Root Structure And Function

Joseph G. Dubrovsky and Gretchen B. North

Introduction Structure

Primary Structure Secondary Structure Root Types Development and Growth Indeterminate Root Growth Determinate Root Growth Lateral Root Development Root System Development Adaptations to Deserts and Other Arid Environments Root Distribution in the Soil Environmental Effects on Root Development Developmental Adaptations Water and Mineral Uptake Root Hydraulic Conductivity Mineral Uptake

Mycorrhizal and Bacterial Associations Carbon Relations Conclusions and Future Prospects Literature Cited

Introduction

From the first moments of a plant's life cycle, including germination, roots are essential for water uptake, mineral acquisition, and plant anchorage. These functions are especially significant for cacti, because both desert species and epiphytes in the cactus family are faced with limited and variable soil resources, strong winds, and frequently rocky or sandy habitats. The goals of this chapter are to review the literature on the root biology of cacti and to present some recent findings. First, root structure, growth, and development are considered, then structural and developmental adaptations to desiccating environments, such as deserts and tropical tree canopies, are analyzed, and finally the functions of roots as organs of water and mineral uptake are explored.

Structure

Cactus roots are less overtly specialized in structure than are cactus shoots. Even so, root structural properties are fundamental to the ability of cacti to take up water and nutrients quickly, and to endure and recover from drought. An understanding of the relationship between root structure and function is essential to understanding how cacti are able to occupy some of the driest, most nutrient-poor habitats on earth.

Primary Structure

During embryogenesis, an embryonic root, or radicle, is formed. In most cactus species, the radicle is relatively small; for example, for Echinocactusplatyacanthus the radicle is 320 pm long with a compact root cap of four cell layers covering the tip (Lux et al. 1995). Similarly, a small radicle is a typical feature in Astrophytum myriostigma, Thelocactus bicolor (Engelman 1960), and Stenocereusgum-mosus (Dubrovsky 1997b). Meristematic activity at the radicle apex begins approximately 12 hours after the radicle emerges from the seed coat for S. gummosus and Fero-cactuspeninsulae var. townsendianus (Dubrovsky 1997b). As a result of activity in the root apical meristem, roots grow in length, and the primary root tissues are formed (Esau 1977). The organization of the root apical meristem has been analyzed fully for Opuntia basilaris (Freeman 1969) and illustrated for a few other species. The roots of most cacti appear to have a closed apical organization in which each tissue can be traced to initial cells at the apex, as seen for O. basilaris (Freeman 1969), O. arenaria (Boke 1979), and E. platyacanthus (Lux et al. 1995).

Probably the best-studied species with respect to root development and structure is O. ficus-indica. The radial pattern of the primary root structure in O. ficus-indica does not differ significantly from that of most other dicotyledonous species (North and Nobel 1996). For this species, the external tissue—the epidermis—is composed of compact cells, some of which produce root hairs (Fig. 3.1A). Underlying the epidermis is the cortical tissue complex, which includes the hypodermis (the outermost cortical layer), the cortex proper, and the endodermis (the innermost cortical cell layer). The tissue complex located inward from the endo-dermis is the vascular cylinder. It comprises a two- or three-cell-layered pericycle and the vascular system, consisting of the xylem, the phloem, and the vascular parenchyma. The root vasculature is polyarch, usually with five to seven xylem poles in cylindropuntias (Hamilton 1970) and with four to eight xylem poles in platyopuntias (Freeman 1969). The pith is composed of parenchyma cells, as seen in O. basilaris

(Freeman 1969). Occasionally, mucilage cells are found in the primary root (Hamilton 1970).

Differentiation of primary tissues starts soon after cell division stops in the meristem. For O. basilaris, the pro-tophloem is first evident at 340 pm from the root cap-root body junction; the protoxylem is first evident at 500 pm and is fully differentiated at 1,400 pm. Casparian strips in the endodermis occur at 500 pm from the junction. The metaxylem begins to develop at the base of the transition zone (region between the root and the hypocotyl) 4 to 5 days after germination and later can be found 1.2 mm from the root apex (Freeman 1969). Primary tissue development is unusually rapid in that as early as 6 days after germination the pericycle cells start to produce the periderm (Freeman 1969), which is the first secondary tissue to develop in platyopuntia roots.

