Length analysis of the whole root system
Roots harvested as whole root systems in the 10 harvests were divided into embryonic roots and different whorls of nodal roots. The node in the coleoptile was defined as the first node. Roots were scanned with a scanner (Epson 1680, Indonesia). For scanning, the root sample was placed in a glass rectangular dish (200 mm × 150 mm) with a layer of water about 4–5 mm deep to untangle the roots and minimize root overlap. When necessary, a root was separated into subsamples until they could be placed into the rectangular dish. Because the root of the plants harvested after silking was very large, in the last 4 harvests, besides scanning the embryonic root and all of the nodal roots from the 1st to 3rd nodal root whorls, 2 representative nodal roots from the 4th to 8th nodal root whorls were selected for scanning. After multiplying by the actual numbers of the nodal roots in the 4th to 8th nodal root whorls, the total length of each whorl of node roots could be obtained. The images were analyzed using the software WinRHIZO version 5.0 (Regent Instruments Inc., Quebec City, Canada). The axial root was discriminated from lateral roots by setting an appropriate diameter. After calculation, the total root length of each whorl of nodal roots and of the whole root system was obtained.