Figure 4.4 Root systems of young (left) wheat and (right) lupin plants. Wheat, a monocot, has a dual root system. Seminal roots emerge from the seed and nodal roots (thicker roots on the outside of the picture) emerge from the crown, a group of closely packed nodes from which tillers emerge. Lupin, a dicot, has a tap root from which lateral roots emerge and which thickens with time as continued cambial activity leads to secondary growth.
The root system architecture is the arrangement of different roots in solid space. Just like a building has walls, roof, and floors, plant root systems also contain different structures including root types (primary, lateral, adventitious), root hairs, and specialized features such as nodules and cluster roots (see case study). In contrast to a fixed structure like a building, the root system is dynamic with new structures forming as needed to explore the soil and old structures breaking down when their use has expired. This four dimensional architecture within soil can now be visualized using technology such as X-ray microscale computed tomography (microCT) and magnetic resonance imaging (MRI).
In order to understand root architecture it is important to understand the different structures that make up the root system. This section will focus on the different root types while cluster roots are explained in the case study that follows. Root hairs are described in the section on the rhizosphere (Section 4.2) and the formation of N2-fixing nodules in Section 4.4.
Figure 4.5 MicroCT images of crop root systems. A, Single wheat plant with primary root (P), lateral roots (L) and seminal roots (S). Roots are false-coloured in white, and soil is false-coloured in brown. Scale bar = 1 cm. (Reproduced from Atkinson et al. (2014) Plant Physiol 166: 538-550. doi:10.1104/pp.114.245423. Copyright American Society of Plant Biologists). B) Two wheat plants grown in the same pot. Roots are false-coloured in blue or orange; each colour shows a single root system. (Image courtesy S. Mairhofer)