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Data are based on the uptake of a vital dye (neutral red) by cells in tissues exposed to the extreme temperature for 1 hour for plants that have been gradually adjusted to low or high day/night air temperatures. The indicated temperature will cause

Data are based on the uptake of a vital dye (neutral red) by cells in tissues exposed to the extreme temperature for 1 hour for plants that have been gradually adjusted to low or high day/night air temperatures. The indicated temperature will cause immediate death of over half of the cells, generally leading to death of the plants. Data are from Nobel (1982a, 1988, 1996); Smith et al. (1984); Nobel et al. (1986); and Loik and Nobel (1993).

between freezing-point depression and osmotic pressure (Nobel 1999), the mechanism of low temperature damage to cacti is not due to the wholesale freezing of cellular water (Nobel 1988). Rather, the initial ice crystals form outside the cells, following a super cooling of the stem (lowering of the stem temperature below the freezing point of the cell sap). Intracellular water molecules are then progressively transferred to the growing extracellular crystals, leading to the shrinkage of protoplasts and the desiccation of the cells. The continuing dehydration of the cells affects their membranes and enzymes, leading to disruption of metabolic processes and eventual cellular death.

How many genes are involved in the multitude of factors leading to greater tolerance of low temperature? This is an important question for breeders and biotechnologists (Chapter 15) dealing with commercialized cacti, as domesticated cultivars tend to have a poorer tolerance of low temperatures compared with many native species (Russell and Felker 1987; Table 4.3). Indeed, one of the major limitations in the expansion of the regions where O. ficus-indica and other platyopuntias can be cultivated is episodic low temperatures, suggesting that global climate warming will be favorable to increased cultivation of such cacti (Nobel 1996).

Figure 4.6. Opuntia humifusa on a snow-covered hillside in Ontario, Canada. The wrinkled appearance of the cladodes is due to dehydration, a response to exposure to low temperatures. Photograph is courtesy of Michael E. Loik.

Although low temperature tolerance has major effects on the natural distribution of cacti and the regions where they can be successfully cultivated, high temperatures are generally not a major limiting factor. For instance, of the 17 species that have been assessed quantitatively, the tolerated high temperature averages 68°C (Table 4.3). This is an incredibly high temperature to survive, as metabolic processes are severely disrupted at 55 to 6o°C for most plants (Nobel 1988). Tolerated high temperatures are remarkably similar among stem types, being essentially the same for barrel cacti, columnar cacti, and opuntias (Table 4.3). The cellular and genetic bases for the high-temperature tolerance of cacti have not been described. In any case, high-temperature damage generally occurs for cacti where the stem contacts the soil, which can have surface temperatures of up to 70°C in deserts (Nobel 1988). Hence nurse plants are often important for ensuring the establishment of cactus seedlings in nature, and care must be exercised in the seasonal timing for planting young plants or cladodes of cultivated species to avoid stem overheating.

Water

Related to the tolerance by cacti of cellular desiccation during subzero episodes (Fig. 4.6) is the ability of cacti to withstand dehydration caused by drought. For O. acan-thocarpa, O. basilaris, and O. bigelovii, uprooted plants can survive 3 years without water (Szarek and Ting 1975; Smith and Madhaven 1982). Moreover, Copiapoa cinerea can survive outdoors (Gulmon et al. 1979) and Ferocactus wislizenii indoors (MacDougal et al. 1915) for 6 years without water. Carnegiea gigantea, F. acanthodes, and O. basilaris can lose 80% of their stem water and still survive (Barcikowski and Nobel 1984), and Coryphantha vivipara can lose 91% of its stem water when exposed to drought and live (Nobel 1981). Indeed, the ability of cacti to tolerate cellular water loss, and hence drought, is correlated with their ability to tolerate intracellular water loss during subzero temperature episodes accompanied by extracellular ice formation.

Cacti can store an immense amount of water in their succulent stems. This has ramifications at the tissue level; during drought, water is shuttled from the internal whitish water-storage parenchyma to the greenish photosynthetic chlorenchyma, thereby allowing net CO2 uptake to proceed for extended periods. For instance, the water-storage parenchyma in Carnegiea gigantea and Ferocactus acan-thodes loses four times more water than does their chlorenchyma during drought (Barcikowski and Nobel 1984). Similar patterns of water loss are also exhibited by O. basilaris and O. ficus-indica (Barcikowski and Nobel 1984; Goldstein et al. 1991). Likewise, the water storage parenchyma loses more water than does the chlorenchyma in O. humifusa in response to cooling from day/night temperatures of 25°/i5°C to 5°/-5°C (Loik and Nobel 1991).

