Park S. NOBEL
Park S. NOBEL
Department of Organismic Biology, Ecology, and Evolution
The physiological basis of the ecological success and agricultural usefulness of opuntias as a forage in large measure reflects their daily pattern of stomatal opening (stomata are fine pores in the leaf or stem surface that regulate the exchange of gases between a plant and its environment). Most plants have daytime stomatal opening so that CO2 uptake occurs concomitantly with photosynthesis, which uses the energy of light to incorporate CO2 from the atmosphere into a carbohydrate. Plants like Opuntia ficus-indica, however, have nocturnal stomatal opening, so net CO2 uptake and water loss occur during the cooler part of the 24-hour cycle. This gas exchange pattern is referred to as Crassulacean Acid Metabolism (CAM) because it was studied extensively in the Crassulaceae, although apparently first recognized in the Cactaceae (Ting, 1985; Nobel, 1988). CAM plants are native to arid and semi-arid regions, as well as to periodically dry microhabitats such as those occupied by epiphytes (most of the 20 000 species of CAM plants are epiphytes growing on trees in tropical forests (Winter, 1985; Nobel, 1991a).
As just indicated, O. ficus-indica takes up CO2 primarily at night (Figure 1A). Under wet conditions and moderate temperatures, net CO2 uptake becomes positive in the late afternoon, when daytime temperatures have decreased substantially, and reaches its maximum value a few hours after dusk. Generally, a small burst of net CO2 uptake occurs at dawn (Figure 1A), when the availability of light allows for direct incorporation of atmospheric CO2 into carbohydrates using the C3 pathway of photosynthesis during the coolest part of the daytime. The daily pattern for water vapour loss via transpiration for O. ficus-indica (Figure 1B) is similar to the pattern for net CO2 uptake, reflecting the requirement of appreciable stomatal opening to get substantial exchange of either gas with the environment.
The CO2 taken up by a CAM plant at night is bound to a three-carbon compound to form a four-carbon organic acid, such as malate. The accumulating organic acids are stored overnight in large vacuoles within cells of the chlorenchyma (the greenish chlorophyll-containing region), so the tissue becomes progressively more acidic during the course of the night. CO2 is released from the organic acids during the next daytime, causing the tissue acidity to decrease. This released CO2, which is prevented from leaving a CAM plant by daytime stomatal closure, is then incorporated into photosynthetic products in the chlorenchyma cells in the presence of light. The daily oscillations of acidity, which is characteristic of CAM plants, requires large vacuoles for the sequestering and short-term storage of the organic acids.
A useful benefit:cost index for gas exchange by plants is the ratio of CO fixed by photosynthesis to water lost by transpiration, which is referred to as the water-use efficiency (WUE). For the gas exchange data presented in Figure 1, net CO2 uptake integrated over the 24-hour period is 1.14 mol/m2/day and the water loss is 51.3 mol/m2/day. Thus the WUE is 0.022 mol CO2 fixed per mol H2O lost for this CAM plant. This WUE is about triple that found for highly productive C4 plants (such as maize or sugar cane) under similar environmental conditions. C4 plants have daytime net CO uptake initially into four-carbon organic acids, and 5-fold higher than for highly productive C3 plants (such as alfalfa, cotton, or wheat), which also have daytime net CO2 uptake but whose initial photosynthetic product is a three-carbon compound (Nobel, 1995).
Figure 1. Net CO2 uptake (A) and transpiration (B) for Opuntia ficus-indica over a 24-hour period under conditions of wet soil, moderate temperatures, and high light levels
(Source: Nobel, 1988, 1995)
The much higher WUE for CAM plants relates to the reduced difference in water vapour concentration between the plant and the atmosphere during the period of substantial stomatal opening. In particular, the water vapour content in leaves and stems is within 1% of the saturation value in air at the tissue temperature (Nobel, 1999); tissue temperatures tend to be much lower at night, and the water vapour saturation value of air increases nearly exponentially with temperature. For instance, the water vapour content for saturated air is 0.52 mol/m3 at 10°C, 0.96 mol/m3 at 20°C, and 1.69 mol/m3 at 30°C. If the water vapour content of the air is 0.38 mol/m3 (40% relative humidity at 20°C), then the drop in water vapour concentration from the plant to the atmosphere, which represents the driving force for water loss from a plant, is the difference between 0.52 and 0.38, or 0.14 mol/m3 at 10°C; 0.96 - 0.38, or 0.58 mol/m3 at 20°C; and 1.69 - 0.38, or 1.31 mol/m3 at 30°C. For the same degree of stomatal opening, the driving force for water loss then is 0.58/0.14, or 4.1-fold higher at 20°C than at 10°C, and 1.31/0.58 or 2.3-fold higher at 30°C than at 20°C. Because tissue temperatures typically average at least 10°C lower at night than during the daytime in many locations, CAM plants tend to lose only 20 to 35% as much water as do C3 or C4 plants for a given degree of stomatal opening. This is a key feature in their utility as forage crops in arid and semi-arid regions.
