Growth direction of nodal roots with reference to root cap development in field-grown maize (Zea mays)

The growth direction of roots is an important factor determining the morphology of the whole root system in higher plants. However, plagiogravitropism of cereal crops has rarely been studied with reference to root cap development. In this paper, we examined the growth direction of each nodal root soon after emergence from different nodes in relation to root cap morphology in maize (Zea mays L.). Nodal roots were sampled several times to record their growth angles, and longitudinal sections of their root tips were stained with PAS reaction and observed using fluorescence microscopy. The size of the root cap and columella, and the number and cross-section area of amyloplasts in the columella cells, were measured. The growth angle of nodal roots that emerged from the lower nodes was almost constant until they reached about 25 cm in length. With root elongation, the length of the root cap and columella in these lower nodal roots became shorter. However, the growth angle of nodal roots from the middle nodes increased with root length, which indicated that the nodal roots were bending downwards during elongation. The size of the root caps in roots that emerged from the middle nodes was almost constant or became larger during their elongation. We examined the relation between root cap morphology and the growth angle of each nodal root and found intimate relationships, especially in roots from the middle nodes. Although there was no correlation between the growth angle and number of amyloplasts in columella cells, significant correlation was found between the size of each amyloplast and growth angle. In conclusion, the development of the root cap, especially the size of amyloplasts in the columella cells, has a strong influence on determining the growth angle of each nodal root in the early stages of maize root elongation.