Introduction and Background

The best known crassulacean acid metabolism (CAM) crop species is the bromeliad pineapple (Ananas comosus), which is cultivated for its fruit on 720,000 ha in about 40 countries (Table 14.1; Bartholomew and Rohrbach, 1993). However, another CAM species (Opuntia ficus-indica), referred to as prickly pear, prickly pear cactus, cactus pear and nopal, is cultivated on just over 1 Mha in about 30 countries (Table 14.1; Russell and Felker, 1987; Nobel, 1996a; Mizrahi et al, 1997). Most such cultivation is for its stem segments (termed cladodes) that are used both for forage and fodder for cattle, goats and sheep; for example, about 400,000 ha are so utilized in Brazil. Cladodes are also harvested as a vegetable for human consumption, especially in Mexico. About 10% of the area for cultivation of O. ficus-indica and closely related opuntias is for their fruit, which has long been an important crop in Sicily and is cultivated most extensively in Mexico. About 20 other species in six genera of cacti are also cultivated for their fruits. These include species of Stenocereus and other columnar cacti in Mexico (Pimienta-Barrios and Nobel, 1994; P.S. Nobel, unpublished observations) and Hylocereus and Seleniocereus in Colombia, Israel, Mexico, USA, Vietnam, and other countries (Gibson and Nobel, 1986; Mizrahi et al., 1997). This market is relatively small (Table 14.1) but expanding rapidly.

In the early part of the 20th century in various countries in eastern Africa, in substantial areas in India, and in the Americas, another CAM species, Agave sisalana, was extensively cultivated for the fibre in its leaves (Gentry, 1982). Agave fourcroydes was also cultivated in Mexico, mostly in the Yucatan peninsula. Because of the advent of synthetic fibres that were cheaper and more tolerant of moisture, the cultivation of these two agave species has declined substantially. However, its cultivation is still appreciable (Table 14.1) and certain related species are being considered as new fibre crops

©CAB International 2000. Climate Change and Global Crop Productivity (eds K.R. Reddy and H.F. Hodges)

Table 14.1. Principal cultivated CAM species, with part harvested and worldwide cultivation area indicated. Data involve extrapolations and estimates.

Harvested part

Area of

Species

or purpose

cultivation (ha) References

Agave fourcroydes,

Leaf fibre

400,000

Gentry, 1982; FAO

A. sisalana

FAOSTAT Statistics Data

base (http://apps.fao.org/)

Agave mapisaga, A.

Beverages,

88,000

Gentry, 1982; Nobel,

salmiana, A. tequilana,

some fodder

1994

various other agaves

Ananas comosus

Fruit

720,000

Bartholomew and

Rohrbach, 1993; FAO

FAOSTAT Statistics

Database

Hylocereus, Seleniocereus

Fruit

14,000

Mizrahi et al., 1997;

and Stenocereus species,

Pimienta-Barrios and

various other cacti

Nobel, 1994

Opuntia ficus-indica, other

Fodder, forage,

900,000

Barbera et al., 1995;

opuntias

vegetable

Nobel, 1994

Fruit

95,000

Cochineal dye,

50,000

chemicals

Total

2,267,000

(McLaughlin, 1993; Ravetta and McLaughlin, 1996). Other agaves are currently cultivated for alcoholic beverages, especially Agave tequilana for tequila. In addition, about ten species of agave are cultivated for mescal (also spelled mezcal), which, like tequila, is a distilled beverage, and for pulque, a fermented beverage. Commercial production of all these beverages occurs in Mexico.

The present worldwide cultivation of agaves as crops totals nearly 500,000 ha (Table 14.1). To this number can be added those agaves used as ornamental plants or for fences and erosion control. In addition to agaves, many species of cacti are also so utilized for these purposes. Because none of these CAM crops is particularly tolerant of freezing temperatures, nearly all cultivation of agaves, cacti and pineapple occurs within 30° latitude of the Equator (Nobel, 1988; Bartholomew and Rohrbach, 1993; Bartholomew and Malézieux, 1994). If temperatures rise as predicted during global climatic change, the regions suitable for the cultivation of such CAM plants will expand (Nobel, 1996b).

