Roots and the Architecture of Root Systems

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Roots are complex structures that exist in diverse forms and exhibit a wide range of interactions with the media in which they live. They also exhibit a very wide range of associations with other living organisms with which they have co-evolved. Laboratory and field studies have revealed a great deal about this complexity, especially during the last 20 years or so when there have been several national programmes of research around the world focusing on below-ground processes. The purpose of this chapter is to describe the essential anatomical and morphological features of roots as a background to understanding the diverse forms of root systems and their functioning which follow in later chapters.

2.1 Nomenclature and types of root

Terrestrial plants produce roots of many types (e.g. aerial roots, storage roots, etc.), but in this book the focus is on roots that generally either originate from plant tissues located below ground or which function principally below ground or both. Many names are used to describe roots, some of which are confusing and inconsistently used. In part this is because appreciation of the full diversity of root types has emerged only slowly and the terms available differentiate only gross differences. For example, the word 'primary' is used variously in the literature to describe the first root to appear at germination, the first-formed branches of roots, and the largest root. The term 'adventitious root' is also commonly used elsewhere but is not used in this book because such roots are the norm in many plants (Groff and Kaplan, 1988). In this book, the nomenclature suggested by the International Society for Root Research (ISRR) has been adopted wherever possible, although because it has been common practice in the literature to use slightly different nomenclature for monocotyledonous and dicotyledonous plants, these names have also been employed if they were used by the original author of the work cited and are not confusing. For example, because the distinction between seminal (i.e. laid down in the seed) and other root axes is commonplace in the literature, this book adopts the terms seminal and nodal axes for graminaceous species. The ISRR nomenclature attempts to provide a uniform terminology based on the part of the plant from which the root grew rather than relying on knowledge of the tissue from which the root was initiated.

In most plants, the emergence of a root is the first sign of germination. The first root axis arises from cells laid down in the seed and for dicotyledonous plants is called the tap root; this term is also applied to any replacement root that may take over the role of this root if the original tap root is damaged. Subsequent root axes may arise from the mesocotyl or hypocotyl and are called basal roots, with roots arising from shoot tissues above ground called shoot-borne roots (Fig. 2.1a). In graminaceous plants such as maize, wheat and barley, the predominant nomenclature has been to refer to the first root and the other root axes arising from the scutellar node as the seminal axes, with axes arising from the mesocotyl called nodal axes because they arise from nodes at the bases of leaves; other terms such as crown, basal or adventitious have also been used for these axes in the literature (Fig. 2.1b). Although different names have been employed for the axes of monocotyledonous and dicotyledonous plants, these differences do not persist in naming the subsequent branches. Lateral roots (first order laterals) arise from the axes, and from these laterals other, second order, laterals arise. Hackett (1968) proposed an alternative nomenclature for laterals that is also widely used in the literature in which the first branches were termed primary, with subsequent branches being secondary and so on. The axis and its associated laterals are called a root, and all the roots of a plant together form the root system.

In gymnosperms and dicotyledons, the tap root and its associated laterals comprise the root system, and some workers, particularly in the ecological literature, have referred to such plants as tap-rooted species. In graminaceous plants, the multiple root axes and their laterals give the appearance of a more finely distributed or fibrous root system; this latter term is also used by some workers to distinguish types of root system. Some botanical textbooks state that the seminal roots of cereals live for only a short time; this despite many field measurements which show this statement is incorrect (Gregory et al., 1978; McCully, 1999).

Some roots or root parts are specialized for a particular function (Plate 2.1). Such specialisms include the following.

Generic Photo Shoot

Fig. 2.1 Diagrammatic representation of generic (a) dicotyledonous, and (b) monocotyledonous plants with commonly used root nomenclature. (Redrawn from unpublished work of the International Society for Root Research.)

Fig. 2.1 Diagrammatic representation of generic (a) dicotyledonous, and (b) monocotyledonous plants with commonly used root nomenclature. (Redrawn from unpublished work of the International Society for Root Research.)