Besides using CAM, with its inherently high WUE, O. ficus-indica has other adaptations that lead to water conservation. For instance, the waxy cuticle on its stems is relatively thick, generally 5 to 30 µm (Conde, 1975; Pimienta-BarRíos et al., 1992, 1993; North et al., 1995). This helps prevent water loss from the plants to the environment. In addition, the stomatal frequency is usually low for opuntias, generally 20 to 30 per square millimetre (Conde, 1975; Pimienta-BarRíos et al., 1992). Consequently, the fraction of the surface area of the stems through which water vapour can move from the plants to the atmosphere is relatively low. Moreover, the stems contain a large volume of whitish water-storage parenchyma, which acts as a water reservoir for the chlorenchyma, where the initial CO2 fixation at night via CAM and the daytime photosynthesis take place. For instance, during a drought lasting three months, the chlorenchyma in the stems of O. ficus-indica decreases in thickness by 13%, while the water-storage parenchyma decreases in thickness by 50%, indicating a greater water loss from the latter tissue (Goldstein et al., 1991). As another adaptation, the roots of O. ficus-indica tend to be shallow with mean depths near 15 cm, facilitating a quick response to light rainfall. For instance, it can form new roots within 24 hours of wetting of a dry soil (Kausch, 1965). Its various water-conserving strategies lead to a need for a small root system; indeed, roots compose only about 12% of the total plant biomass for O. ficus-indica (Nobel, 1988).
Drought, which physiologically commences when the plants can no longer take up water from the soil (because the soil water potential is then less than the plant water potential), leads to a decrease in the ability of the stems to take up CO2 from the atmosphere (Figure 2A). Little change in net CO2 uptake ability occurs during the first week of drought for O. ficus-indica, reflecting water storage in the stem and the inherently low water requirement for CAM. Also, the waxy cuticle and low stomatal frequency allow 20% of the maximal net CO2 uptake to be present even one month after the plants are under drought conditions (Figure 2A). After the initial week of drought, the net CO2 uptake over the next month averages about half of the maximal value (Figure 2A); after about two months, a small daily net CO2 loss occurs, as respiration becomes greater than net photosynthesis, whereas most C3 and C4 crops begin having a net loss of CO2 within one week of the commencement of drought. Thus the net CO2 uptake ability of O. ficus-indica and certain other CAM plants is extremely well suited to arid and semi-arid regions. Nevertheless, soil water is the major limiting factor for net CO2 uptake by O. ficus-indica in such regions, where irrigation may not be economically feasible.
Figure 2. Influence of drought length (A), night-time temperature (B), and light (C) on net CO2 uptake over 24-hour periods for O. ficus-indica. Except as indicated, plants were well watered, maintained at night-time temperatures near 15°C, and had a PPF of about 25 mol/m2/day incident on the cladode surfaces
(Source: Nobel and Hartsock, 1983, 1984; Israel and Nobel, 1995)
Temperature not only influences metabolic processes and hence daily net CO2 uptake but extreme temperatures can also lead to injury and even death of plants. In this regard, O. ficus-indica is extremely tolerant of high air temperatures, but not of air temperatures substantially below freezing. When plants are acclimatized to high day/night air temperatures of 50°C/40°C, their chlorenchyma cells are not seriously injured by 1 hour at 60°C, and most cells survive 1 hour at 65°C (Nobel, 1988). Indeed, high-temperature damage for O. ficus-indica in the field is generally only observed near the soil surface, where temperatures in deserts can reach 70°C; young plants or newly planted cladodes are especially vulnerable to injury. In contrast, cell injury in the field occurs at freezing temperatures of -5°C to -10°C. Damage varies with the cultivar (Russell and Felker, 1987b), with the rapidity of the onset of freezing and hence the time for low-temperature acclimatizion or hardening (Nobel, 1988), and with the stem water content, as a lower water content leads to better tolerance of lower air and stem temperatures (Cui and Nobel, 1994a; Nobel et al., 1995).
