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tivated cactus such as Opuntia ficus-indica may grow in a particular region (Garcia de Cortázar and Nobel 1991). The Temperature Index can be used to evaluate maritime or el-evational influences on net CO2 uptake for a particular site. The Water Index can be used to quantify the influence of the timing of irrigation on net CO2 uptake. However, the component index that has had the greatest influence on agronomic practices for O. ficus-indica is the PPF Index, which can be directly related to plant spacing. In particular, the nonlinear response to total daily PPF for net CO2 uptake by individual stem surfaces (Fig. 4.2C) suggests that excessive shading should be avoided, i.e., when the stem area index—total area of both sides of all cladodes per unit ground area—becomes very high, the average PPF incident on the cladodes decreases, leading to less total daily net CO2 uptake per unit cladode area (Fig. 4.5). However, although spacing plants at large distances maximizes net CO2 uptake per plant, it leads to little net CO2 per unit ground area. Computer models dividing the stems of O. ficus-indica into many surfaces facing in different directions and using hourly values for PPF throughout a year have indicated an optimal spacing for net CO2 uptake per unit ground area (Garcia de Cortázar et al. 1985; Garcia de Cortázar and Nobel 1991). In particular, the stem area index should be about 4 to 5 for optimal biomass productivity (Fig. 4.5). Although some modification of such close spacing is necessary for management practices, such as picking of fruit, the simulation modeling indicates that much closer spacing than had been traditionally used in various locations greatly enhances biomass productivity per area.

Based on calculations using EPI, experimental field plots were established to ascertain the maximal productivity of O. ficus-indica under environmental conditions approaching those optimal for net CO2 uptake. To meet those conditions, sites were chosen with moderate nighttime temperatures (and hence a high Temperature Index) and with nutrient-rich soils. The plants were optimally spaced or pruned to keep the stem area index at 4 to 5 and were irrigated to maintain the Water Index near 1.00. Under such conditions, O. ficus-indica and O. amyclaea had a annual biomass productivities of 45 to 50 tons dry mass hectare-1 year-1 (Garcia de Cortazar and Nobel 1992; Nobel et al. 1992). Such extremely high biomass productivities are exceeded by those of only a few cultivated C3 and C4 species. Indeed, the annual biomass productivities are 38 tons hectare-1 year-1 for the four most productive C3 crops, 41 tons hectare-1 year-1 for the four most productive C3 trees, and 56 tons hectare-1 year-1 for the four most productive C4 crops (Nobel 1991). Although most cacti are relatively slow growing, with modest net CO2 uptake, certain

stem area index (cladode area/ground area)

Figure 4.5. Biomass productivity for O. ficus-indica as a function of plant spacing as quantified by the stem area index (SAI). Optimal refers to conditions that include irrigation, leading to a Water Index of 1.00; Chile refers to natural conditions near Santiago, Chile; Semiarid refers to conditions that are discussed in the text. Data are adapted from Garcia de Cortázar et al. (1985); Garcia de Cortázar and Nobel (1991); and Nobel (1991).

stem area index (cladode area/ground area)

Figure 4.5. Biomass productivity for O. ficus-indica as a function of plant spacing as quantified by the stem area index (SAI). Optimal refers to conditions that include irrigation, leading to a Water Index of 1.00; Chile refers to natural conditions near Santiago, Chile; Semiarid refers to conditions that are discussed in the text. Data are adapted from Garcia de Cortázar et al. (1985); Garcia de Cortázar and Nobel (1991); and Nobel (1991).

opuntias have great potential for biomass productivity and could even be used for sequestering massive amounts of carbon, an international goal for mitigating the increasing atmospheric CO2 concentrations and the accompanying global climatic warming.

Besides indicating conditions for maximal productivity, EPI can also be used to estimate productivity under more typical field conditions. For instance, predictions can be made for a region near Santiago, Chile, where O. ficus-indica is grown without irrigation for fruit production (Fig. 4.5). Moreover, the stem area index for O. ficus-indica may be near 2 to allow pathways in the field for plant maintenance and harvesting of fruits or cladodes, which would lead to 62% of maximal net CO2 uptake per unit ground area (Fig. 4.5). A site chosen for the cultivation of O. ficus-indica might not have ideal temperatures, at least for certain seasons. Actually, total daily CO2 uptake does not vary tremendously with mean nighttime temperature, being within 40% of the maximal values for mean nighttime temperatures from 2°C to 25°C (Fig. 4.2A), and so the Temperature Index may average 0.80 on an annual basis. If the cultivation site is semiarid with two wet periods per year, the Water Index might average 0.25 on an annual basis. Hence, for a stem area index of 2, EPI may average (0.62X0.80X0.25) or 0.12. If 50 tons dry mass hectare-1 year-1 is taken as the maximum productivity, then the predicted productivity for the site would be (0.12X50) or 6 tons hectare-1 year-1 (Fig. 4.5), which is representative of biomass productivities of O. ficus-indica in semiarid regions without irrigation. Moreover, cacti, with their high water-use efficiency, have a distinct advantage over C3 and C4 species in such regions; e.g., O. ficus-indica can be cultivated in regions with too little soil moisture for Zea mays and other such crops.

Survival

Temperature

Temperature influences every process in plants, from photosynthesis and respiration to growth and survival. With respect to survival, low temperatures are more critical with regard to the distribution of native species of cacti and the cultivation of opuntias than are high temperatures. For instance, the northern limits of the columnar cacti Carnegiea gigantea, Lophocereus schottii, and Stenocereus thurberi in Arizona are dictated by episodic low temperatures (Turnage and Hinckley 1938; Nobel 1988). In particular, these three species can be killed by stem temperatures of -7 to -i0°C, similar to the lethal temperatures for cultivated arborescent species such as Opuntia ficus-indica and Stenocereus quere-taroensis (Table 4.3). In contrast, certain lower growing species can tolerate about -20°C, e. g., Coryphantha vivipara, Opuntia humifusa, and Pediocactus simpsonii (Table 4.3), which are all native to regions in the northern United States that receive appreciable snowfall. Moreover, Opuntia fragilis, which occurs north of 56° north latitude in western Canada, can tolerate extremely low stem temperatures of -48°C (Loik and Nobel 1993).

Cacti that tolerate extremely low temperatures exhibit superior ability for low-temperature hardening (increase in the tolerance of subzero temperatures as the ambient temperature is gradually decreased over a period of weeks; Nobel 1988). Cladodes with lower water content tend to tolerate lower temperatures (Nobel 1988; Loik and Nobel 1991; Nobel et al. 1995). For example, tissue water content for O. humifusa (Fig. 4.6), which is native to southeastern Canada and the eastern United States, generally decreases by 35% in the winter, and winter kill is limited to plants with greater water content, suggesting that cold tolerance for cacti might depend on increases in osmotic pressure (Koch and Kennedy 1980). In fact, when day/night temperatures decrease from 30/20°C to i0/0°C, the osmotic pressure for O. humifusa increases four times more than do the osmotic pressures for the subtropical and less freezing-tolerant O. ficus-indica and O. streptacantha, due to a greater synthesis of simple sugars and the production of mannitol within the cells of O. humifusa (Goldstein and Nobel 1994).

Because the cellular contents of cacti would freeze at -1 to -2°C based on their osmotic pressure and the relation

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