Figure 3.4. Root tips of Stenocereusgummosus at (A) 24 hours, (B) 36 hours, and (C) and (D) 48 hours after the start of radicle protrusion. Root hairs (arrows) approach the tip in (A) and (B) and later cover the tip completely in (C). In (D), the five most apical cells (arrow) have not formed root hairs. Scale bars: A = 400 pm, B-C = 100 pm, D = 50 pm.
root and subsequently cover it completely (Fig. 3.4; Dubrovsky 1997b).
The size of the root apical meristem for cacti with determinate root growth is relatively small, with 12 to 21 cells in a cell file (Table 3.1). The cells in the root apical meristem divide relatively quickly, every 10 to 14 hours for Stenocereus gummosus and 12 to 17 hours for Ferocactus peninsulae (Dubrovsky et al. 1998a). A comparison of the duration of the period of steady-state growth (Dubrovsky i997a,b) and the duration of the cell division cycle in the root apical meristem (Dubrovsky et al. 1998a) shows that, on average, only two cell division cycles occur in the root apical meristem in both species. Assuming that meristem-atic activity is maintained until the meristem is exhausted, the maximum number of cycles is four in S. gummosus and five in Fpeninsulae. Thus, the determinate pattern of root growth in these cactus species represents a normal developmental path, during which only a few cell division cycles take place.
For angiosperms, lateral roots originate mainly from the pericycle of a parent root. For O. basilaris, the pericycle cells opposite the protoxylem give rise to lateral root pri-mordia early in seedling development (Freeman 1969). For F. peninsulae and S. gummosus, lateral root primordia can be detected 4 to 5 days after germination (Dubrovsky i997a,b). The time from primordium initiation to lateral-root emergence is relatively short for these species and comparable to that of other angiosperms (Dubrovsky 1997b). During lateral root development, a vascular junction between the parent root and a lateral root is established, consisting of relatively short tracheary elements with large areas of non-lignified primary cell walls (North et al. i992), phloem elements, and vascular parenchyma cells. Early in development, lateral root primordia are internal to the periderm, cortex, and epidermis of the parent root (North et al. 1992; North and Nobel 1992). For Ferocactus acanthodes, the inner layers of the periderm (the phelloderm layers) of the parent root are continuous with the base of the lateral root, whereas the suberized layers of the periderm (the phellem layers) of the parent root are ruptured as the lateral root elongates (North et al. 1992).
The initiation of lateral root primordia is apparently promoted by drought. For example, the number of second-order lateral root primordia for F acanthodes is significantly greater for plants subjected to drought than for plants under wet conditions (North et al. 1993). In distal root segments of O. ficus-indica, four to five more lateral root primordia develop during soil drying than under wet conditions (Dubrovsky et al. 1998b). Similarly, the initiation of lateral root primordia in the epiphytes E. phyl-lanthus and Rhipsalis baccifera is stimulated by soil drying (North and Nobel 1994). Because the rate of root growth decreases during drought (Dubrovsky et al. 1998b), the occurrence of primordia closer to the root tip than under wet conditions may reflect reduction in parent root growth, induction of lateral roots, or both. The developmental signals for lateral-root initiation may also include changes in hormone levels due to the death of the parent root tip.
Lateral root elongation after drought is induced by rain or watering, and such lateral roots have been called "rain" roots, although a more general term is "ephemeral roots," because such roots tend to be short-lived (Nobel 1988). Ephemeral roots can emerge rapidly, for example, within 8 hours of watering for both Opuntia puberula (Kausch 1965) and F. acanthodes (Nobel and Sanderson 1984). Within 24 hours of watering O. ficus-indica after 14 days of drought, lateral roots are 2 to 4 mm long, and lateral root growth rate during the second day after emergence is 9.7 mm day-1 (Dubrovsky et al. 1998b). Apparently these roots emerge from primordia formed during drought, but further analysis is needed. Ephemeral roots are important for a rapid increase in absorbing surface area without a substantial increase in the distance for water transport (Cannon 1911; Jordan and Nobel 1984; Dubrovsky et al. 1998b). For example, for F acanthodes the total root length increases by 27% because of ephemeral root formation (Jordan and Nobel 1984). During subsequent drought, fine lateral roots can abscise (North et al. 1993), and have thus been called "deciduous roots" (Cannon 1911).