Because CO2 uptake for CAM plants occurs primarily at night, night-time temperatures are far more important than are daytime ones for daily net CO2 uptake by O. ficus-indica (Figure 2B). Moreover, the optimal night-time temperature is relatively low, 15°C, and temperatures from 5°C to 20°C all lead to at least 80% of the maximal net CO2 uptake. Such low temperatures also lead to low rates of transpiration. As night-time temperatures rise, stomata tend to close for O. ficus-indica; e.g., at 30°C the stomata are only one-third as open as at 20°C (Nobel and Hartsock, 1984), which helps reduce net CO2 uptake at the higher temperature (Figure 2B). Except for night-time temperatures substantially below freezing or above 30°C, temperature is generally not a major limiting factor for net CO2 uptake by O. ficus-indica, especially in seasons when water from rainfall is available, which is fortunate, because manipulation of air temperatures in the field is expensive.
Another environmental parameter affecting net CO2 uptake is light; the light incident on individual stems can be readily manipulated by the spacing of plants, although tradeoffs occur between maximizing net CO2 per plant versus net CO2 uptake per unit ground area (García de Cortázar and Nobel, 1991). The stems of O. ficus-indica are opaque, contrary to the case for the leaves of most C3 and C4 plants, so orientations of both sides must be considered when evaluating light absorption. Also, the light that is relevant is that absorbed by photosynthetic pigments, mainly chlorophyll, which is referred to as the photosynthetic photon flux (PPF; 400 to 700 nm wavelength; also referred to as the photosynthetic photon flux density and the photosynthetically active radiation (PAR); Nobel, 1999).
When the plants are maintained in the dark, only respiration occurs, so there is a slight loss of CO2 (Figure 2C). As the daily PPF increases, the daily net CO2 uptake by O. ficus-indica increases. Saturation by light is approached at a total daily PPF of about 25 mol/m2/day (Figure 2C; for comparison, the total daily PPF on a horizontal surface for a clear day during which the sun passes overhead is about 65 mol/m2/day; Nobel, 1988). Because of the opaque nature of the stems, some of their surfaces are not favourably oriented with respect to interception of sunlight; also interplant shading will reduce daily net CO2 uptake. Thus net CO2 uptake per plant is greatest when the plants are far apart and do not shade each other. However, net CO2 uptake and hence productivity per unit ground area is then minimal. If the plants are very close together, shading is excessive and much of the stem area receives less than 5 mol/m2/day total daily PPF, for which net CO2 uptake is substantially reduced (Figure 2C). Indeed, net CO2 uptake per unit ground area for O. ficus-indica is maximal when the total area of the stem (including both sides of their flattened stem segments, or cladodes) is 4 to 6 times the ground area (García de Cortázar and Nobel, 1991). When the ratio of total stem area to ground area, termed the stem area index (SAI), is 1, 2 and 3, the net CO2 per unit ground area for O. ficus-indica is 35%, 62%, and 85% of maximal, respectively (Nobel, 1991a).
Net CO2 uptake, growth and productivity for O. ficus-indica are influenced by macronutrients and micronutrients in the soil, as well as by salinity and soil texture (Nobel, 1988; Hatzmann et al., 1991). For instance, growth in sandy loam is about 25% of maximal at a nitrogen content of 0.03% by dry mass, 50% of maximal at 0.07% N, 75% of maximal at 0.15% N, and approaches maximal near 0.3% N (Nobel, 1989a). Because the N content in native sandy soils in arid and semi-arid regions is generally below 0.07%, nitrogen fertilization usually increases the growth of O. ficus-indica and other opuntias in such areas (Nobel et al., 1987). The protocol for nitrogen fertilization of O. ficus-indica has followed traditional practices developed for other crops (Barbera et al., 1992; Nerd et al., 1993), where the main form taken up from the soil is nitrate (Nerd and Nobel, 1995). Although N is generally the major limiting nutrient, growth of opuntias usually is also stimulated by phosphorus and potassium fertilization (Nobel, 1989b). A soil level of only 5 parts per million by dry mass (ppm) P leads to half-maximal growth for O. ficus-indica (Nobel, 1989b), but the stems produced are below the nutritional needs of cattle for phosphorus. Indeed, stems of most opuntias contain about 1% N by dry mass in nutrient-poor native soils, which is below the nutritional need of cattle for nitrogen, but about 2% when grown on periodically fertilized agricultural land (Nobel, 1988).
