Root size and spatial distribution in the soil are not limiting factors for N acquisition under sufficient N supplies because of the high mobility of N in the soil (Sinclair and Vadez, 2002). In arable soil, however, N mobility in soils might be limited by water shortage. In this case, longer roots and better root distribution in the soil will be important for utilization of the heterogeneously distributed N and especially of newly mineralized N (Marschner, 1998). In the present study, the change in total root length was more dramatic than that in total root dry weight during the whole growth period, especially after reaching the highest levels during the grain-filling stage (Figs. 2, 5 and 6). These results indicated that lateral roots only comprised a small portion of the total root dry weight but constituted the majority of the total root length. In