Secondary Structure

For O. ficus-indica, Ferocactus acanthodes, and two epiphytic cacti, Epiphyllum phyllanthus and Rhipsalis baccifera, periderm layers (radially flattened cells just outside the pericycle) are well developed at about 150 to 200 mm from the root tip in young roots. Even young seedlings of cylindropuntias have roots with several corky (suberized) layers (Hamilton 1970). Such layers are more numerous and more heavily suberized closer to the tip of roots that have experienced drought than is the case for roots of well-watered plants (North and Nobel 1992). Back from the root tip, in regions approximately 2 to 4 months old, the cortex external to the periderm dies and is shed (Fig. 3.1B), a process that is also hastened by soil drying. Later in development, the outermost layers of the periderm are also shed as the vascular cylinder enlarges due to secondary growth. For the epiphyte R. baccifera, radial fissures open in the outer suberized layers of the periderm as roots swell upon re-watering after drought, thereby enhancing water uptake (North and Nobel 1994).

Within the vascular cylinder of most cactus roots, secondary growth produces wedge-shaped regions of vessels and fibers, separated by rays of parenchyma (Fig. 3.1C). For several species, including platyopuntias such as O. ficus-indica, large mucilage cells develop in the parenchyma rays, with a possible consequence for regulating water relations within the vascular cylinder (Preston 1901b; Gibson 1973; North and Nobel 1992; Loza-Cornejo and Terrazas 1996). Other characteristics associated with the parenchyma in the secondary xylem can be the occurrence of calcium oxalate crystals (Fig. 3.1D), the storage of starch, and the development of succulence. With respect to the xylem vessels themselves, secondary growth leads to a nearly

Root Structure Cactus

Figure 3.1. Median cross-sections of (A) a 1-month-old root of Opuntia ficus-indica, showing primary root tissues; (B) a 3-month-old root of Epiphyllum phyllanthus, with cortex separating from the periderm; (C) a 3-month-old root of O. ficus-indica, showing secondary growth; and (D) a 5-month-old root of Rhipsalis baccifera, with arrow indicating a calcium oxalate crystal. Cell types shown are epidermis (ep), hypodermis (h), cortex (c), endodermis (en), pericycle (p), periderm (per), and xylem (x). Scale bars: A = 50 pm, B = 500 pm, C-D = 100 pm.

Figure 3.1. Median cross-sections of (A) a 1-month-old root of Opuntia ficus-indica, showing primary root tissues; (B) a 3-month-old root of Epiphyllum phyllanthus, with cortex separating from the periderm; (C) a 3-month-old root of O. ficus-indica, showing secondary growth; and (D) a 5-month-old root of Rhipsalis baccifera, with arrow indicating a calcium oxalate crystal. Cell types shown are epidermis (ep), hypodermis (h), cortex (c), endodermis (en), pericycle (p), periderm (per), and xylem (x). Scale bars: A = 50 pm, B = 500 pm, C-D = 100 pm.

threefold increase in mean vessel diameter for O. ficus-indica and F7 acanthodes, and a seven- to tenfold increase in vessel number during 12 months of growth (North and Nobel 1992). For the epiphytes E. phyllanthus and R. bac-cifera, mean vessel diameter increases only slightly during 3 months of growth, but vessel number also increases about tenfold (North and Nobel 1994). Such increases in vessel diameter and number are accompanied by large increases in the rate of water transport in the xylem (North and Nobel 1992, 1994).

