Net CO2 uptake is negative for stems maintained continually in the dark (Fig. 4.2C), as only respiration occurs under such circumstances because photosynthesis requires light. Net CO2 uptake increases as the total daily PPF increases and becomes half-maximal at about i3 mol m-2 day-1 for O. ficus-indica (Fig. 4.2C; because most CO2 uptake occurs at night, data on the light responses of cacti are generally based on the total daily PPF). Net CO2 uptake approaches light saturation at about 30 mol m-2 day-1, which is approximately half the total daily PPF incident on a horizontal surface on a clear day with the sun passing overhead at noon (the instantaneous PPF is then about 2,000 pmol m-2 s-1) but about equal to that incident on a vertical surface of a cactus stem facing east or west on such a day (Nobel 1988). For the shade-tolerant hemiepiphyte Hylocereus undatus, on the other hand, net CO2 is halfmaximal at only 4 mol m-2 day-1, maximal at 20 mol m-2 day-1, and is 40% of maximal at 30 mol m-2 day-1 (Raveh et al. 1995). For the holoepiphyte Schlumbergera truncata, photosynthesis is inhibited at even lower light levels, with daily net CO2 uptake being i9% lower for plants exposed to a total daily PPF of 17 mol m-2 day-1 than for those exposed to 9 mol m-2 day-1 (Nobel and Hartsock 1990). Indeed, epiphytic cacti generally do better under shaded conditions, both in the field and under cultivation (Mizrahi et al. 1997).

One reason that unshaded nonepiphytic cacti generally do not exhibit chronic photoinhibition (damage to the light-dependent processes of photosynthesis due to excess PPF; Taiz and Zeiger 1998) is that most of their photosyn-thetic surfaces are vertical and thus not perpendicular to the incoming PPF. Spines and pubescence can also create considerable shading for the stems of many cacti; in particular, the apices of certain ecotypes of Ferocactus acan-thodes are exposed to only 10% of the incoming PPF (Nobel 1980). The recycling of internal CO2 can also help prevent chronic photoinhibition by maintaining photo-synthetic activity, thereby avoiding harmful effects of absorbed light (Nobel 1988). Although chronic photoinhibition is uncommon in cacti, dynamic photoinhibition, which results in a temporary decrease in photosynthetic efficiency due to the radiationless dissipation of absorbed PPF (Taiz and Zeiger 1998), is common, especially during drought and in the late afternoon under favorable conditions, when endogenous CO2 levels are low (Adams et al. 1989; Barker and Adams 1997). The excess PPF that cannot be used in photosynthesis is dissipated in reactions involving xanthophylls and other carotenoids. For O. macrorhiza, the concentrations of carotenoids in a clado-de surface correlate with orientation, the greatest concentrations occurring in cladodes facing south or west, which are the surfaces that receive the most sunlight in the afternoon (Barker and Adams 1997).

Influences of plant architecture on the interception of PPF differ for the massive opaque stems of cacti that exhibit CAM, compared with the relatively thin, flat leaves of C3 and C4 species, for which light incident on either surface can be distributed throughout the leaf. For instance, the face of a cladode of a platyopuntia or the side of a roughly spherical barrel cactus facing poleward can receive far less than optimal total daily PPF when the opposite equatorially facing side is approaching PPF saturation of daily net CO2 uptake. Hence, because some of the plant's surfaces face away from direct sunlight essentially the entire day, some face sunward, and others face in intermediate directions, net CO2 uptake by whole plants of such species under unshaded conditions tends to increase up to full sunlight. In other words, because of the architecture of cacti, part of the stems, and hence the plants themselves, are always light-limited with respect to CO2 uptake. Moreover, the phototropism of O. leucotricha mentioned in the Introduction also occurs for O. ficus-indica, as newly developing cladodes can rotate 16° to become more perpendicular to a unidirectional light beam (Nobel 1982b).

In the field, cladodes tend to be favorably oriented to intercept more PPF and to have more net CO2 uptake (Fig. 4.2C) and hence more biomass productivity. Such cladode orientation is dependent on the latitude of the plants and the timing of cladode initiation. For instance, because of the path of the sun in the sky, an east-west orientation tends to be favored for plants close to the equator and decreases with distance from the equator. Cladodes initiated during the winter tend to have a north-south orientation, whereas cladodes that are initiated during the summer face east-west. Because daughter cladodes of various platyo-puntias tend to be oriented in the same plane as the underlying mother cladodes, orientation tendencies can be observed at the whole plant level (Nobel 1982c, 1988).

Nutrients and Salinity

Net CO2 uptake, and thus nocturnal acid accumulation, for the stems of cacti can also be affected by nutrient levels in the soil and hence in the stem (Nobel 1989). Of the nutrients investigated, nitrogen has the greatest positive effect on net CO2 uptake. For O. ficus-indica, nocturnal net CO2 uptake, as reflected in increases in tissue acidity, more than doubles as the nitrogen level in the chlorenchy-ma increases from 1% of the dry mass to over 2% (Nobel 1983). Among nine other species of cacti for which the nitrogen content in the chlorenchyma ranges from 1.0% to 2.5% of the dry mass, maximal nocturnal acid accumulation increases with average nitrogen content (Nobel 1983). Low stem nitrogen content may be one of the reasons that Stenocereus queretaroensis has a lower net CO2 uptake than do most other cultivated cacti (Nobel and Pimienta-Barrios 1995). For O. engelmannii and O. rastrera, dry mass increases 73% when fields are fertilized with 160 kg nitrogen per hectare (Nobel et al. 1987). Other nutrients that show a positive effect on net CO2 uptake by cacti are potassium, phosphorous, and boron (Nobel et al. 1987; Nobel 1989).

