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Data are from North and Nobel (1992, 1994).

Data are from North and Nobel (1992, 1994).

water flow across the root tissues up to the xylem (and which generally is similar in value to Lp), and axial conductivity, which determines the rate of flow through the xylem.

Despite the ability of most cactus roots to endure drought, their Lp under wet soil conditions (Table 3.2) is comparable to that of many mesophytic species, such as Zea mays (1.0-2.8 X 10-7 m s-1 MPa-1; Steudle et al. 1987) and Phaseolus vulgaris (1.4-4.0 X10-7 m s-1 MPa-1; Sands et al. 1982). Three- to 5-month-old roots of both terrestrial and epiphytic cacti have a higher Lp than do i-month-old roots under wet conditions, due to the higher resistance of the cortex and the immaturity in the xylem of younger roots (North and Nobel 1992, 1994, 1997). During 3 to 4 weeks of soil drying, Lp decreases to about 50% of its value under wet conditions, averaged for young and older roots of the four cactus species (Table 3.2). The decrease is primarily due to reductions in radial conductivity, caused by increased suberization in the periderm, and secondarily to decreases in axial conductivity, due to embolism in the xylem (North and Nobel 1992, 1994, 1996). During the same period of soil drying, root diameter shrinks by i9% for roots of Opuntia ficus-indica with rhizosheaths, by 26% for bare roots of this species, and by i3% for older bare roots of Epiphyllum phyllanthus (North and Nobel 1992, 1994). Such shrinkage causes air gaps to develop between the root and the soil, which decrease water loss from the root that would otherwise occur due to the higher water potential of the root than of the soil during drought (Nobel and Cui 1992).

In response to soil rewetting after drought, Lp's for roots of both terrestrial and epiphytic cacti increase to equal or exceed their pre-drought values, with the exception of older roots of Ferocactus acanthodes and O. ficus-indica (Table 3.2). The layers of periderm in these older roots increases in number and in extent of suberization during drought, with a concomitant reduction in their permeability to water (North and Nobel 1992, 1996). Younger root regions lack a periderm, and their permeability is restored by simple rehydration of tissues that, in addition, are protected during drought by the rhizosheaths (North and Nobel 1997). For the younger roots, and for root regions near the junctions between main and lateral roots, numerous lateral root primordia arise during drought and elongate upon rewetting. As these new lateral roots emerge from the parent root, they break through the suberized layers of periderm, thereby increasing root permeability and Lp (North et al. 1993; Dubrovsky et al. 1998b). Once new lateral roots have emerged, the root system for these cacti is capable of rapid water uptake, allowing depleted storage reservoirs to be refilled.

Mineral Uptake

The ability of roots to take up minerals is directly related to their growth. In addition, root growth depends on the nutrient status of the soil. When a mineral resource is in limited supply in the soil, root systems increase in length to explore more area around the plant. As a consequence, relative root biomass (or the root/shoot ratio) tends to be higher in poor soil than in richer soils (Marschner 1986).

Such is the case for cactus plants. In the Sonoran Desert, trees with relatively large canopies (e.g., Prosopis articula-ta) that serve as nurse plants for many cactus species (Nobel 1988) tend to be "islands" rich in nutrients. The concentration of nitrogen (N), phosphorous (P), and carbon (C) in such island soil is 1.4, 1.6, and 1.8 times greater, respectively, than in a treeless region; correspondingly, the root/shoot ratios for plants of Pachycereus pringlei grown in island soil are smaller than for plants grown in soil from a treeless region (Carrillo-Garcia et al. 2000).