Storage roots. Parts of roots of plants such as carrot (Daucus carota), sugar beet (Beta vulgaris), sweet potato (Ipomoea batatas) and yams (Dioscorea spp.) are specifically adapted to store products photosynthesized in the shoot. The products are synthesized above ground and transported to the root in the phloem where they reside until needed to complete the life cycle. In biennial plants such as carrot and sugar beet, the storage organs are frequently harvested for human use before the life cycle is complete, but if allowed to mature, the stored materials are retranslocated to the shoot where they are used to produce flowers, fruits and seeds. The development of storage roots is similar to that of non-storage roots except that parenchyma cells predominate in the secondary xylem and phloem of the storage roots.

Aerial roots. Aerial or shoot-borne roots originate from a range of above-ground structures. In grasses such as maize, these roots act to prop or brace the stem but when they grow into the soil they may branch and also function in the absorption of water and nutrients (McCully, 1999). Many trees also produce prop roots - including the spectacular banyan tree (Ficus macrophylla) - which gradually invades new ground and 'takes over' surrounding trees. In plants such as ivy (Hedera helix), the aerial roots cling to objects like walls and provide support to the climbing stem. There are many adaptations in the aerial roots of epiphytes which allow the plants to live on, but not parasitize, other plants. In some genera (e.g. Ansiella, Cyrtopodium and Grammatophyllum) fine aerial roots grow upright to form a basket which collects humus which is then penetrated by other roots which utilize the nutrients. In epiphytic orchids, root tip cells contain chloroplasts as, in many cases, do the cortical cells. These perform photosynthesis and, in the case of leafless orchids of the genera Taeniophyllum and Chiloschista, are the only photosynthetic tissue of the plant (Goh and Kluge, 1989). A characteristic feature of the aerial roots of orchids is the outer layer of dead cells forming the velamen (Benzing et al., 1982). Many of the cells are water-absorbing while others are filled with air and facilitate the exchange of gases with the inner cortex. The physiological role of the velamen is not known with certainty. In some species it appears to aid the uptake of water and nutrients while in others it appears to be a structure for water conservation.

Air roots. In some trees that live in swamps, such as mangroves, parts of the roots develop extensions which grow upward into the air. These air roots or pneumatophores grow above the surface of the water and allow oxygen to be transported to the inner cortex of the root system, and CO2 to escape from the root interior (Geissler et al., 2002). The primary structures allowing gas exchange through the pneumatophores are the lenticels, but several other structures, such as horizontal structures close to the apex, specific to particular species have also been identified (Hovenden and Allaway, 1994; Geissler et al., 2002).

Hair roots. These are produced by many heathland plants such as the Ericaceae and Epacridaceae and are the finest roots known (typically 20-70 pm diameter and <10 mm long). They are characterized by a reduction of vascular and cortical tissues, by the absence of root hairs, and by the presence, in what would be the root hair zone of other plants, of swollen epidermal cells occupied by mycorrhizal fungi (Read, 1996). The hairs develop as first order branches on normal root axes or as second or higher order branches on other hair roots (Allaway and Ashford, 1996). The hair roots form a dense fibrous root system and when excavated from soil, the roots have a coating of tightly bound soil particles (a rhizosheath - see section 2.4.2).

Proteoid or cluster roots. These specialized roots were first identified in members of the Proteaceae, but are now known to occur in other species from a diverse range of families (Purnell, 1960; Watt and Evans, 1999). Agriculturally important species include white lupin (Lupinus albus) and yellow lupin (Lupinus cosentinii Guss.). Cluster roots are bottle-brush-like clusters of hairy rootlets (each rootlet typically 5-10 mm long), and may appear as ellipsoidal-shaped clusters of roots (like small bunches of bananas) at intervals. There are many roots in each cluster, and the clusters may be separated by normally branched regions (Lamont, 2003). The formation of proteoid roots is typically associated with soils that are low in phosphate and/or iron, although lupins will form them in soils to which P fertilizers have been applied (Watt and Evans, 2003). Phosphate uptake in soils low in P is assisted by the exudation of a range of carboxylates with malate, malonate, lactate and citrate being common (Roelofs et al., 2001) (see section 7.2.2 for more details).

