Achievable productivities of certain CAM plants: basis for high values compared with C3 and C4 plants

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. CONTENTS Summary 183 I. Introduction 184 II. Biochemistry of C₃, C₄, and CAM plants 185 III. CO₂ uptake rates 188 IV. Canopy architecture and light absorption 193 V. Measured biomass productivity 198 VI. Conclusions 200 Acknowledgement 202 References 202