Mineral uptake by roots can be assessed indirectly from an analysis of the contents of different elements in cactus stem tissues. By abundance, the elements in the stem tissue rank as follows: Ca > K > N > Mg > Na > P > Fe > B > Mn > Zn > Cu > Mo, with the chlorenchyma having higher levels of Ca, Mg, B, and Zn than are found in most agronomic plants (Berry and Nobel 1985). Element concentrations in the cactus stem tissues can be more than 103 times greater than in the root substrate, implicating active uptake and transport of ions by the roots (Berry and Nobel 1985; Kolberg and Lajtha 1997). Like cactus stems, roots also accumulate certain elements in their tissues. For example, when boron (B) is applied at a concentration of 15 ppm to soil in which O. ficus-indica and F. acanthodes are grown, the content of the element in the roots of both species is about 145 ppm. Interestingly, the level of B increases to 2,000 ppm in stem tissues of O. ficus-indica and to only 220 ppm in Ff acanthodes (Berry and Nobel 1985), indicating species differences in element translocation from the root to the shoot. Different species can also vary in their sensitivity to heavy metals. For example, when high concentrations of copper (Cu) and zinc (Zn) are added together to a substrate, the root dry weight of O. ficus-indica is reduced more than that of F acanthodes (Berry and Nobel 1985).

Mycorrhizal and Bacterial Associations

Mineral acquisition is frequently related to the activity of fungi and bacteria in the rhizospere. Mycorrhizal associations, important for mineral uptake in many plant species, occur in the root systems of a number of cacti. The characteristic structures indicating infection by vesicular-arbuscular mycorrhizae can be detected in a cleared, longitudinally dissected root of E pringlei. During the first stages of root colonization, the fungus forms an adherent apressorium on the root surface. Subsequently, fungal hy-phae penetrate the root, apparently multiplying the internal surface area available for the absorption of limiting mineral nutrients such as P and iron (Fe). In the Sonoran Desert, the level of mycorrhizal colonization ranges from less than 10% of the roots examined in Mammillaria dioica, P. pringlei, Stenocereus gummosus, and S. thurberi, to 30 to 70% in Ff peninsulae, to more than 70% in Cochemia poselgeri, Lophocereus schottii, Opuntia cholla, and O. lind-sayi (Carrillo-Garcia et al. 1999). Extensive colonization by three different fungal species in the genus Glomus also occurs for roots of Echinocactus acanthodes, Echinocereus en-gelmannii, O. acanthocarpa, O. basilaris, O. bigelovii, and O. echinocarpa (Bethlenfalvay et al. 1984). Mycorrhizal associations are found in tropical forest cacti as well, including Nopalea karwinskiana, O. excelsa, and O. puberula (Allen et al. 1998). For these forest species, mycorrhizal infection increases in proportion to fine root production, which, in turn, is determined by the rainfall pattern (Allen et al. 1998).

Free-living nitrogen-fixing bacteria from the genus Azospirillum, present in the rhizosphere of many plant species (Kapulnik 1996), have been isolated from cactus roots as well. For example, A. lipoferum occurs in the rhi-zosphere of species of Opuntia growing in India (Rao and Venkateswarlu 1982) and Mexico (Mascarua-Esprarza et al. 1988). Another species, A. brasilense, which occurs in the rhizosphere of O. ficus-indica, S. pruinosus, and S. stellatus, shows nitrogenase activity (ability to fix atmospheric nitrogen) and also exudes the plant hormone auxin, which may induce root branching (Mascarua-Esprarza et al. 1988). When young seedlings of P. pringlei are inoculated with A. brasilense, the bacteria survives in the plant rhi-zosphere for up to 300 days (Puente and Bashan 1993). In another experiment, inoculation with A. brasilense increases root length but not shoot size, and nitrogenase activity is not detected (Carrillo-Garcia et al. 2000). However, bacteria showing acetylene reduction activity (indicative of nitrogenase activity) are eleven times more abundant in the rhizosphere of ten species of cacti in Mexico than in adjacent bare soil (Loera et al. 1996). A likely role for rhizosheaths in providing conditions favorable to the growth of beneficial bacteria has yet to be explored for cacti.

