MODELING THE EFFECTS OF VERNALIZATION ON PROGRESS TO FINAL LEAF APPEARANCE IN WINTER-WHEAT

A simple model of vernalization, originally developed to quantify the vernalization response of field-grown carrots, was fitted to previousl y published experimental results for winter wheat cv. Norin 27. The op timum temperature for vernalization indicated by the model was c. 5.2 degrees C, as this induced the fastest progress to final leaf appearan ce, expressed as the reciprocal of number of days from sowing to final leaf. This rate decreased linearly with temperature rise or fall on e ither side of the optimum, extrapolating to zero at -4.8 degrees C (T- min) and 26.6 degrees C (T-max). When all the treatment temperatures a nd durations were expressed as vernalizing degree days > -4.8 degrees C (V degrees C d), there was a linear increase in post-treatment devel opment rate with increasing vernalization up to c. 275 V degrees C d. Ending the effective treatment duration for vernalization at the estim ated time of initiation of the final leaf primordia brought many of th e data points closer to the linear trend which described the rest of t he data. Effects of using leaf number, which is linearly related to th ermal time, instead of days as the unit of time to compensate for temp erature differences in the original experiment were examined. Unvernal ized plants had the potential to produce Is leaves before flowering an d therefore rates were expressed as the fraction of the potential tota l 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 as sumed 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 treatm ent. The relationship between these rates and treatment temperatures w as similar to that for rates based on post-treatment durations. There was an optimum temperature c. 5.5 degrees C and T-min and T-max of -5. 1 and 18.8 degrees C estimated by extrapolating the decreasing linear trends to the base rate of 1/18. When plotted against V degrees C d ca lculated from these temperatures, the rates from the full data set wer e well represented by the model line which had been fitted to the data from just one treatment duration.