Contractile roots. Contractile roots are widely distributed among monocotyledons and herbaceous perennial dicotyledons and serve to pull the shoot closer to the ground or, in bulbs, deeper into the soil. Contraction in many monocotyledonous species occurs when the inner cells of the cortex (contractile parenchyma) expand radially and contract longitudinally (Reyneke and van der Schijff, 1974). The consequence of the contraction in these cells is that the inner vascular tissues and the outer cortical cells become buckled longitudinally and the root appears wrinkled. This mechanism, though, is not universal in all species with contractile roots. Similarly, the factors which induce contractile root activity differ between species. For example, while light and temperature fluctuations appear important in inducing contractile behaviour in species such as Nothoscordum inodorum, Narcissus tazetta and Sauromatum guttatum, this is not the case in the ornamental day lily, Hemerocallis fulva, in which contraction appears to be a basic characteristic (Pütz, 2002).

Parasitic roots. In parasitic associations between higher plants, the connection between two plants is established via haustoria formation by the parasite (see section 6.3.3). For example, in Striga species, when the radicle makes contact with a host root, elongation ceases and the tip of the radicle swells to form a pre-haustorium. Sticky hairs develop on this structure, which results in parasite-host adhesion. After this, intrusive cells develop at the root tip, which penetrate the cortex and endodermis of the host root by secreting enzymes that cause separation of the host cells rather than effecting intra-cell penetration. Once in the stele, there is a rapid development of links between the parasite and the host xylem (Parker and Riches, 1993).

2.2 Root structure

The anatomy of roots is complex with very variable structures both between and within plant species. There are considerable differences among species (especially between an-giosperms and gymnosperms), among habitats, and along the length of individual roots. Common examples of differences in structure include death of the epidermis and in some species the entire cortex, development of aerenchyma in the cortex, development of the endodermis and exodermis (with their Casparian bands, suberin lamellae, and thickened, modified walls), and the production of a periderm (Steudle and Peterson, 1998). It is important to note that most published work on root structure has been conducted with young plants grown in 'clean' environments. Whether such studies are useful in describing how a root growing in soil will look or understanding how a root system growing in soil functions are topics of lively scientific debate (e.g. McCully, 1995, 1999). This section draws mainly from literature on juvenile plants grown in solutions or sand.

2.2.1 Primary structure

In their primary stage of growth, roots show a clear separation between three types of tissue systems - the epidermis (dermal tissue system), the cortex (ground tissue system) and the vascular tissues (vascular tissue system). In most roots, the vascular tissues form a central cylinder, but in some monocotyledons they form a hollow cylinder around a central pith (Esau, 1977). These three tissue systems form a range of cells visible in transverse and longitudinal sections (Fig. 2.2).

Dermal tissue. In young roots, the epidermis is a specialized absorbing tissue containing root hairs which are themselves specialized projections from modified epidermal cells known as trichoblasts (Bibikova and Gilroy, 2003). Root hair formation is a complex process (see section 2.3.3) regulated by many genes and is also responsive to a variety of environmental stimuli. Root hairs markedly extend the absorbing surface of the root but they are often considered to be short-lived and confined to the zone of maturation. A thin cuticle may develop on the epidermis, and in some herbaceous species the cell walls thicken, suberin is deposited in them, and the epidermis remains intact for a long time as a protective tissue.

Ground tissue. In young plants, the cortex usually occupies the largest volume of most roots and consists mainly of highly vacuolated parenchyma cells with intercellular spaces between. The innermost cell layer differentiates as an endodermis and one or more layers at the periphery may differentiate as a hypodermis/exodermis.