As for most cacti, O. ficus-indica is sensitive to soil salinity. Inhibition of growth is often linear with sodium content, with 150 ppm Na leading to approximately 50% inhibition of biomass accumulation by O. ficus-indica (Nobel, 1989b). Its roots are more affected by salinity than are its shoots; e.g. watering with 60 millimolar (mM) NaCl (about 12% of the salinity of seawater) for six months reduces root growth by 84% and shoot growth by 50% (Berry and Nobel, 1985). Exposing the entire root system of O. ficus-indica to 100 mM NaCl for 10 weeks reduces root growth by 38% (Nerd et al., 1991) but after only 4 weeks, growth of a single root exposed to 100 mM NaCl can be reduced by 93% (Gersani et al., 1993). Also, Na is not readily transferred from the roots to the shoot or from basal cladodes to new daughter cladodes (Berry and Nobel, 1985). As is the case for nearly all plant parts, the Na content of the cladodes of O. ficus-indica does not meet the nutritional needs of cattle for this element.
The atmospheric CO2 level is currently increasing by about 2 ppm by volume annually, which can lead to an increase in daily net CO2 uptake by O. ficus-indica. For instance, a doubling of the current atmospheric CO2 level causes net CO2 uptake by two-month-old cladodes to increase by 49% and their WUE to increase by 55% compared with the current atmospheric CO2 level (about 360 ppm; Cui et al., 1993). The aboveground-dry-mass productivity by O. ficus-indica in the field is 37-40% higher for a doubled CO2 level versus the current CO2 level (Nobel and Israel, 1994). Although the nitrogen content of older cladodes remains near 1% of the dry mass, the N content of three-month-old cladodes averages 1.47% of the dry mass at current ambient atmospheric CO2 levels but 1.26% at doubled atmospheric CO2 levels (Cui and Nobel, 1994b); the lower N content at the higher atmospheric CO2 level reflects a lower amount of photosynthetic enzymes, as is also found for other species.
The responses of daily net CO2 uptake to environmental factors over 24-hour periods under controlled conditions can lead to predictions for plant productivity in the field, as CO2 uptake leads to growth and hence to an increase in plant biomass (Nobel, 1988, 1991b; García de Cortázar and Nobel, 1991). Such responses, especially with regard to plant spacing leading to a high-planting density that maximizes productivity per unit ground area, have been used to predict maximal productivity. This has led to experimental cultivation of O. ficus-indica near Santiago, Chile, and Saltillo, Coahuila, Mexico, under wet soil conditions (generated by year-round irrigation), moderate temperatures close to those optimal for net CO2 uptake, SAIs of 4 to 6, and non-limiting soil nutrients (Nobel, 1991a; García de Cortázar and Nobel, 1991, 1992; Nobel et al., 1992). For these ideal conditions, the measured dry mass productivity is 50 t/ha/yr. Considerably lower productivity is expected, however, under more typical (non-ideal) field conditions, productivities that can be predicted using Figure 2 to obtain relative net CO2 uptake responses.