The type and extent of root systems in cacti can vary in response to both external and internal factors. For example, cladodes of Opuntia versicolor planted in adobe (clay-containing) soil produce some vertically oriented anchoring roots and some horizontally oriented absorbing roots, whereas cladodes planted in sand produce roots at seemingly random angles (Cannon 1925). The formation of a root system is not necessarily a continuous process in a desert. For example, a seedling may develop roots, then lose them during a subsequent drought, and develop another root system upon re-watering. The duration of drought, the shoot biomass (and thus water storage capacity), and the ability to form sequential root systems all affect seedling survival for Stenocereus thurberi (Dubrovsky 1996, 1998, 1999). Most seedlings having fresh weights of 25 to 75 mg lose their root systems completely during a 40-day drought. After rehydration, adventitious roots grow from the basal portion of the hypocotyl, forming a new root system that is larger than the original one (Dubrovsky 1999). Seedlings and young plants in the desert may develop a number of root systems before forming a lasting, adult root system.
The architecture of a root system is partially determined by whether roots exhibit determinate or indeterminate growth. For plants with determinate primary root growth (e.g., S. gummosus), some lateral roots appear to have indeterminate growth. In an adult plant of this species, long rope-like roots can be found, extending 5 to 6 m away from the plant (Dubrovsky 1999). Upon closer inspection, however, such roots are not formed by continuous growth of the root apical meristem, but instead represent a series of interconnected lateral roots (J. G. Dubrovsky, unpublished observations), similar to sympo-dially branched roots described by Boke (1979).
Adaptations to Deserts and
Other Arid Environments
Most desert cacti can be classified as shallow-rooted perennials (Rundel and Nobel 1991). In the Sonoran Desert, the roots of most cacti usually grow no deeper than 15 to 30 cm below the soil surface, although the roots of some species can extend laterally more than 10 m away from the plant base (Cannon 1911). Not surprisingly, the deepest roots are found for columnar cacti. A young plant of Carnegiea gigantea, 1.2 m tall, had a stout taproot that penetrated 30 cm and lateral roots that extended 1.5 to 5 m away from the plant, whereas an older, 6.8-m-tall plant had lateral roots up to 9.7 m long that penetrated to a depth of 77 cm (Cannon 1911), perhaps the deepest cactus roots on record.
Root proliferation and elongation is essential for continued water and mineral uptake, and for competition for these resources with other plants. For Ferocactus acanthodes at a site in the Sonoran Desert, the dry weight of the whole root system averages only 14% that of the shoot; however, the total surface area of the root system is about 3 times greater than that of the shoot, and the total length of the main roots in the root system per plant averages 182 m (Jordan and Nobel 1984). Roots of Opuntia polyacantha growing in the shortgrass steppe of Colorado have a median root depth of less than 2.5 cm (Dougherty et al. 1996). For O. polyacantha, as little as 2.5 to 5 mm of precipitation significantly increases cladode biomass. Indeed, the frequency of rain is more important than the absolute amount of rain, due in part to competition with more deeply rooted grasses that capture water from deeper soil levels (Dougherty et al. 1996). In this case, a shallow root distribution not only helps to exploit light rainfall, but also gives the cactus an edge in competition with other plants.
The root elongation rate depends on temperature. For F. acanthodes, root growth in response to temperature can be described by a bell-shaped curve, with maximal elongation at 3o°C (Jordan and Nobel 1984). Cactus roots in natural environments are frequently exposed to temperatures higher than optimal; for example, in the northeastern Sonoran Desert, the maximum temperature 5 to 10 cm below the soil surface can be 40 to 5o°C (Jordan and Nobel 1984). Cannon (1916) reported that at 43°C, roots cease growing. At 6o°C, the root cells of F acanthodes die, although high temperature tolerance of roots in this species can be increased by acclimation. Roots of plants acclimated at day/night temperatures of 45/35^ survive at temperatures 4°C higher than those of plants acclimated at 3o/2o°C (Jordan and Nobel 1984).
