COLD DAMAGE AND DEVELOPMENT OF RICE - A CONCEPTUAL-MODEL

Loss of yield in rice owing to exposure to suboptimal (<20-degrees-C) temperatures is a widespread phenomenon; in Australia there is circums tantial evidence that cold damage is substantial and that it can occur at any time during the growing season. Damage may take the form of re duced potential yield, abnormal spikelet development, and reduced spik elet fertility. In extreme cases plants may be completely infertile. W e are using a conceptual model based on the daily cycle of temperature to quantify a cooling index and the impact of cold. The daily impact of cold varies with the phenology of the crop (i.e. some stages of dev elopment are particularly cold-sensitive), and the total or cumulative impact depends on the summation of thermal time below a critical mini mum temperature for cold damage. We are in the process of quantifying the variables of the model for 2 cultivars of rice. This aims at predi ction of yield loss in response to seasonal variation in temperature, and identifying the likely benefit which will arise from using particu lar parent lines in later breeding for cold tolerance.