Root Growth Theory In Medicine

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From Read and Gregory, 1997.

From Read and Gregory, 1997.

charides and not bound within polysaccharides. However, high glucose contents were reported in unfractionated rice mucilage (Chaboud and Rougier, 1984). Such materials not only form a substrate for microorganisms close to the root, but the polysaccharide fraction with its large molecules which can twist and flex also contributes to the viscoelastic behaviour of mucilages described earlier.

While the mucilage per se has almost no capacity to store water in the rhizosphere (McCully and Boyer, 1997), the chemical and physical properties of mucilage influence the supply of water to the root. The water potential of root mucilage when fully hydrated is about -7 to -10 kPa (Guinel and McCully, 1986; Read et al., 1999), and water is rapidly lost as the soil dries. Read and Gregory (1997) showed that the surface tension of both maize and lupin mucilages was reduced to about 48 mN m-1 at total solute concentrations >0.7 mg ml-1, indicating the presence of powerful surfactants. Fatty acids and lipids (both surfactants) are common components of plant tissues and mucilage (e.g. Sukhija et al., 1976) and subsequent analysis of maize, lupin and wheat mucilages showed that most of the plant-produced lipid present was phosphatidylcholine (Read et al., 2003). Reduced surface tension will enhance the ability of the mucilage to wet the surrounding soil particles and may also change the moisture characteristic curve of the soil to reduce the water content at any particular tension by 10-50% depending on particle size distribution (Read et al., 2003). Such alterations of physical properties suggest that if the root can maintain sufficient concentrations of surfactant close to the root, then it may be able to access water and nutrients from smaller soil pores than would otherwise be accessible to it.

Many functions have been ascribed to mucilage, but one of the most common is that it acts as a lubricant to ease the passage of the root through the soil. This role has been questioned (e.g. McCully, 1999), but it now seems likely that it is the presence of root cap cells within the mucilage that facilitates this role. The resistance to root penetration in a soil is the sum of the frictional resistance to root penetration plus the pressure required to form a cavity. Friction can be 80% of the penetration resistance experienced by roots as they move through soil, so reducing this resistance would be advantageous to plants in their exploration of soil resources. Bengough and McKenzie (1997) measured the frictional resistance experienced by metal rods, roots pushed into soil and growing roots, and found that the frictional resistance experienced by roots was a small, but not negligible, fraction of that experienced by metal probes. For example, in soil with a bulk density of 1.3 mg m-3, the penetration force was about 330, 200 and 150 mN for a metal probe, pushed root, and growing root, respectively. They postulated that the friction was probably relieved by the detachment of root cap cells to form a low-friction lining to the cavity enlarged by the root. The border cells might act as a cushion between the soil particles and the root, to reduce local stresses and allow the maintenance of an intact mucilage layer over the root surface (Hawes et al., 2003). For this concept to work in practice, there need to be sufficient border cells to cover a substantial proportion of the root cap surface. Iijima et al. (2000) found that the number of root cap cells of maize sloughed into sand increased 12-fold (from 60 to >700 per mm of root extension) as penetrometer resistance increased from 0.29 to 5.2 MPa. This increase in cell production was estimated to cover the whole of the root cap with detached cells in the compacted sand, compared with about 7% of the surface area of the cap in loose sand. In this case (see also Iijima et al., 2003), sufficient border cells were released that this lubricating layer of sloughed cells and mucilage probably decreased the frictional resistance to root penetration; whether this occurs in species other than maize is not currently known.