A high-planting density with an SAI of 4 to 6 causes the root systems of individual plants to overlap. Thus a more typical field situation might be an SAI of 2, which also allows for the pathways in the field necessary for plant maintenance and the harvesting of cladodes. The weather may not lead to ideal temperatures, which are essentially controllable only by changing the location of the fields. Instead of year-round irrigation, two lesser water availability situations will be considered, which are more typical of Mediterranean climates or regions where encroaching desertification favour the use of O. ficus-indica as a forage, namely, where the seasonal rainfall leads to wet soil conditions for nearly two months in the winter and where the rainfall leads to two wet periods of nearly one month each. Using an SAI of 2 leads to 62% of the maximal CO2 uptake per unit ground area based on PPF interception (Figure 2C) and the field temperatures may lead to 80% of the maximal daily net CO2 uptake (Figure 2B). For the single winter wet period and using the response of O. ficus-indica to drought (Figure 2A), the plants would have a maximal net CO2 uptake for two months, plus half of the maximal for a month more during drought or (2.5/12)(100%) or 21% of the maximal net CO2 uptake that would be obtained under year-round wet conditions. Because responses to these three environmental factors are multiplicative (Nobel, 1984, 1988, 1991a), the predicted productivity is 0.62 × 0.80 × 0.21 × 50 t/ha/yr = 5.2 t/ha/yr. For the two wet periods per year plus the daily net CO2 uptake responses of O. ficus-indica to drought (Figure 2A), water limitations would lead to [(1.5 + 1.5)/12] × 100 = 25% of the maximal annual net CO2 uptake, so the predicted productivity is (0.62) × (0.80) × (0.25) × (50 t/ha/yr) = 6.2 t/ha/yr. A more precise estimate can be obtained by using monthly or even daily values for the limitations caused by water, temperature and light on daily net CO2 uptake (Figure 2). In any case, environmental conditions in the field can be used to predict productivity for O. ficus-indica using responses of daily net CO2 uptake to soil water, air temperature and PPF determined under controlled conditions in the laboratory.
Although most ecophysiological studies on opuntias have been done with O. ficus-indica, similar results occur for other opuntias and other CAM plants (Nobel, 1988, 1994). For instance, Opuntia amyclea can have a high annual biomass productivity of 45 t dry mass/ha/yr at an optimal SAI and under irrigation in Saltillo, Coahuila, Mexico (Nobel et al., 1992; actually, species status for O. amyclea is uncertain but it is morphologically distinct from O. ficus-indica). Among other CAM plants, certain agaves used commercially in Mexico, namely, Agave mapisaga and A. salmiana, have high biomass productivities, averaging 40 t/ha/yr (Nobel et al., 1992). In comparison, the four highest-yielding C3 crops have an average productivity of 38 t/ha/yr, the four fastest-growing C3 trees average 41 t/ha/yr, and the four highest-yielding C4 crops average 56 t/ha/yr (Nobel, 1991a). Of greater importance for forage considerations in arid and semi-arid regions is the biomass productivity when rainfall is severely limiting. Under such circumstances the advantages of CAM become apparent for water conservation, as agaves and opuntias have a higher WUE, leading to a higher biomass productivity per unit ground area than do C3 or C4 plants under the same conditions (Nobel, 1994).
Agaves and other cacti also have other ecophysiological responses similar to those for O. ficus-indica (Nobel, 1988, 1994). For instance, net CO2 uptake, growth and biomass productivity respond favourably to N fertilization, and generally also to P and K fertilization, and nearly all species are inhibited by increasing soil salinity (Nobel, 1989b). As for O. ficus-indica, increasing the atmospheric CO2 level also increases the biomass productivity for agaves. A doubling of the current CO2 level leads to about 50% more biomass for Agave salmiana over 4.5 months (Nobel et al., 1996) and nearly 90% more biomass for Agave deserti over 17 months (Graham and Nobel, 1996). Doubling the atmospheric CO2 level for A. deserti increases daily net CO2 uptake per unit leaf area by 49% while reducing daily transpiration by 24%, leading to a 110% higher WUE. As for O. ficus-indica, other commercial CAM plants are also sensitive to freezing temperatures, but highly tolerant of high temperatures (Nobel, 1988). For instance, -8°C for 1 hour had similar deleterious effects on chlorenchyma cells of A. salmiana and O. ficus-indica (Nobel, 1996). Thus the extent of cultivation of both species can increase because of the rising air temperatures that are predicted to accompany global climate change, and the increasing atmospheric CO2 levels will increase their biomass productivity.
Clearly, O. ficus-indica and certain other commercial CAM species are well suited for forage crops in arid and semi-arid regions, generally as a result of their nocturnal stomatal opening that leads to nocturnal net CO2 uptake. The responses of their daily net CO2 uptake to soil water content, air temperature and PPF are known or can be measured, allowing predictions of their biomass productivity in various regions. Although extremely high (50 t dry mass/ha/yr) productivity is possible for O. ficus-indica, predicted productivity of 5 to 6 t/ha/yr under water-limited conditions can still surpass productivity of C3 and C4 species used for forage. Specifically, O. ficus-indica can have a WUE that is 3 to 5 times higher than for C3 and C4 species. In addition, a low stomatal frequency and a thick cuticle reduce