Detailed information on the timing of terminal spikelet formation, anthesis, maturity and the rate of leaf appearance is crucial to the predictive accuracy of the AFRCWHEAT2 growth simulation model for wheat. To obtain appropriate data under different growing conditions for two spring wheat (Triticum aestivum L.) cultivars (cvs Minaret and Canon), a main field experiment was conducted comprising eight sowing dates between mid-March and late June 1994; data from single sowings of Minaret in 1992 and 1995 were also included in the analysis. Minaret is the focus of a major European study of the impact of climate change factors on the growth and development of wheat, and data of the type reported here were essential to support the modelling aspects of the project involving AFRCWHEAT2.
The accumulated thermal time from emergence to critical developmental stages was similar for all sowings of both cultivars, even though biomass was greatly reduced in the later sowings. Although final leaf numbers were comparable for all sowing dates, the rate of leaf appearance was correlated with the rate of change of day length at crop emergence in Minaret. The unmodified AFRCWHEAT2 model did not predict the timing of critical growth stages and leaf numbers well, but removal of the vernalization and photoperiod factors from the developmental sub-model greatly improved the accuracy of such predictions. These results strongly suggest that fixed quantities of thermal time may be employed successfully to predict the timing of critical developmental stages of these spring wheat cultivars over a range of sowing dates, geographic locations and climatic conditions, since development appeared to be a linear function of temperature.
Despite removing the photoperiod and vernalization functions, the model consistently over-predicted final leaf number because leaf production by the model continued until 1·8 phyllochrons before anthesis, as compared to the three phyllochrons actually recorded for Minaret. AFRCWHEAT2 could therefore be further modified to reproduce more accurately the smaller number of leaves typical of these spring cultivars.