PREDICTION OF LEAF APPEARANCE IN WHEAT - A QUESTION OF TEMPERATURE

The rate of leaf appearance in wheat depends on temperature. When the rate is expressed in thermal time based on air temperature, a marked contrast is apparent between autumn and spring sowings. This variation has often been attributed to a preconditioning response that determines the thermal phyllochron at about the time of emergence, either directly through an effect of daylength or its rate of change. However, in this paper we show that invoking such a response is unnecessary. Leaf appearance rate was found to be well predicted based only on temperature near the apical meristem (near-surface soil temperature until stem extension began, then canopy temperature) and leaf number. It was not necessary to include a response to daylength or its rate of change. A model of leaf appearance based on estimates of near-surface soil temperature and canopy temperature gave superior predictions than others based on air temperature alone or modified by rate of change of daylength.