Rate and duration of grain filling and grain nitrogen accumulation of wheat cultivars grown in different environments

In Australia, the period during grain filling for wheat (Triticum aestivum L.) is often associated with increasing ambient temperatures and diminishing soil moisture conditions. This can affect grain size and grain protein concentration. In this study, grain filling and nitrogen accumulation were investigated by sampling every 7 days during grain filling for 4 cultivars: Rosella, Hartog, Halberd, and Eradu. The plants were grown in trials with and without irrigation in 1991 and 1992, which were seasons with divergent temperatures and rainfall during this period. A 4-term logistic function, using both days after anthesis (DAA) and growing degree days (GDD), was fitted to data to estimate maximum rate and duration of grain filling. The logistic model proved to be, in most cases, an accurate method to determine the rate and duration of grain filling and nitrogen accumulation. Probably because of differences in availability of soil moisture, the use of GDD did not improve on the estimates obtained from using DAA as the independent variable. In 1991, a relatively dry year, non-irrigated plants showed signs of wilting. In that year, grain nitrogen contents on a per grain basis were similar in both the irrigated and non-irrigated (dryland) environments, but grain weights were much higher from the irrigated environment. The maximum rate and duration of grain filling were lower in the dryland environment. In contrast, whereas the duration of nitrogen accumulation was similarly shorter in the dryland environment, the maximum rate of accumulation was substantially higher. In 1992, a cooler and wetter year, differences between irrigated and non-irrigated environments were smaller, but the trends for rate and duration were similar. We concluded that, under stress conditions, higher rates of accumulation of grain nitrogen and lower rates of accumulation of carbohydrate, rather than differences in duration of accumulation, were primarily responsible for increased grain nitrogen concentrations. In general, cultivars used in this study were ranked similarly for rate and duration of grain filling across environments, with the exception of Hartog, which had a significantly lower maximum rate in the dryland treatment in 1991. This suggests that Hartog may be more sensitive to drier conditions than the other cultivars. In almost all environments, Eradu had a higher maximum rate of accumulation of grain nitrogen than other cultivars; however, the duration was reduced, so that in the mature grain, grain nitrogen concentration was not significantly different.