Sirius: a mechanistic model of wheat response to environmental variation

Sirius is a wheat simulation model that calculates biomass production from intercepted photosynthetically active radiation (PAR) and grain growth from simple partitioning rules. Leaf area index (LAI) is developed from a thermal time sub-model. Phenological development is calculated from the mainstem leaf appearance rate and final leaf number, with the latter determined by responses to daylength and vernalisation. Effects of water and N deficits are calculated through their influences on LAI development and radiation-use efficiency. This paper describes the model and its validation using data from independent and near independent experiments at Lincoln, New Zealand, and Rothamsted, UK. Despite there being no calculation of tiller dynamics or grain number, the model accurately simulated the behaviour of crops exposed to a wide range of conditions. We conclude that the accurate prediction of phenological development and LAI is much more important for grain yield prediction than are the components of yield. Although grain population is not a necessary step in yield calculation in Sirius, the model proved useful in investigating the effects of stress in setting grain number. The analysis showed that the influence of stress on partitioning of biomass to the ear during pre-anthesis ear growth was much more important in determining grain number than was the effect on biomass accumulation during the same phase. (C) 1998 Elsevier Science B.V.