Achievable productivities of certain CAM plants: basis for high values compared with C 3 and C 4 plants

summary CAM species, which taxonomically are at least five times more numerous than C 4 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 CO 2 fixation. For every CO 2 fixed photosynthetically, C 3 plants require 3 ATP and 2 NADPH, whereas the extra enzymatic reactions and compartmentation complexity for C 4 plants require 4 or 5 ATP and 2 NADPH, and CAM plants require 5.5–6.5 ATP and 2 NADPH. Photorespiration in C 8 plants can release some of the CO 2 , fixed and also has an energetic‐cost, whereas photorespiration is much less in C 4 and CAM plants. Therefore, CAM plants can perform net CO 2 fixation 15% more efficiently than C 3 , plants, although 10% less efficiently than C 4 plants. Using a simple model that assumes 8 photons per CO 2 fixed and a processing time per excitation of 5 ms, a maximum instantaneous rate for net CO 2 , uptake of 55 μmol m −2 s −1 is predicted. Measured maximal rates average 48μmol m −2 s −1 for leaves of six C 3 species with the highest rates and 64 μmol m −2 s −1 for six such C 4 species; CAM plants take up CO 2 mainly at night, which is not directly related to the instantaneous rate of photon absorption. Net CO 2 uptake integrated over 24 h, which is more pertinent to productivity than are instantaneous CO 2 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 CO 2 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 CO 2 uptake can be quantified using an environmental productivity index (EPI), which predicts the fractional limitation on net CO 2 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 C 4 species, 35 Mg ha −1 yr −1 for sis agronomic C 3 species, and 39 Mg ha −1 yr −1 for six C 3 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 C 3 species, may increase the cultivation of such CAM plants in various areas in the future.CONTENTSSummary 183 I. Introduction 184 II. Biochemistry of C 3 , C 4 , and CAM plants 185 III. CO 2 uptake rates 188 IV. Canopy architecture and light absorption 193 V. Measured biomass productivity 198 VI. Conclusions 200Acknowledgement 202References 202