Carbon Relations

In comparison with most other plants, cacti invest relatively little carbon into the construction and maintenance of roots. This is partly due to the extremely small root/ shoot ratio of most succulents (Nobel 1988; Rundel and Nobel 1991), particularly when expressed on a fresh weight basis. It is also due to the relatively low rates of root respiration. Specifically, root respiration, as measured by total CO2 efflux, is 0.7 and 0.3 mol CO2 kg-1 day-1 for young and older roots, respectively, of Ferocactus acanthodes, and i.i and 0.5 mol CO2 kg-1 day-1 for young and older roots of Opuntia ficus-indica (Palta and Nobel 1989). Comparable rates for the roots of twelve nondesert angiosperms average 4.8 mol CO2 kg-1 day-1 (Lambers 1979). Under drying conditions, root respiration for F. acanthodes and O. ficus-indica declines even further, averaging 14% of the rate under wet conditions 8 days after water is withheld (Palta and Nobel 1989). The rate of growth respiration, measured as CO2 given off by newly initiated roots, is also low for cacti in comparison to other plants, averaging about 9 mol CO2 kg-1 day-1 for F7 acanthodes and O. ficus-indica (Nobel et al. 1992a), in contrast to 24 mol CO2 kg-1 day-1 for non-desert angiosperms (Lambers 1979). Carbon costs are also involved with maintaining mycorrhizal associations and with the creation of rhizosheaths, although young sheathed roots of O. ficus-indica exude only about 1% of newly fixed carbon to the soil (Huang et al. 1993).

Conclusions and Future Prospects

Roots and root systems of cacti have evolved structural and physiological features that permit them to withstand environmental stresses, such as high temperatures, prolonged drought, nutrient-poor soils, and strong winds. Developmental adaptations, such as the early formation of root hairs, lateral roots, and periderm, are most significant during the critical period of seedling establishment. The development of rhizosheaths is important for taking up water from moist soil and reducing water loss to dry soil, and the formation of lateral root primordia during drought hastens plant recovery when soil moisture is restored. The shallow distribution of roots in desert and grassland soils helps cacti to exploit limited rainfall, at times in competition with more deeply rooted neighboring plants. Root associations with fungi and bacteria can help in the efficient capture of limited mineral nutrients.

A number of structures and processes in roots of the Cactaceae deserve further investigation. For example, a century ago it was known that roots of Opuntia arbuscula are capable of producing shoots (Preston 1901a), and similar "root buds" have been described for O. arenaria (Boke 1979). New shoots also appear to arise from the roots of Myrtillocactusgeometrizans (J. G. Dubrovsky, unpublished observations). Although root buds are a known phenomenon in angiosperms (Peterson i975), their occurrence in cacti has not been studied. Despite many accounts of ephemeral roots, little is known about root phenology and root plasticity in cacti. The relationship between root growth and shoot activity and how it is affected by environmental variables, such as precipitation, needs to be investigated, particularly in the field. As an example, an un demanding of how cactus roots respond to rain occurring in the middle of a summer drought is important for predicting how desert communities will respond to possible climate changes. Studies of mycorrhizal and bacterial associations with cactus roots will also help elucidate phenomena that are less well known for deserts and for tropical canopies than for other plant communities. The nurse-plant association between cacti and other perennial species deserves to be investigated from the perspectives of root competition and root communication, both processes that may also be influenced by fungal and bacterial activity.

In addition to the ecological questions remaining to be addressed for cactus roots, certain basic developmental and physiological processes should be explored for species that can withstand prolonged water stress, such as Opuntia ficus-indica. For example, the effects of soil drying on pro-teinaceous water channels (aquaporins) in the cell membranes of cactus roots can add to the current understanding of such channels in more mesophytic species. The external and internal signals that trigger the initiation of lateral root primordia and other developmental processes, such as determinate root growth and early root hair formation, can be studied in cacti from a wide range of habitats. The role of cactus roots as intermediaries between relatively stable, succulent shoots and heterogeneous, often desiccating soil suggests numerous stimulating possibilities for future research.

Acknowledgment

J.G.B. thanks CONACyT, Mexico (grants 5277-N and 31832-N) for support.

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