Productivity

Most productivity studies on agronomic C3 and C4 species were done before 1975 (Loomis and Gerakis, 1975). After that time, the focus has been on increasing their harvest index (fraction of the plant harvested), as well as on aspects of plant performance that can be affected by molecular biology/

biotechnology. Very little such attention has been devoted to CAM plants, although efforts to preserve the germplasm with field plantings in various countries have been made with prospects for increased productivity and future biotechnology in mind. CAM plants in nature are generally relatively slow-growing, but productivity for certain cultivated CAM species can be high. For example, the dry weight productivity of two cultivated CAM species, Opuntia amyclea and O. ficus-indica, equals or exceeds the highest values for all C3 crops and trees and is exceeded by only a few C4 crops (Table 14.2; Nobel, 1991a, 1996a). Moreover, because of the water-conserving characteristics of CAM (high WUE), the productivity of CAM plants can be substantially higher

Table 14.2. Maximal net CO2 uptake and productivity of cultivated CAM plants, with comparisons for the C3 and C4 species with the highest productivities.

Maximal

Maximal

Maximal net

daily net CO2

productivity

CO2 uptake rate

uptake (mmol

(t dry weight

Species

(mmol m-2 s-1)

m-2 day-1)

ha-1 year-1)

References

Agave

11

360-400

about 15

Nobel, 1985;

fourcroydes,

Nobel and

A. sisalana,

Valenzuela, 1987;

A. tequilana

Nobel, 1988

Agave mapisaga,

29-34

1050-1170

38-42

Nobel et al., 1992

A. salmiana

Ananas comosus

S

240-310

20-31

Bartholomew and Kadzimin, 1977; Bartholomew, 1982; Medina et al., 1991; Bartholomew and Rohrbach, 1993; Zhu et al, 1997a

Opuntia

26-28

940

45-50

Nobel et al, 1992;

amyclea, O.

Garcia de Cortázar

ficus-indica

and Nobel, 1992

Stenocereus

1G

320

Nobel and

queretaroensis

Pimienta-Barrios, 1995

Six highest C3

av. 39

av. about 900

29-45

Loomis and

crops

Gerakis, 1975; Loomis, 1983; Nobel, 1991a

Six highest C3

-

-

36-44

Jarvis and

trees

Leverenz, 1983; Nobel, 1991a

Six highest C4

av. 46

av. about 1200

32-70

Loomis and

crops

Gerakis, 1975; Loomis, 1983; Nobel, 1991a

than that of C3 and C4 plants under conditions of low availability of soil water, an increasingly important aspect for crops worldwide.

Productivity per unit ground area per year, which is crucial for assessing agronomic potential, can be related to the net CO2 uptake characteristics of plants (Nobel, 1988). In this regard, A. mapisaga, A. salmiana, O. amyclea, and O. ficus-indica have the highest maximal net CO2 uptake rates among CAM crops, averaging 29 |mmol m-2 s-1, which is only 30% less than the average reported maximal rates for the C3 and C4 crops with the highest rates (Table 14.2). Net CO2 uptake for CAM plants is generally measured and expressed for the surface most exposed to the incident PPF (one side of a leaf or a stem, which is usually opaque). For the thin leaves of C3 and C4 plants, net CO2 uptake is measured for both sides of a leaf but is expressed per unit area of one side (the projected leaf area). If net CO2 uptake by the sides of leaves or stems of CAM plants facing away from the highest PPF is also included, then the maximal rates are actually rather similar among plants of the three photosynthetic pathways. Because net CO2 uptake by CAM plants can occur during both day and night (Fig. 14.1), compared with C3 and C4 plants which have net CO2 uptake only during the day, the maximal daily net CO2 uptake for these four highly productive CAM species is intermediate between the six C3 crops with the highest values and the equivalent six C4 species, even though a different area basis is conventionally used (Table 14.2). Moreover, for the CAM crops, the maximal total daily net CO2 uptake is highly correlated with the maximal instantaneous net CO2 uptake rate. Plant productivity per unit ground area for the various CAM crop plants considered is also highly correlated with the total daily net CO2 uptake per unit photosynthetic surface area, although the actual productivity in the field depends on interplant spacing.

CAM plants minimize the energetically expensive release of CO2 via photorespiration by presenting high levels of CO2 to the pivotal enzyme, ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) (Nobel, 1988, 1991a, 1994). Thus photorespiration is relatively low for CAM plants, as is also the case for C4 plants. Based on energy costs for the production of ATP and NADPH and the number of these molecules required for the fixation of CO2 into carbohydrates, the total cost per carbon molecule fixed for CAM species is lower than that for C3 species and only slightly higher than that for C4 species (Nobel, 1991a, 1996a). Thus the intermediate values of total daily net CO2 uptake and of long-term productivity by highly productive CAM species compared with similar C3 and C4 crops (Table 14.2) can be explained at a cellular level. This increases confidence in predictions that certain CAM crops will be more extensively cultivated in the future (Nobel, 1994, 1996a,b; Mizrahi et al., 1997), because the CAM pathway can lead to high productivity.

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