S U M M A R YA simple model of vernalization, originally developed to quantify the vernalization response of fieldgrown carrots, was fitted to previously published experimental results for winter wheat cv. Norin 27. The optimum temperature for vernalization indicated by the model was c. 5-2 °C, as this induced the fastest progress to final leaf appearance, expressed as the reciprocal of number of days from sowing to final leaf. This rate decreased linearly with temperature rise or fall on either side of the optimum, extrapolating to zero at -4-8 °C (T min ) and 26-6 °C (7" max ). When all the treatment temperatures and durations were expressed as vernalizing degree days > -4-8 °C (V °C d), there was a linear increase in post-treatment development rate with increasing vernalization up to c. 275 V °C d. Ending the effective treatment duration for vernalization at the estimated time of initiation of the final leaf primordia brought many of the data points closer to the linear trend which described the rest of the data.Effects of using leaf number, which is linearly related to thermal time, instead of days as the unit of time to compensate for temperature differences in the original experiment were examined. Unvernalized plants had the potential to produce 18 leaves before flowering and therefore rates were expressed as the fraction of the potential total leaf number that each new leaf represented. All plants were assumed to have an initial development rate of 1/18 per leaf. This rate was assumed to increase linearly with time during the vernalizing treatment periods and then remain constant after treatment until the final leaf appeared. Leaf numbers reported from the original experiment were used with these assumptions to estimate the rate at the end of each treatment. The relationship between these rates and treatment temperatures was similar to that for rates based on post-treatment durations. There was an optimum temperature c. 5-5 °C and T mla and T max of -51 and 18-8 °C estimated by extrapolating the decreasing linear trends to the base rate of 1/18. When plotted against V °C d calculated from these temperatures, the rates from the full data set were well represented by the model line which had been fitted to the data from just one treatment duration. I N T R O D U C T I O NC 0 0 ' t e m P e r a t u r e s : fi rst t o a range of temperatures and second to the durations of these temperatures Many wheat varieties respond to a period of cool experienced by the plant. Ideally it should be based on temperatures early in their development by hastening data for the same variety. Such data remain scarce, their subsequent progress towards final leaf appear-Previous studies have used functions to describe ance and flower initiation. This vernalization response vernalization which combined data from experiments is strongest for winter wheats and is weak or absent and observations on winter rye and a number of for spring varieties. Models that attempt to predict winter wheat varieties (Lumsden 1980; Weir el al. the grow...

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