Root Types

Different types of roots can be classified according to their developmental origin. For example, a root that develops from the embryonic radicle is termed a primary root. Later, when the primary root reaches a certain length, lateral roots are formed. Any root formed on another root is considered a lateral root. When a root is formed on an organ other than a root, it is termed an adventitious root. Cladodes of O. ficus-indica readily produce adventitious roots at or near areoles (Fabbri et al. 1996; Dubrovsky et al. 1998b), reflecting localized activity in the vascular cambium (Villalobos 1995). For Pereskia, adventitious roots can be formed on leaf petioles (Carvalho et al. 1989). Adventitious roots form along the stems of many decumbent, prostrate, and epiphytic cacti, most of which never develop elongated primary roots (Gibson and Nobel 1986). Adventitious rooting of fallen stem segments allows desert species, such as O. bigelovii, to reproduce vegetatively, and the larger water storage capacity of such rooted segments assures greater drought tolerance than is the case for much smaller seedlings. For epiphytic cacti, adventitious rooting along stems can improve anchorage in the canopy, and enables dislodged stem segments to take root where they land on host species (Andrade and Nobel 1997). The ability to produce adventitious roots is also useful for clonal propagation of O. ficus-indica and other agronomic species (Le Houérou 1996).

Cactus roots can also be classified according to their function and position within a root system. A century ago, Carleton Preston from Harvard University defined anchoring versus absorbing roots in different cactus species and found some anatomical differences in these root types related primarily to the thickness of the vascular cylinder (Preston 1900, 1901b). William Cannon from the Desert Botanical Laboratory also used these terms, stating that anchoring roots can be: (1) vertically oriented, deeply penetrating, taproots; or (2) horizontally oriented, supporting roots (Cannon 1911). Cannon divided absorbing roots into two categories: (1) rope-like roots and (2) filamentous, relatively thin roots (Cannon 1911). This functional descriptive classification is not absolute, because each root type can have a few functions simultaneously (Preston 1900; Cannon 1911).

Two other root types with morphological modifications are succulent roots and tuberous storage roots, each of which can have some characteristics of the other. Cannon (1911) reported fleshy roots in O. vivípara, and first recognized water storage capabilities of the roots of some cactus species. Thick succulent roots (that occasionally include the hypocotyl-root transition zone) can be found in small cacti, such as species of Ariocarpus (Britton and Rose 1963; Bravo-Hollis and Sanchez-Mejorada 1978), Aztekium (Porembski 1996), Leuchtenbergia (Britton and Rose 1963), and Lophophora (Nobel 1994). For the columnar cactus Pachycereuspringlei, the fleshy taproot can be 18 cm thick near its base (J. G. Dubrovsky, unpublished observations). Succulence develops within the secondary xylem in Maihuenia patogonica, Nyctocereus serpentinus, Opuntia macrorhiza, O. marenae, Pereskia humboldtii, Pterocactus tuberosus, and Tephrocactus russellii, and in cortical ground tissue in Neoevansia diguetii and Peniocereus greggii (Gibson 1978). Generally, water storage capacity (capacitance) is relatively small in cactus roots compared to shoots (Nobel 1996). For succulent roots, however, the capacitance is greater than for nonsucculent roots, and may be comparable to that of the water-storage parenchyma in stems. Water-storage tissue in succulent roots has the abil ity to withstand a high degree of dehydration without irreversible damage, and may also help prevent water loss and decrease root shrinkage during drought.

In addition to storing water, cactus roots frequently accumulate starch. To accommodate starch reserves, the roots of some species acquire a distinct morphology. A relatively large, subterranean storage root is characteristic of cacti that are geophytes; such roots give rise to aboveground annual shoots that shrivel and die during drought and are regenerated the following year, when water is available (Gibson 1978; Gibson and Nobel 1986). Typical geophytes in North America are species of Neoevansia, O. chaffeyi, Peniocereus, and Wilcoxia, and in South America, Pterocactus tuberosus (Gibson 1978). Tuberous roots of Wilcoxia poselgeri and W. tamaulipensis are characterized by starch-storing parenchyma, primarily in the cortex, along with mucilage cells in the pith, the cortex, and the vascular tissue complex (Loza-Cornejo and Terrazas 1996). Tuberous roots can be sizable; e.g., those of Peniocereus greggii grow up to 60 cm in diameter, 15 to 20 cm long, and have a weight of 27 to 56 kg (Britton and Rose 1963). Non-geo-phytes can also develop one or a few tuberous or tuber-like roots. Groups of tuber-like roots 1 to 2.5 cm in diameter occur for O. arbuscula (Cannon 1911), O. marenae, and O. reflexispina (Felger and Moser 1985). Single tuber-like taproots occur for other non-geophyte species, such as Ancistrocactus megarhizus (Britton and Rose 1963), Escobaria henricksonii (Glass 1998), Thelocactus mandragora (Bravo-Hollis and Sanchez-Mejorada 1978), and T subter-raneus (Higgins 1948).