Roots that undergo significant amounts of secondary growth (see next section), such as gymnosperms, often shed their cortex early in life, but in other species, the cortical cells develop secondary walls that become lignified. The intercellular spaces allow the movement of gases in the root and under particular conditions may develop into large lacunae (aerenchyma) in some species (e.g. rice, see section 5.4.3). The cortical cells have numerous interconnections both via the cell walls and via the plasmodesmata which link the protoplasm of each cell (Roberts and Oparka, 2003). Substances can move across the root, then, either via the cell walls (the apoplastic pathway) or via the cell contents (the symplastic pathway); these pathways are assumed to be important in the internal transport of water and nutrients (see section 4.2.3). In some species such as grasses, the epidermis may be shed together with all or part of the cortex as a normal part of the ageing process of roots or in response to adverse soil conditions (Wenzel and McCully, 1991). For example, when wheat roots were grown in dry soil, the upper portion of seminal axes had collapsed epidermal and cortical cells (Brady et al., 1995). On re-wetting, dormant lateral roots grew rapidly to take up water and N but the seminal axes themselves did not appear capable of significant N uptake.

In contrast to the rest of the cortex, the endodermis lacks air spaces and the cell walls contain suberin in a band (the Casparian strip) that extends around the radial and transverse cell walls, which are perpendicular to the surface of the root. Three stages in the development of the endodermis can be discerned. First, radial and transverse endodermal cell walls are impregnated with lipophilic and aromatic compounds (Casparian strips) which restrict,

Fig. 2.2 Diagrammatic representation of the early stages of primary development of a root. The region of cell division extends for some distance behind the apical meristem and may overlap with the regions of cell elongation and cell differentiation/maturation. (Based on original work by Esau (1941); redrawn and reproduced with permission from Torrey, American Journal of Botany; Botanical Society of America Inc., 1953.)

Fig. 2.2 Diagrammatic representation of the early stages of primary development of a root. The region of cell division extends for some distance behind the apical meristem and may overlap with the regions of cell elongation and cell differentiation/maturation. (Based on original work by Esau (1941); redrawn and reproduced with permission from Torrey, American Journal of Botany; Botanical Society of America Inc., 1953.)

but do not altogether stop, apoplastic movement of water and ions (see section 4.2.3). The second stage occurs especially in species in which the epidermis and cortex are shed and lateral roots emerge, and is characterized by the deposition of a thin, lipophilic suberin lamella to the inner surface of radial and tangential walls of endodermal cells. Finally, there may be considerably more deposition on the inner tangential and radial cell walls evident as U-shaped wall thickening (Schreiber et al., 1999). These changes in the endodermis begin opposite the phloem strands and spread towards the protoxylem. Opposite the protoxylem, the cells may remain thin-walled with Casparian strips; these are called passage cells (Peterson and Enstone, 1996).

In many angiosperms, an exodermis differentiates from peripheral cortical cells. In a survey of >200 angiosperm species, 94% of all plants possessed a hypodermis and about 90% had a hypodermis with Casparian strips (Perumalla and Peterson, 1990; Peterson and Perumalla, 1990). In a small proportion of plants, the exodermis comprised more than one cell type with long, suberised cells and short cells in which deposition of suberin was delayed; these short cells act as passage cells for water and ions (Peterson, 1991). As cells in the epidermis mature, a hypodermis may differentiate in the outer cortical cells (Plate 2.2). Like the young endo-dermis, these cell walls are impregnated with lipophilic and aromatic compounds. Moreover, in response to environmental stresses such as drought, aeration and potentially toxic metals, Casparian strips and suberin lamellae form in the hypodermis just as occurs in the first stage of endodermis formation. A hypodermis with Casparian strips is called an exodermis (Perumalla and Peterson, 1986), and this forms a barrier of variable resistance to the flow of both water and nutrients across the root (Hose et al., 2001). In summary, Casparian strips are a characteristic feature of primary endodermal cell walls, whereas they only form in hypodermal cell walls as a reaction to environmental factors (Schreiber et al., 1999).