Internal water redistribution reflects differences in the cellular properties of water-storage parenchyma and chlorenchyma. For C. gigantea and F. acanthodes exposed to drought, solutes are lost from the water storage parenchyma, thus creating a difference in the osmotic pressure between the two tissues and facilitating water movement into the chlorenchyma (Barcikowski and Nobel 1984). When O. ficus-indica is exposed to drought, the osmotic pressure in the water storage parenchyma becomes lower than in the chlorenchyma due to a polymerization of sugars leading to the formation of starch grains in the water-storage parenchyma (Goldstein et al. 1991). Water-storage parenchyma can also survive at a lower relative water content than chlorenchyma. Mucilage, which has a very high water-binding capacity, also occurs in greater amounts in the intercellular air spaces of water storage tissue than in the chlorenchyma (Goldstein et al. 1991). In addition, the elastic modulus of the cell walls of water storage parenchyma is only 40% of that of the chlorenchyma, allowing the water-storage parenchyma to maintain turgor over a large range of water contents.

Other aspects of water conservation and drought tolerance by cacti include relatively low stem stomatal frequencies of 20 to 80 per mm2 compared with i00 to 300 per mm2 for leaves of C3 and C4 species (Conde 1975;

Pimienta-Barrios et al. 1993; Nobel 1994). Therefore, only a small fraction of the surface area of cacti is available for water loss to the atmosphere. Also, the waxy cuticles covering the stems tend to be relatively thick for cacti—5 to 30 pm—compared with cuticles that are only 0.4 to 2 pm thick on the leaves of representative C3 and C4 species (Conde 1975; Pimienta-Barrios et al. 1993; North et al. 1995). As another adaptation, cacti have shallow roots, with a mean depth of only 10 to 15 cm, which facilitates responses to light rainfall; in addition, new roots develop rapidly once the soil is wet (Nobel 1988).

Salinity

Another factor that can be lethal for cacti is soil salinity. Indeed, cacti generally do not thrive in soils that are high in sodium chloride or calcium carbonate, which affects native populations and where opuntias can be cultivated. Cereus validus is native to the salt flats of Salinas Grandes, Argentina; this area experiences high salinity during the dry season, when its root system withers as water evaporates and salts collect near the soil surface (Nobel 1988). During the dry season, the sodium concentration increases in its roots and is progressively lower toward the apex of the stem (Nobel et al. 1984), as also occurs for O. ficus-indica (Berry and Nobel 1985). The increasing concentration of NaCl in the chlorenchyma of these two species as the soil salinity increases is accompanied by a decrease in net CO2 uptake, as also occurs for O. humifusa (Silverman et al. 1988), and can eventually lead to plant death

Inhibition of growth for O. ficus-indica is approximately linear with soil sodium content, with 150 ppm by mass of Na leading to approximately 50% inhibition of shoot growth (Nobel 1989). A similar 50% inhibition of shoot growth can be caused by watering with 60 mM NaCl (12% of the salinity of seawater) for 6 months, which leads to 84% inhibition of root growth (Berry and Nobel

1985). Plant dry weight for O. ficus-indica is 60% less for plants irrigated with 200 mM NaCl compared with 5 mM NaCl (Nerd et al. 1991). Cladode water content is also 10% lower, and cladode osmotic pressure doubles for plants under the higher NaCl regime. For Ferocactus acanthodes, watering with 60 mM NaCl has relatively little effect on existing shoot biomass but reduces existing root biomass by 40% (Berry and Nobel 1985), again indicating that root growth is more sensitive to salinity than is shoot growth.

Gravity and Wind

Due to their mechanical strength, imparted primarily by the wood content in their stems (Gibson and Nobel

1986), cacti do not deflect greatly due to gravity or wind.

The wood of the columnar cactus Carnegiea gigantea may get stiffer with plant age, making the stem resist deflection and buckling more per amount of wood area and allowing this species to reach heights of 15 m (Niklas and Buchman 1994). Also, stems of another columnar cactus, Pachycereus pringlei, are able to resist bending moments due to xylem accumulation being greater at the base of the plant, much in the same manner as typical dicotyledonous trees (Niklas et al. 1999). The ribs of this species also provide support, especially for young plants and younger tissue on older plants.

