root biomass, length and N in the top soil layer. The study highlights the potential value of improved APSIM NG model to target phenotype, offering potential targets for genotype selecting in water-limited conditions. Further improvement in the model components in N uptake and root growth in the top soil layer is still required for the APSIM NG.
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