CAM species, which taxonomically are at least five times more numerous than C4 species, often grow-slowly, as is the case for various short-statured cacti and many epiphytes in several families, However, slow growth is not a necessary corollary of the CAM photosynthetic pathway, as can be appreciated by considering the energetics of CO2 fixation. For every CO2 fixed photosynthetically, C3 plants require 3 ATP and 2 NADPH, whereas the extra enzymatic reactions and compartmentation complexity for C4 plants require 4 or 5 ATP and 2 NADPH, and CAM plants require 5.5–6.5 ATP and 2 NADPH. Photorespiration in C8 plants can release some of the CO2, fixed and also has an energetic-cost, whereas photorespiration is much less in C4 and CAM plants. Therefore, CAM plants can perform net CO2 fixation 15% more efficiently than C3, plants, although 10% less efficiently than C4 plants.
Using a simple model that assumes 8 photons per CO2 fixed and a processing time per excitation of 5 ms, a maximum instantaneous rate for net CO2, uptake of 55 μmol m−2 s−1 is predicted. Measured maximal rates average 48μmol m−2 s−1 for leaves of six C3 species with the highest rates and 64 μmol m−2 s−1 for six such C4 species; CAM plants take up CO2 mainly at night, which is not directly related to the instantaneous rate of photon absorption. Net CO2 uptake integrated over 24 h, which is more pertinent to productivity than are instantaneous CO2 uptake rates, is similar for the three pathways, although the higher water-use efficiency of CAM plants can be an advantage during drought.
Canopy architecture is crucial for the distribution of the photosynthetic photon flux density (PPFD) over the shoot, which determines net CO2 uptake per unit ground area and hence determines productivity. Maximal productivity for idealized canopies under optimal conditions is predicted to be about 100 Mg d. wt ha−1 yr−1 (1 Mg = 1 tonne), whereas actual values of environmental factors in the field approximately halve this prediction. The influence of environmental factors on net CO2 uptake can be quantified using an environmental productivity index (EPI), which predicts the fractional limitation on net CO2 uptake and is the product of a water index, a temperature index, and a PPFD index (nutrient effects can also be included).
Using EPI with a ray-tracing technique to determine the PPFD index and taking into account respiration and carbon incorporated structurally, maximal productivity of CAM plants is predicted to occur at leaf or stem area indices of 4–5. In experiments designed using such shoot area indices, annual above-ground dry-weight productivities averaging 43 Mg ha−1 yr−1 have recently been observed for certain agaves and plutyopuntias. In comparison, the measured average annual productivity of the most productive plants is 49 Mg ha−1 yr−1 for six agronomic C4 species, 35 Mg ha−1 yr−1 for sis agronomic C3 species, and 39 Mg ha−1 yr−1 for six C3 tree species. Thus, CAM plants are capable of similar high productivities, which can become especially advantageous in regions of substantial water stress. Recognition of the high potential productivity of certain CAM species under optimal environmental conditions, exceeding that of most C3 species, may increase the cultivation of such CAM plants in various areas in the future.
Crassulacean acid metabolism
environmental productivity index
leaf area index
photosynthetic photon flux density
stem area index