Branches of platyopuntias are able to resist applied forces with little deflection or deformation, even though the cladodes of a branch are connected by a junction that is usually small in cross-sectional area compared to the middle of the individual segments. For instance, for a cladode of O. ficus-indica that is 30 cm in length, the angular deflection of the junction with the underlying clad-ode, plus the cladode itself, is 6° when loaded by a force equal to the cladode mass perpendicular to the face of the cladode at the center of mass, 2° when the applied mass is parallel to the cladode's face, and 2° for a relatively high windspeed of 10 m s-1 (36 km hour-1; Nobel and Meyer 1991). Thus, even though cladodes are thin compared with stems of many woody plants and the cladode junctions are relatively small in area compared with other regions of the branch, the shoots of O. ficus-indica are quite rigid.

For other cacti, stem failure under static or dynamic loading may actually be advantageous because it results in vegetative reproduction. It is believed that when the semi-erect shrub Stenocereus gummosus reaches its structural height limit, the failure of the stems leads to vegetative reproduction because the stems root after they come into contact with the ground, eventually forming dense colonies (Molina-Freaner et al. 1998). A hybrid platy-opuntia in southern California also exhibits this type of vegetative reproduction (Bobich and Nobel 2001). In particular, the cladode junctions of the hybrid O. "occidental-is" are weaker than the junctions of either of its putative parent species, O. ficus-indica and O. littoralis, allowing O. "occidentalis" to form large thickets. These thickets are also large enough to survive fires that engulf isolated plants of O. ficus-indica and O. littoralis, thus giving O. "occidental-is" a selective advantage over its parent species in a chaparral region subject to periodic fires.

Conclusions and Future Prospects

Since the early research performed at the Desert Botanical Laboratory, information on the environmental biology of cacti has grown substantially. Interest in the physiology of cacti is spurred by their extraordinary ability to survive high temperatures, high PPF, and especially drought. The utilization of CAM by most of the species in the family in coping with these environmental stresses has added to the interest in these plants and has led to investigations of net CO2 uptake for species from all three subfamilies, as well as knowledge of water-use efficiency and productivity for select species. However, net CO2 uptake and productivity have been investigated for only slightly more than 2% of the species in the family. Also, the environmental biology of cacti from certain regions, such as the Atacama Desert of Chile, is lacking, suggesting that much more field research is needed.

As ecological and agricultural interest in cacti grows, greater insight into their optimal growth conditions will become essential. This will require detailed analyses of the effects that temperature, light, and water have on CO2 uptake for particular species. Greater attention should also be paid to how mineral nutrition affects net CO2 uptake, because soil elements other than sodium have been studied only for their effects on nocturnal acid accumulation (Nobel 1983) or on productivity (Nobel et al. 1987; Nobel 1989). Moreover, the optimal soil properties, such as the sand, silt, and clay fractions, should be further investigated because some species have the ability to grow on various types of soil while others are restricted to special soil types (Benson 1982).

Although comparisons of net CO2 uptake and productivity have been made between crops and certain cacti, particularly the highly productive O. ficus-indica (Nobel, 1988), comparisons of uncultivated cacti with sympatric species in their native habitat are rare. For example, Opuntia humifusa has received attention because it is the most widespread cactus in North America and is exposed to many different environmental stresses (Silverman et al. 1988; Loik and Nobel 1991). Yet, little is known about the net CO2 uptake for O. humifusa compared to that of the many species with which it coexists or about which characteristics allow this species to compete successfully for resources in such a wide variety of habitats. Similarly, little is known about the productivity for most uncultivated epiphytic cacti, even though over 10% of the Cactaceae are epiphytes (Gibson and Nobel 1986). With the large amount of information on the utilization of CAM by other vascular epiphytes (Griffiths 1989; Zotz and Ziegler 1997), studies comparing their CO2 uptake to that of epiphytic cacti in response to various environmental factors would aid in understanding the distribution and frequency of epiphytic cacti in the canopies of tropical forests.

New research possibilities for the environmental biol ogy of cacti go far beyond the aforementioned topics. Improvements in instrumentation will facilitate investigations of the photosynthetic strategies for cacti and their ability to survive harsh conditions. Moreover, members of the Cactaceae occur natively from southern Argentina and Chile to Canada, and from coastal strand communities to tropical alpine environments, and are cultivated in more than 30 countries (Gibson and Nobel 1986; Nobel 1994). Almost endless possibilities exist for interesting environmental studies involving cacti.

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