Accurate prediction of leaf appearance rate is required in maize (Zea mays L.) simulation models to estimate leaf area development, biomass and yield. Plant temperature is closely related to the development rate, but the air temperature record used to estimate plant temperature is often biased during the early stages of maize development because the growing plant parts are below the soil surface. A field study was conducted in 1996 at East Lansing, Michigan, on a Capac loam soil, to determine the effect of soil, air and apex temperatures on maize development and to evaluate their utility in the improvement of leaf developmental predictions. Seasonal variation in leaf tip appearance rates was observed for four sowing dates spaced about one month apart. Solar radiation and temperature of the air, apex and soil (0.01 m, 0.03 m and 0.05 m depths) were recorded on half-hourly intervals. Apex temperature was found to be close to the soil temperature at 0.03 m or 0.05 m when the apex was below the surface. When the apex was above the surface, its temperature was close to the air temperature. The phyllochron (degree-days between leaf appearance events) was found to be higher (52.4°C/leaf tip) than values used in most existing maize models, a possible reason of over-prediction of leaf development rates.
A functional model was developed to estimate mean daily apex temperature using inputs of daily maximum and minimum air temperatures and solar radiation. The model was based on individual differences between apex daylight and apex night time temperatures and maximum and minimum air temperatures. Solar radiation was used only during the daylight period. The model was tested using an independent weather data set. The resulting estimates had a root mean square error (RMSE) of 1.31°C per day and mean bias error (MBE) of -0.06°C respectively. After stem elongation pushed the apex above the soil surface, mean air temperature was close enough to the mean apex temperature to assume that they were equal. The model provided improved accuracy for predicting maize vegetative development rate.