Vascular tissue. The vascular cylinder (stele) consists of vascular tissues (xylem and phloem) and one or more layers of non-vascular tissues, the pericycle, which surrounds the vascular tissues (Fig. 2.2).

The central vascular cylinder of most dicotyledonous roots consists of a core of primary xylem from which ridge-like projections of xylem extend towards the pericycle (Esau, 1977). Between the ridges are strands of primary phloem. If the xylem does not differentiate in the centre of the root (as happens in many monocotyledons), a pith of parenchyma or sclerenchyma (parenchyma cells with secondary walls) is present. The number of xylem ridges varies between species and among roots of the same species (see McCully, 1999, for a description of xylem development in maize) and this variation is captured by referring to roots as diarch, triarch, tetrarch, etc., depending on the number of ridges. The first xylem elements to lose their cell contents and mature (the protoxylem) are those next to the pericycle, while those closer to the centre are the typically wider metaxylem elements which mature later and commonly have secondary walls with bordered pits. As with the xylem, the phloem shows a centripetal order of differentiation with protophloem nearest the pericycle and metaphloem nearer the centre. Companion cells accompany the metaphloem but are less frequent in the protophloem, although in grasses each protophloem element is associated with two companion cells giving a consistent, symmetrical pattern in transverse sections. In contrast to the xylem, the phloem consists of living cells.

The pericycle is composed of parenchyma cells with primary walls but these may develop secondary walls as the plant ages. Lateral roots arise in the pericycle (see section 2.3.2), and in roots undergoing secondary growth, the pericycle contributes to the vascular cambium opposite the protoxylem and generally gives rise to the first cork cambium.

Maturation of the xylem so that it can conduct water may take some time and lignification is not a good indicator of maturity (McCully, 1995, 1999). For example, St Aubin et al. (1986) found that the large vessels of actively growing maize root did not mature and become open for conduction until at least 150 mm, and sometimes >400 mm behind the root tip. The narrower vessels started to mature about 40-90 mm from the tip and the very narrow protoxylem at about 10-20 mm (McCully, 1999). Similar results were summarized for other species including barley, banana, soyabean and wheat by McCully (1995), with a range of 110-1300 mm for the distance from the root tip at which late metaxylem vessels became open tubes. Immature, living xylem cells are highly vacuolated with a thin layer of cytoplasm. They do not conduct water but accumulate ions, especially potassium, in high concentration in the vacuole (McCully, 1994). The point of transition from closed to open (living to dead) large xylem vessels is important because it affects both water and nutrient uptake and transport in roots. Detached roots (used in many laboratory studies) tend to differentiate their tissues rapidly so that their behaviour may not represent that of more slowly maturing roots grown in field conditions.

2.2.2 Secondary structure

Secondary growth is characteristic of roots of gymnosperms and of most dicotyledons but is commonly absent from most monocotyledons. Secondary growth consists of (i) the formation of secondary vascular tissues dividing and expanding in the radial direction, and (ii) the formation of periderm, composed of cork tissue (Esau, 1977). Growth within the secondary vascular system is driven by the cambium, which consists of two morphologically distinct cell types: fusiform initials (greatly elongated in the axis of the root) and ray initials (which are cuboid) (Chaffey, 2002). Both cell types are thin-walled and highly vacuolated. The process starts with the initiation of vascular cambium by divisions of procambial cells that remain undifferentiated between the primary xylem and primary phloem. Thus, depending on the number of xylem and phloem groups present in the root, two or more regions of cambial activity are initiated. Soon the pericycle cells opposite the protoxylem elements also divide and become active as cambium so that cambium quickly surrounds the core of xylem (Plate 2.3). The vascular cambium opposite the phloem strands begins to produce secondary xylem toward the inside, so that the strands of primary phloem are displaced outwards. By the time that the cambium opposite the protoxylem is actively dividing, the cambium is circular and the primary phloem and xylem have been separated. By repeated divisions, secondary xylem and secondary phloem are added and files of parenchyma cells within these form rays. As the secondary xylem and phloem increase in width, so the primary phloem is crushed and disappears.

