Physiological traits such as increased kernel number, early flowering, and an increased rate and duration of kernel filling, are thought to increase the potential yield of wheat in certain environments. A crop simulation model (APSIM-NWHEAT) was used to evaluate the role of these traits in improving potential yield under a range of environmental conditions. APSIM-NWHEAT calculates the potential yield for a specific environment limited only by temperature, solar radiation, water and nitrogen supply. The model predicted crop production, soil water and N dynamics for low and high-yielding environments in Australia, The Netherlands and China. Applying the model to the various traits showed that a 20% increase in kernel number, early anthesis and a 20% increase in kernel filling rate had no or only minor effects on increasing potential yield in high- and extremely high-yielding environments. In these environments, when a maximum specific kernel size was imposed, a yield increase resulted mainly from a 20% extension in the duration of kernel filling, suggesting that the crop is usually source limited. In a low-yielding environment, each trait, except early flowering showed a small positive effect on average on increasing potential yield. Early flowering reduced the potential yield on a low water-holding capacity soil. The effect of the various traits on potential yield was also very variable in a low-yielding environment, varying with both seasonal rainfall and soil type, and all traits, except the increased rate of kernel filling, reduced potential yields in some seasons. The effect of the various traits on yield in the low-yielding environment depended on biomass accumulation at anthesis and water availability during kernel filling. When all the traits were simulated together, the relative increase in potential yield was of similar magnitude in both low- and high-yielding environments. The analysis suggested that an increase in the potential yield from 9 to 22% can be expected from selection for the above physiological traits, but selection in the appropriate environment is important. (C) 2002 Elsevier Science B.V. All rights reserved.