The most convincing evidence indicating that agaves, cacti and pineapple are CAM plants is the substantial nocturnal CO2 uptake by their photosynthetic organs, which occurs in all three groups (Joshi et al., 1965; Neales, 1973; Sale and Neales, 1980; Nose et al, 1986; Nobel, 1988; Borland and Griffiths, 1989; Medina et al., 1991). Because photosynthesis cannot occur without light, the CO2 taken up at night cannot be immediately fixed into photosynthetic products such as glucose and sucrose. Rather, the CO2 is incorporated into phosphoenolpyruvate (PEP) by the enzyme PEP carboxylase, leading to the formation of an organic acid such as malate. Nocturnal acidification of the chlorenchyma can be easily tested to demonstrate CAM in various species, including agaves, cacti and pineapple (Neales, 1973; Friend and Lydon, 1979; Nobel, 1988; Medina et al., 1991, 1993). A sophisticated but indirect method for determining whether or not a particular species uses CAM is to find the ratio of various carbon isotopes in the plant tissues. In particular, differences in the enzymes involved in the initial fixation of carbon and the subsequent biochemical processing of the fixed carbon lead to unique isotopic signatures for the three photosynthetic pathways: CAM (which is the only photosynthetic type with nocturnal stomatal opening), C3 and C4. Thus, isotopic analysis of carbon by mass spectroscopy of tissue samples can indicate the photosynthetic pathway. This has been done for all three groups of CAM crops (agaves, cacti and pineapple; Nobel, 1988; Medina et al., 1994).

A relevant question with respect to global climatic change and its influences on productivity (the major theme of this book) is what is the effect of increasing atmospheric CO2 concentrations ([CO2]) on net CO2 uptake by CAM plants? Another question about CAM plants concerns effects on their gas exchange and productivity caused by changes in other environmental factors, such as air temperature, photosynthetic photon flux (PPF, wavelengths of 400-700 nm that are absorbed by photosynthetic pigments) and soil water status (quantified by the soil water potential and reflecting the effects of rainfall). Environmental effects on net CO2 uptake and biomass productivity among CAM plants have been studied most extensively for O. ficus-indica, but sufficient data exist for predictions for other commercial CAM species as well.

One way to quantify the effects of environmental factors on net CO2 uptake is to use an environmental productivity index (EPI; Nobel, 1984, 1988, 1991b). The individual environmental factors affect net CO2 uptake multiplicatively, not additively. For instance, if prolonged drought causes daily stomatal opening to cease, then no net CO2 uptake will occur, regardless of whether or not light levels and temperatures are optimal for CO2 uptake. EPI can be represented as follows:

EPI = fraction of maximal daily net CO2 uptake (Eqn 14.1)

= water index x temperature index x PPF index

The water index, which like the other indices ranges from 0 when it is totally limiting for daily net CO2 uptake to 1 when it does not limit CO2 uptake, represents the fractional limitation due to soil water availability; hence, it progressively decreases during drought. The temperature index quantifies the limitations of temperature on daily net CO2 uptake, and the PPF index quantifies the effects of light. Because CAM plants take up CO2 primarily at night, the photosynthetic responses represented by the PPF index relate total daily net CO2 uptake over 24 h periods (mol CO2 m-2 day-1) to the total daily PPF (mol photons m-2 day-1). All the indices in equation 14.1 are determined over 24 h periods.

A fourth multiplicative index can be incorporated into equation 14.1 to quantify the effects of nutrients on daily net CO2 uptake. Such a nutrient index has been determined for various species of agaves and cacti, for which five elements (N, P, K, B and Na) have major influences on daily net CO2 uptake (Nobel, 1989). For plants maintained under various ambient CO2 levels for prolonged periods (months), the effects of [CO2] are most readily incorporated into predictions of net CO2 uptake by making measurements over 24 h under conditions of wet soil, optimal temperatures and saturating PPF. Increases in [CO2] can also affect the influences of other environmental factors on net CO2 uptake by CAM plants.

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