The variation in root DW was higher in N0 treatment than in the other N treatments throughout the grain-filling stage (Fig. 3). Across years, the reduction in root DW in both N120 and N240 was 14% (Table 3). This reduction was mostly shown in 2011. In 2013, there was a temporally increase in root DW from silking to 20 DAS. Thereafter, the root DW decreased gradually to a similar value as at silking. In N0 treatment, root DW increased by 6% across year. The reason is that root DW in the N0 treatment in 2013 continued to increase from the beginning of silking until 30 DAS, and then decreased (Fig. 3). At maturity, the root DW of the N0 treatment had increased by 22% compared to that at silking.
The change in root length of the early developed nodal roots compared to the late-developed nodal roots was investigated in more detail (Fig. 2). The reduction in root length was greater in early developed nodal roots than in late-developed nodal roots. Across years and N treatments, the root length of early developed nodal roots decreased by 44 to 71% and that of late developed nodal roots decreased by 8 to 43% (Table 3). Compared with the N120 treatment, the N240 treatment did not delay the reduction in root length of both the early- and the late-developed nodal roots (Fig. 2, Table 3).
In agreement to that found in root length, the reduction in DW was stronger in the early-developed roots than in the late-developed roots, but to a much smaller extent (Table 3, Fig. 3). Nitrogen effect was not significant on the reduction of the early-development roots. For the late-developed roots, root DW even increased in the N0 treatment in 2013 (Fig. 3).