Summary Crassulacean acid metabolism (CAM) is a specialized mode of photosynthesis that features nocturnal CO2 uptake, facilitates increased water‐use efficiency (WUE), and enables CAM plants to inhabit water‐limited environments such as semi‐arid deserts or seasonally dry forests. Human population growth and global climate change now present challenges for agricultural production systems to increase food, feed, forage, fiber, and fuel production. One approach to meet these challenges is to increase reliance on CAM crops, such as Agave and Opuntia, for biomass production on semi‐arid, abandoned, marginal, or degraded agricultural lands. Major research efforts are now underway to assess the productivity of CAM crop species and to harness the WUE of CAM by engineering this pathway into existing food, feed, and bioenergy crops. An improved understanding of CAM has potential for high returns on research investment. To exploit the potential of CAM crops and CAM bioengineering, it will be necessary to elucidate the evolution, genomic features, and regulatory mechanisms of CAM. Field trials and predictive models will be required to assess the productivity of CAM crops, while new synthetic biology approaches need to be developed for CAM engineering. Infrastructure will be needed for CAM model systems, field trials, mutant collections, and data management.

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SummaryA system dynamics (SD) approach was taken to model crassulacean acid metabolism (CAM) expression from measured biochemical and physiological constants. SD emphasizes statedependent feedback interaction to describe the emergent properties of a complex system. These mechanisms maintain biological systems with homeostatic limits on a temporal basis.Previous empirical studies on CAM have correlated biological constants (e.g. enzyme kinetic parameters) with expression over the CAM diel cycle. The SD model integrates these constants within the architecture of the CAM 'system'. This allowed quantitative causal connections to be established between biological inputs and the four distinct phases of CAM delineated by gas exchange and malic acid accumulation traits.Regulation at flow junctions (e.g. stomatal and mesophyll conductance, and malic acid transport across the tonoplast) that are subject to feedback control (e.g. stomatal aperture, malic acid inhibition of phosphoenolpyruvate carboxylase, and enzyme kinetics) was simulated. Simulated expression for the leaf-succulent Kalancho€ e daigremontiana and more succulent tissues of Agave tequilana showed strong correlation with measured gas exchange and malic acid accumulation (R 2 = 0.912 and 0.937, respectively, for K. daigremontiana and R 2 = 0.928 and 0.942, respectively, for A. tequilana). Sensitivity analyses were conducted to quantitatively identify determinants of diel CO 2 uptake. The transition in CAM expression from low to high volume/area tissues (elimination of phase II-IV carbon-uptake signatures) was achieved largely by the manipulation three input parameters.

Biomass production on low-grade land is needed to meet future energy demands and minimize resource conflicts. This, however, requires improvements in plant water-use efficiency (WUE) that are beyond conventional C3 and C4 dedicated bioenergy crops. Here we present the first global-scale geographic information system (GIS)-based productivity model of two highly water-efficient crassulacean acid metabolism (CAM) candidates: Agave tequilana and Opuntia ficus-indica. Features of these plants that translate to WUE advantages over C3 and C4 bioenergy crops include nocturnal stomatal opening, rapid rectifier-like root hydraulic conductivity responses to fluctuating soil water potential and the capacity to buffer against periods of drought. Yield simulations for the year 2070 were performed under the four representative concentration pathway (RCPs) scenarios presented in the IPCC's 5th Assessment Report. Simulations on low-grade land suggest that O. ficus-indica alone has the capacity to meet 'extreme' bioenergy demand scenarios (>600 EJ yr À1) and is highly resilient to climate change (À1%). Agave tequilana is moderately impacted (À11%). These results are significant because bioenergy demand scenarios >600 EJ yr À1 could be met without significantly increasing conflicts with food production and contributing to deforestation. Both CAM candidates outperformed the C4 bioenergy crop, Panicum virgatum L.(switchgrass) in arid zones in the latitudinal range 30°S-30°N.

The Nobel environmental productivity index (EPI) was used as a framework for the development of a predictive geospatial model to estimate the bioethanol yield potential of four crassulacean acid metabolism (CAM) candidates in Australia (Agave fourcroydes, Agave salmiana, Agave tequilana, and Opuntia ficus-indica). GIS software was used to integrate climate datasets with titratable acidity responses to changes in photosynthetically active radiation (PAR), temperature, and water availability. Additional refinements to Nobel's approach were made to accommodate spatial and temporal fluctuations in soil water potential (ψs) as a function of soil particle size distribution and precipitation, and CO 2 uptake response to a range of day and night temperatures. A scalar factor for CO 2 persistence during periods of drought was also introduced to model the capacity of succulent species of Agave to buffer against fluctuations in ψs. Macro-scale criteria were applied to estimate environmentally responsible (ER) bioethanol yield potential on lands that are not suitable for food production. Consideration was given to indigenous vascular plant species richness and endemism scores at ER sites of interest. ). This research indicated the CAM pathway may produce significant yields (≥ 5 kL ha -1 yr -1 ) at ER sites totalling 57,700 km 2 (0.7% land area of Australia).