2.5 Architecture of root systems

Root architecture, the spatial configuration of a root system in the soil, is used to describe distinct aspects of the shape of root systems. Lynch (1995) states that studies of root architecture do not usually include fine details such as root hairs, but are concerned typically with an entire root system of an individual plant. From the architecture both the topology (a description of how individual roots are connected through branching) and the distribution (the presence of roots in a spatial framework) can be derived, whereas neither topology nor distribution can be used to derive architecture. From the brief description of root branching given in section 2.3.2, it will be clear that root architecture is quite complex and varies between and within plant species. Drawings of excavated root systems of crops and other species show the differences in shape between monocotyledons and dicotyledons and allow some broad generalizations to be made about the depth of rooting and the relative distribution of roots (Kutschera, 1960) (Fig. 2.7). Nearly all such drawings show that, with the exception of the tap root which grows almost vertically throughout, most other root axes grow initially at some angle relative to the vertical but gradually become more vertically orientated. Gravitropic responses combined with responses to light, water and touch together, perhaps, with the predominance of vertical cracks in deeper soil layers, produce these patterns.

Root architecture's importance lies in the fact that many of the resources that plants need from soil are heterogeneously distributed and/or are subject to local depletion (Robinson, 1994). In such circumstances, in contrast to the shapes shown in Fig. 2.7, the development and growth of root systems may become highly asymmetric, and the spatial arrangement of the root system will substantially determine the ability of a plant to secure those resources (Lynch, 1995). Such ideas have been investigated in a series of experiments and models using common bean (Bonser et al., 1996; Ge et al., 2000). While root trajectories are essentially under genetic control, phosphorus deficiency was found to decrease the

Wurzelatlas Kutschera

Fig. 2.7 Drawings of excavated root systems: (a) maize, Zeamays; (b) ryegrass, Loliummultiflorum; (c) oilseed rape, Brassica napus; and (d) sugar beet, Beta vulgaris. (Reproduced with permission from Kutschera, Wurzelatlas; DLG-Verlags-GmbH, 1960.)

gravitropic sensitivity of both the tap root and the basal roots, resulting in a shallower root system. It was hypothesized that the shallower root system was a positive adaptive response to low soil P availability by: first, concentrating roots in the surface soil layers where soil P availability was highest; and second, reducing spatial competition for P among roots of the same plant. This hypothesis was tested by modelling root growth and P acquisition by bean plants with nine contrasting root systems in which basal root angle was varied but not root length or degree of branching (Fig. 2.8). Shallower root systems acquired more P per unit carbon cost than deeper root systems and in soils with higher P availability in the surface layers, shallower root systems acquired more P than deeper root systems because of less inter-root competition as well as increased root exploration of the upper soil (Ge et al., 2000). In practice, of course, the plant may have multiple resource constraints to contend with (e.g. heterogeneously distributed P and soil water) and will try to optimize its investment in roots. Ho et al. (2004) investigated this optimization with respect to beans grown under different combinations of water and P availability. They postulated that an optimizing plant would grow roots deeper into the profile until the marginal benefit exactly equalled the marginal cost and by modelling found that the basal root angle would be shallower for localized shallow P, and deeper for localized deep water than that obtained in the case of uniformly distributed water and P. When P was concentrated in the surface and water was located deep, the optimal basal root angle depended on the relative rates of change with depth in the values ascribed to the available resources. While useful in indicating general principles, it should be remembered that not all of the responses of roots to a heterogeneous environment (e.g. changes in branching frequency and root hair growth) are yet captured in such models; this is a substantial challenge.

The branching patterns (topology) of individual roots have implications not only for resource capture but also for the construction costs of roots (Fitter et al., 1991). In topological analysis, the root can be considered as any other mathematical branching tree, with links that are either exterior (ending in meristems) or interior (i.e. internodes). The links have geometrical properties, including length, radius, angle and direction of growth, and are distributed in a defined pattern; as in most branching trees (e.g. the trachea in the lung), the diameter increases with increasing magnitude of the individual link. Fitter et al. (1991) em-

Wurzelatlas Kutschera

Fig. 2.8 Geometric simulation modelling of bean root systems that vary in basal root angle but are otherwise identical in length and branching. The variation illustrated is present among different genotypes of Phaseolus vulgaris and has been shown to be influenced by soil P availability. The scale on the right is from 0 to 40 cm. (Reproduced with permission from Ge et al., Plant and Soil; Springer Science and Business Media, 2000.)

