TIMING OF CRITICAL DEVELOPMENTAL STAGES AND LEAF PRODUCTION IN FIELD-GROWN SPRING WHEAT FOR USE IN CROP MODELS

Detailed information on the timing of terminal spikelet formation, ant hesis, maturity and the rate of leaf appearance is crucial to the pred ictive accuracy of the AFRCWHEAT2 growth simulation model for wheat. T o obtain appropriate data under different growing conditions for two s pring wheat (Triticum Aestivum L.) cultivars (cvs Minaret and Canon), a main field experiment was conducted comprising eight sowing dates be tween mid-March and late June 1994; data from single sowings of Minare t 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 factor s on the growth and development of wheat, and data of the type reporte d here were essential to support the modelling aspects of the project involving AFRCWHEAT2. The accumulated thermal time from emergence to c ritical developmental stages was similar for all sowings of both culti vars, even though biomass was greatly reduced in the later sowings. Al though final leaf numbers were comparable for all sowing dates, the ra te of leaf appearance was correlated with the rate of change of day le ngth at crop emergence in Minaret. The unmodified AFRCWHEAT2 model did not predict the timing of critical growth stages and leaf numbers wel l, but removal of the vernalization and photoperiod factors from the d evelopmental sub-model greatly improved the accuracy of such predictio ns. These results strongly suggest that fixed quantities of thermal ti me may be employed successfully to predict the timing of critical deve lopmental stages of these spring wheat cultivars over a range of sowin g dates, geographic locations and climatic conditions, since developme nt appeared to be a linear function of temperature. Despite removing t he photoperiod and vernalization functions, the model consistently ove rpredicted final leaf number because leaf production by the model cont inued until 1.8 phyllochrons before anthesis, as compared to the three phyllochrons actually recorded for Minaret. AFRCWHEAT2 could therefor e be further modified to reproduce more accurately the smaller number of leaves typical of these spring cultivars.