Soil salinity has a negative affect on net CO2 uptake. Net CO2 uptake for O. ficus-indica decreases by about 50% after exposure to a 150 mM NaCl solution and 83% after exposure to a 200 mM solution for 10 weeks (Fig. 4.3). Longer term exposure to high concentrations of NaCl has an even more profound effect, with exposure to a solution of 100 mM for six months causing a net CO2 efflux for O. ficus-indica (Hatzmann et al. 1991). The concentration of NaCl in the chlorenchyma also correlates with net CO2 uptake, because CO2 uptake halves as the sodium level in the chlorenchyma increases from 20 ppm by dry mass to 300 ppm for O. ficus-indica (Nobel 1983) and from 10 to 80 ppm for Cereus validus (Nobel et al. 1984). The latter tissue level w 2

S5 0

0 50 100 150 200

applied NaCl concentration (mm)

0 50 100 150 200

applied NaCl concentration (mm)

Figure 4.3. Influence of a 10-week application of NaCl solutions on daily net CO2 uptake for cladodes of O. ficus-indica. Plants were grown in 50% Hoagland solution, plus the indicated NaCl concentration. Data are from Nerd et al. (1991).

can be caused by exposing C. validus to 400 mM NaCl for 16 days. When O. humifusa is exposed to 150 mM NaCl for 6 weeks, total daily net CO2 uptake is reduced 94% for plants from inland populations but 71% for plants from the marine strand that are exposed to higher soil salinity in their native habitat (Silverman et al. 1988).

Atmospheric CO2

As the CO2 concentration in the atmosphere is increased, the driving force for CO2 entry into cacti, and hence net CO2 uptake, tends to increase. Yet C3 plants, when exposed to doubled atmospheric CO2 concentrations, often acclimate to the new conditions after a period of months, leading to less enhancement than observed initially (Drennan and Nobel 2000). All four species of cacti that have been exposed to a doubled atmospheric CO2 concentration have shown increases in both total daily net CO2 uptake and biomass productivity. For instance, the biomass increase over a 12-month period is 30% greater for Ferocactus acanthodes at a doubled compared with the current atmospheric CO2 concentration (Nobel and Hartsock 1986c). For Hylocereus undatus, total daily net CO2 uptake is 34% higher under the doubled atmospheric CO2 concentration (Raveh et al. 1995), and for Stenocereus quere-taroensis it is 36% higher (Nobel 1996).

Opuntia ficus-indica is the most studied CAM species with respect to effects of elevated atmospheric CO2 concentrations (Drennan and Nobel 2000). Its total daily net CO2 uptake for terminal cladodes can be enhanced 41 to 61% by doubling the atmospheric CO2 concentration (Cui et al. 1993). Its biomass increase over a 12-month period is 40% greater under a doubled atmospheric CO2 concentration than under the current one (Nobel and Israel 1994). Other responses of O. ficus-indica to elevated atmospher ic CO2 concentrations include thicker cladodes, a thicker chlorenchyma, increased chlorenchyma cell length, a decrease in PEPCase per unit cladode area, a decrease in stem nitrogen concentration, a decreased stomatal frequency, an increased root:shoot ratio, increased root cell length, and a 60 to 70% increase in water-use efficiency (Cui et al. 1993; Luo and Nobel 1993; North et al. 1995; Drennan and Nobel 2000).

Net CO2 Uptake: Fruits

CO2 uptake for fruits of cultivated platyopuntias follows a typical CAM pattern (Fig. 4.1), although maximum instantaneous net CO2 uptake rates and total daily net CO2 uptake (Table 4.2) are only 10 to 40% as high as for stems (Inglese et al. 1994; Nobel and De la Barrera 2000). In particular, CO2 uptake rates for young fruits of platyopuntias are similar to those for stems of uncultivated cacti (Table 4.1). The daily net CO2 uptake by young fruits accounts for only 6 to 15% of their total daily dry-mass gain (Table 4.2). The contribution to dry-mass gain decreases with fruit age, reflecting decreases in Rubisco and PEPCase activity and in chlorophyll content (Inglese et al. 1994). In contrast, during the first two weeks after initiation, cladodes of Opuntia ficus-indica have a daily net CO2 loss, but at four weeks they have a net CO2 gain that is similar to that of their underlying mature cladodes (Wang et al. 1997). Thus, clado-des make the transition from a carbohydrate sink to a carbohydrate source at an early age, whereas fruits become more of a carbohydrate sink as they mature.

Because the contribution of net CO2 uptake to the dry-mass gain of platyopuntia fruits is small (Table 4.2), during their development fruits must receive considerable carbohydrates from underlying mature cladodes via the phloem (Inglese et al. 1994; Wang et al. 1997; Nobel and De la Barrera 2000). However, the importance of the phloem to fruits lies not only in the supplying of solutes. Platyopuntia fruits have water potentials that are higher (less negative) than those of their underlying cladodes, indicating that water cannot flow passively in the xylem from the cladodes to the fruit (Nobel et al. 1994), as occurs for most developing organs (Nobel 1999). Rather, the phloem is responsible for water transport into the fruits of platyopuntias (Nobel et al. 1994), much as it is for 2-week-old cladodes of O. ficus-indica (Wang et al. 1997). Moreover, the osmotic pressure of the phloem sap entering the fruits of O. ficus-indica is relatively low (0.94 MPa) compared with the phloem sap in other plants (Nobel et al. 1994). The fruits transpire over 60% of the imported water, leading to a large buildup of solutes (Nobel and De la Barrera 2000). To maintain higher water potentials than their un

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