CAM Photosynthesis

CAM photosynthesis is a CO2-concentrating mechanism that uses a C4 cycle of PEP carboxylation followed by C4 acid decarboxylation to concentrate CO2 around Rubisco (Figure 4). In CAM plants, stomata open at night when conditions are relatively cool and humid. PEP carboxy-lase is active at night, fixing inorganic carbon into C4 acids that are stored in large vacuoles. During the day,

Table 1 Characteristics of C3, C4 and CAM plants in the late-twentieth century

Parameter

C3

C4

CAM

Units

Maximum photosynthesis rate

20-50

35-75

5-10

immolm^s-1

Maximum daily production rate

10-40

40-80

6-10

g m-2d-1

Maximum biomass yield

1-5

3-14

0.8-2.5

kg m-2yr-1

Photosynthetic thermal optimum

15-30

30-40

10-15 (night)

°C

30-40 (day)

Photorespiration/Photosynthesis @ 10°C

8%

1-5%

@ 25 °C

25%

1-5%

@ 40 °C

40%

1-5%

Resource-use efficiencies

Water-use efficiency (WUE)

1.5-2.5

3-5

6-15

gDMg-1 H2O

Nitrogen-use efficiency (NUE)

50-280

280-520

mmolCO2mol-1 N

Radiation-use efficiency (RUE)

1.7-1.9

2.5

gDMMJ-1

Principle life-forms

All terrestrial

Grasses, sedges,

Desert

forms

weedy dicots, desert

succulents

shrubs

Tropical

epiphytes

Data for CAM photosynthesis is derived from species in natural stands with drought, where the CAM pathway would be active. DM, dry matter. Adapted from LarcherW(2003) Physiological Plant Ecology. Berlin: Springer; and Sage RF (2001) In: Levin SA(ed.) Encyclopedia of Biodiversity, vol. 1 pp. 575-598. San Diego: Academic Press.

Data for CAM photosynthesis is derived from species in natural stands with drought, where the CAM pathway would be active. DM, dry matter. Adapted from LarcherW(2003) Physiological Plant Ecology. Berlin: Springer; and Sage RF (2001) In: Levin SA(ed.) Encyclopedia of Biodiversity, vol. 1 pp. 575-598. San Diego: Academic Press.

stomata close to save water, and the C4 acids are decar-boxylated, releasing the CO2 that is then refixed by Rubisco in the normal mode of C3 photosynthesis. Both CAM plants and C4 plants use a very similar biochemistry to effect CO2 concentration; however, PEP carboxylation and Rubisco carboxylation are spatially separated in C4 plants, but temporally separated in CAM plants.

Photosynthetic productivity in CAM plants is restricted by the limited capacity of the vacuole for storing organic acids at night. To overcome this limitation, CAM plants form large photosynthetic cells with massive vacuoles to enhance carbon storage. These large cells cause the leaves to have a succulent morphology. Even with this modification, the peak biomass productivity of CAM plants is generally well below that of productive C4 and C3 vegetation. Many CAM plants are not exclusively restricted to CAM metabolism, however, as they can switch into a normal C3 mode when water is readily available. In such species, the CAM mode of photosynthesis is activated during times of severe drought or salinity stress as a survival, rather than a high-productivity mechanism.

The ability to concentrate CO2 at night and close stomata during the day greatly enhances WUE and thus allows for primary productivity in extremely harsh situations. CAM plants exhibit the highest WUE of all plants (Table 2). High WUE enables CAM plants to grow in environments that would otherwise be too dry to support higher plants. CAM plants dominate many of the low-latitude deserts of the world, allowing for species-rich ecosystems in settings that would otherwise have a simple ecosystem of ephemerals plants, mosses, and lichens.

Most CAM species actually occur in tropical rainforests, growing as epiphytes on the branches of C3 trees. Here, CAM photosynthesis provides primary productivity in microenvironments where the lack of soil allows for substantial aridity between rain events. Many of the orchids and bromeliads of the tropical forest are CAM epiphytes, and they allow for complex aboveground habitat in what could otherwise be a barren branch. In the temperate zone where CAM epiphytes are absent due to cold, much simpler lichens and mosses fill the niche of the CAM epiphyte.

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