Fig. 2.8 Geometric simulation modelling of bean root systems that vary in basal root angle but are otherwise identical in length and branching. The variation illustrated is present among different genotypes of Phaseolus vulgaris and has been shown to be influenced by soil P availability. The scale on the right is from 0 to 40 cm. (Reproduced with permission from Ge et al., Plant and Soil; Springer Science and Business Media, 2000.)

ployed a simulation model to demonstrate that a herringbone topology (where branching is on the main axis) with long interior and exterior links is associated with high exploration efficiency, although such a pattern is also characterized by large tissue volumes and hence high construction costs. Fitter et al. (1991) predicted that a herringbone topology would be favourable for slow-growing species in habitats where soil resources are scarce and that nutrient-rich soils or those with a nutrient-rich surface layer would encourage the formation of dichotomous topologies with their associated cheaper construction costs (Fig. 2.9). Such predictions were only partially borne out by the experimental results of Fitter and Stickland (1992) in which Trifolium repens L. became more herringbone-like as soil water content increased (contrary to prediction) but Mercurialis perennis L. responded as expected to both irrigation and N and P additions. Topological considerations alone, though, are unlikely to be the sole adaptive trait to particular soil environments. For example, Bouma et al. (2001) found that roots of Chenopodiaceae in a salt marsh changed from herringbone-like at low elevation to dichotomous at higher elevations but that the Gramineae showed no such relationship. Moreover, root diameter was not related to link magnitude thereby undermining the basis of the estimates of construction efficiency proposed by Fitter et al. (1991).

Not only does root topology and root system architecture respond to soil heterogeneity, but the form of the root system may, indeed, induce soil heterogeneity. In grassland and savanna systems, caespitose (i.e. tussock or bunch) and rhizomatous perennial grasses represent two distinct forms of grass. In rhizomatous grasses, nutrients can accumulate in the rhizomes but do not accumulate in the soil whereas in the caespitose grasses, both carbon and nitrogen accumulate in soils directly beneath plants resulting in fine-grained soil heterogeneity (Derner and Briske, 2001). The 'islands' of nutrients appear to accumulate beneath caespitose grasses even when they are small, suggesting that they are present throughout much of the plant's life. Plant-induced increases in nutrient concentrations do not form beneath the rhizatomous species and the large nutrient pool beneath such species in a semi-arid community was largely a consequence of niche separation for microsites characterized by deeper soils with higher amounts of water and nutrients.

In broad-scale agriculture where single crops are grown with inputs of fertilizers, there has been little consideration until recently of root architecture, but with the increasing emphasis on the more efficient use of water and nutrients in production systems, this is starting to change. For example, in soils where P availability is low, selecting genotypes with appropriate

Fig. 2.9 Diagram showing the distinction between (a) herringbone, and (b) dichotomous branching patterns. (Modified and reproduced with permission from Fitter et al., New Phytologist; New Phytologist Trust, 1991.)

Fig. 2.9 Diagram showing the distinction between (a) herringbone, and (b) dichotomous branching patterns. (Modified and reproduced with permission from Fitter et al., New Phytologist; New Phytologist Trust, 1991.)

architecture may increase soil exploration by roots and raise yields (Lynch and Beebe, 1995). Equally important in other areas is the ability of roots to capture nutrients such as nitrate that might otherwise leach from the soil profile into water courses. Dunbabin et al. (2003) have shown the role that root architecture may play in this regard and the importance of quickly producing a high density of roots in the topsoil on the sandy soils that they studied. In many parts of the world, though, mixed cropping is important either with crops grown together as intercrops or with different crops grown in sequence, as is the growing of trees and crops in agroforestry associations. In such systems, root architecture is important in determining both the spatial competition and spatial complementarity of root systems (van Noordwijk et al., 1996). These ideas will be explored more in Chapter 9.

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Plant Roots: Growth, Activity and Interaction with Soils

Peter J. Gregory Copyright © 2006 Peter Gregory

Chapter 3

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