Periderm (analogous to bark) formation usually follows the initiation of secondary xylem and phloem formation to become the outer, protective covering of the root. Divisions of pericycle cells increase the number of layers of pericycle cells. In the outer cell layers, cork cambium is formed which produces a layer of cork on the outer surface and phello-derm toward its inner surface; these three tissues constitute the periderm. The remaining cells of the pericycle may form tissue that resembles a cortex. Lenticels may differentiate in the periderm to facilitate the passage of gases into and out of the root. With the formation of the periderm, the endodermis, cortex and epidermis are isolated from the rest of the root, die, and are sloughed off. A woody root remains.

While the preceding description applies to many roots, secondary growth may also result in roots with different appearance to that described above. For example, Plate 2.4 shows a young root of Catalpa speciosa in which appreciable secondary growth has occurred. The endodermis has expanded and is intact, and the cortex has outer cells which are differentiating to form a periderm.

2.3 Extension and branching

2.3.1 Extension

The extension growth of roots occurs in the apical regions of roots, and it is this extension of root axes and laterals into new regions of soil that expands the resource base and anchorage ability of the plant. Figure 2.3 shows that the zone of elongation is confined to the apical meristem where cell division occurs, and the region immediately behind this where predominantly longitudinal cell elongation occurs. Towards the tip of the apical meristem is a zone referred to as the 'quiescent centre' where cell division occurs rapidly during very early root growth but then becomes infrequent.

The cells in the root meristem are mainly cytoplasmic and have no clearly defined central vacuole (Barlow, 1987a). The patterns of cell division in this region are precisely regulated and determine the future characteristic form of the root (Fig. 2.3). Many of the cells divide in planes that are parallel to the main axis of the root and, in so doing, create files of cells which subsequently divide transversely to the axis of the root, thereby increasing the number of cells in each file (Barlow, 1987b). Groups of cells (packets) within a cell file can easily be seen and their ontogeny traced. For example, Barlow (1987b) followed the morphogenesis in the tap root of maize and by counting the number of cells in packets determined that the period between each round of cell division was fairly constant except in cortical and stellar cells around the quiescent centre where there was evidence for a steep gradient of rates of cell proliferation.

Cell division does not result in extension but rather provides the raw materials for subsequent cell expansion and so does not, itself, drive growth. In the elongating zone, outside the meristem, cells increase in length accompanied by a large increase in the size of the vacuole and an increase in the area of the lateral walls of the cell. Expansion of root cells requires the co-ordination of many processes including the control of ion (especially potassium) and water uptake into the vacuole, the production of new wall and membrane materials, and the increase in size of the cytoskeleton (Dolan and Davies, 2004). Root elongation occurs, then, as the sum of the individual cell expansions along a file of cells (i.e. in a single directional axis). During cell expansion, changes in cell wall properties enable the walls to be strong enough to cope with the internal pressure of the growing cell but flexible enough to allow growth (Pritchard, 1994). Cell walls can loosen rapidly during periods of accelerating growth and, conversely, tighten after exposure to stresses such as low temperature and high soil strength in ways that are still being explored. The cell wall consists of cellulose microfibrils, hemicellulose and pectin, together with various proteins. The microfibrils both provide a framework for the assembly of other wall components and influence the orientation of cell growth through their interaction with microtubules comprised of polymers of the tubulin protein (Barlow and Baluska, 2000). Cell expansion results from internal hydrostatic pressure (turgor) which expands the cell wall (Pritchard, 1994). However, the turgor pressure has no preferential direction so that the preferential longitudinal expansion which predominates in the zone of elongation is believed to be a consequence of differential depositions or modifications of cell wall materials mediated by microtubule-directed processes (Barlow and Baluska, 2000). Microtubules, together with actin microfilaments, form a cytoskeleton that confers structural order and stability to the

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