Boulders and subterranean rocks, which are common in desert environments, can provide favorable microsites for cactus root systems. Cannon (1911) observed that rocks stimulated root branching of Opuntia phaeacantha var. discata in the Sonoran Desert. Similarly, roots of Echinocereus engelmannii are more commonly found alongside boulders than at increasing distances away, and lateral roots of F acanthodes are 5.5 times longer and 3 times more numerous under rocks than in regions of the soil without rocks (Nobel et al. 1992b). Such increased growth and branching are explained primarily by a longer period of water availability, as the soil water potential decreases sharply with distance away from rocks and, after soil wetting, remains higher under rocks than in rock-free soil (Nobel et al. 1992b). In addition to creating locally moist microsites, rocks may also be associated with regions of higher nutrient concentrations. During active growth, roots exude carbohydrates to the soil (Huang et al. 1993) and, during subsequent drought, ephemeral roots die. Organic matter may thus accumulate in the vicinity of rocks, promoting new root growth near the rocks when water is again available.
Soil drying generally decreases the rate of root growth, although the rate of drying is critical in determining whether apical elongation can continue. For example, when Opuntia ficus-indica is subjected to gradual drying of the substrate, roots have sufficient time for developmental changes to occur, whereas rapid substrate drying leads to death of the apical meristem. The meristem and elongation zone become shorter when the substrate dries gradually,
whereas under rapid drying roots die because insufficient time for such rearrangements is available (Dubrovsky et al. 1998b). Similarly, plants of F acanthodes that have been previously exposed to drought suffer less inhibition in root growth under newly imposed water stress than do plants that have not previously been so exposed (Jordan and Nobel 1984).
For a number of desert and epiphytic cacti, soil drying also appears to enhance the development of rhizosheaths, which are rough cylinders around the roots, composed of soil particles that are bound to root hairs and other epidermal cells by mucilage exuded by the roots (Fig. 3.5; Huang et al. 1993; North and Nobel 1992, 1994). Such rhi-zosheaths become thicker and more cohesive during drought and help improve root water relations, both by ensuring good contact between the root and wet soil and by helping to reduce water loss from the root to a drier soil (North and Nobel 1997).
Determinate root growth can be viewed as a developmental program well suited to desert conditions. For example, meristem exhaustion in the primary root of Stenocereus gummosus coincides with or perhaps triggers the initiation of lateral root primordia. The percentage of roots bearing lateral root primordia is correlated with the percentage of roots in which the root apical meristem is exhausted (Dubrovsky 1997a, 1997b). Determinate primary root growth is thus a developmental mechanism leading to the rapid induction of lateral root formation and root system development. In most desert habitats, the optimum period for seed germination is extremely brief; thus, the rate of root-system formation is a critical factor for successful seedling establishment. Because many of the lateral roots of S. gummosus also have determinate growth, a compact root system is formed that requires limited carbon input but is sufficient for water and mineral uptake (Dubrovsky 1997b, 1998).
A related developmental feature with adaptive significance is the relatively short duration of the cell cycle in the root apical meristem for cacti with both determinate and indeterminate root growth. Rapid root elongation and root branching are possible only when new cells are produced rapidly. A relatively short cell cycle can thus be advantageous, particularly during the critical stage of seedling establishment. At a later stage, when established roots resume both apical growth and branching after a drought, a relatively short cell cycle with its high rate of cell production should also enhance the rate of colonization of new soil regions by the roots.
Early root hair production, as seen for S. gummosus, represents another developmental adaptation. For this species, root hairs develop almost at the onset of seed germination. When the radicle is still very small, root hairs are evident and are frequently longer than the radicle itself (Fig. 3.4A). The basal epidermal cells, embryonic in origin, average 18 pm in length, whereas the root hairs formed by these cells average 100 times longer (1.8 mm). Each epidermal cell is capable of forming a root hair, unlike the usual case for plants. Later, the epidermal cells that are formed due to root meristem activity average 98 pm in length (Table 3.1). Such early root hair formation, occurring even before germination is completed, can maximize the root surface area available for absorption, thereby increasing water and mineral uptake during the relatively short optimum growth period in a desert (J. G. Dubrovsky, unpublished observations).
Water and Mineral Uptake
For both desert and epiphytic cacti, soil moisture varies greatly in both time and space. The success of cacti faced with such heterogeneity in water availability depends on the ability of their roots to conduct water quickly when it is available, to resist water loss when the soil becomes dry, and to resume water uptake upon the cessation of drought. The ability of roots to absorb and transport water is quantified by the root hydraulic conductivity, or Lp. The units of Lp (m s-1 MPa-1, where MPa is 106 pascals) indicate that a volume of water moves across the root surface area per unit time in response to a difference in pressure (such as the difference between the water potentials of the plant and the soil; Nobel 1999). For roots, Lp has two components, the radial conductivity, which determines the rate of
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