Another specialized root type—aerial roots—are rarely produced by desert cacti. However, aerial roots can occur for S. gummosus in the Sonoran Desert (Dubrovsky 1999). This species has decumbent stems that form adventitious roots when branches touch the soil. Aerial roots can develop before such contact, on the lower part of the convex stem or on other portions of the stem (Fig. 3.2). These roots are short, succulent, and sometimes extensively branched, with secondary growth, and can be 3 to 4 mm or more in diameter (Dubrovsky 1999). A possible role for such roots in dew uptake remains to be studied. Under greenhouse conditions, O. arenaria is also capable of forming numerous aerial roots (Boke 1979). In epiphytic and climbing species, aerial root development is a common phenomenon, as in plants from the genera Epiphyllum, Hylocereus, and Selenicereus (Bravo-Hollis and Sánchez-Mejorada 1978).

Root systems can be composed of several different root types and in many combinations. Nevertheless, three basic morphological patterns of root systems are recognized

Figure 3.2. Aerial roots on the shoot of Stenocereus gummosus in the Sonoran Desert.

(Cannon 1911; Gibson and Nobel 1986). The first type is composed of a taproot with few or no lateral roots, as seen for geophytes and species with succulent roots, such as in the genera Lobivia and Lophophora (Gibson and Nobel 1986). The second type of root system is composed of a taproot and horizontal, subsurface lateral roots and/or adventitious roots, as occurs for most columnar cacti and species of Ferocactus (Cannon 1911; Gibson and Nobel 1986). The third type lacks a taproot and consists of roots of different lengths, with small species tending to have numerous branched roots directly beneath the shoot, and larger species tending to have long subsurface roots extended some length from the shoot, as seen in several species of Opuntia (Gibson and Nobel 1986).

Development and Growth

Root development and growth are important, both during the early stages of a plant's life cycle (particularly for seedling establishment) and, later, as continued shoot growth requires that roots invade new areas to obtain water and nutrients. Increases in root surface area are the result of two processes: (1) root elongation, which involves cell production by the root apical meristem; and (2) root branching, or the production of lateral roots. Cells within the root apical meristem can proliferate for an indefinite period, exhibiting indeterminate growth, or they can lose such ability after a limited period, exhibiting determinate growth. The amount and pattern of root branching depends, in part, on whether main roots are characterized by indeterminate or determinate growth.

Indeterminate Root Growth

Indeterminate root growth is common in most flowering plants, including cacti. For example, adventitious roots of Opuntia ficus-indica are characterized by indeterminate growth, insofar as cell production by the root apical meristem continues for a relatively long period. The tips of main roots of O. ficus-indica generally die after a few months of growth; however, death occurs more quickly in dry soil than in wet soil (Dubrovsky et al. 1998b; G. B. North, unpublished observations). By analyzing the cell lengths along the root, three main root zones can be determined for O. ficus-indica: (1) the meristem (where cells are relatively small and are in the cell division cycle), (2) the elongation zone (where cells start and nearly complete rapid elongation), and (3) the differentiation zone (where cells complete their elongation and start to acquire certain tissue characteristics). The root apical meristem of a main adventitious root of O. ficus-indica is relatively large—on average 1.1 mm long—consisting of 82 cortical cells in a cell file (Dubrovsky et al. 1998b), comparable to the root meristem of most crop plants. The growing part of the root (the meristem and the elongation zone) in this species is 5 to 7 mm long. The primary root of a Sonoran Desert species, Pachycereus pringlei, exhibits indeterminate growth only

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