Abstract
Crassulacean acid metabolism (CAM) crops are important agricultural commodities in water-limited environments across the globe, yet modelling of CAM productivity lacks the sophistication of widely used C3 and C4 crop models, in part due to the complex responses of the CAM cycle to environmental conditions. This work builds on recent advances in CAM modelling to provide a framework for estimating CAM biomass yield and water use efficiency from basic principles. These advances, which integrate the CAM circadian rhythm with established models of carbon fixation, stomatal conductance and the soil-plant-atmosphere continuum, are coupled to models of light attenuation, plant respiration and biomass partitioning. Resulting biomass yield and transpiration for Opuntia ficus-indica and Agave tequilana are validated against field data and compared with predictions of CAM productivity obtained using the empirically based environmental productivity index. By representing regulation of the circadian state as a nonlinear oscillator, the modelling approach captures the diurnal dynamics of CAM stomatal conductance, allowing the prediction of CAM transpiration and water use efficiency for the first time at the plot scale. This approach may improve estimates of CAM productivity under light-limiting conditions when compared with previous methods.
Highlights
Due to their unique circadian rhythm of nocturnal carbon dioxide uptake and storage, Crassulacean acid metabolism (CAM) photosynthetic plants regularly achieve a water use efficiency six or more times higher than that of their C3 counterparts (Lambers, Stuart Chapin III, & Pons, 2008), making them promising candidates for food, fodder, and biofuel production in water-stressed ecosystems across the globe
The modeling approach introduced by Bartlett et al (2014) and incorporated in the Photo3 model (Hartzell et al, 2018) couples a mathematical representation of the CAM circadian rhythm as a Van der Pol oscillator with established models of carbon fixation (Farquhar et al, 1980), stomatal conductance (Katul & Oren, 2009; Medlyn et al, 2011), and the soil-plant-atmosphere continuum to calculate CAM carbon assimilation and transpiration on an hourly timescale and at the plant scale based upon photosynthetically active radiation (PAR), temperature, specific humidity, and soil moisture, but does not address resource allocation or dry mass productivity
Model results for O. ficus-indica captured Phases I and III of the observed behavior and suggested a short Phase II and IV uptake of CO2 which have been observed for O. ficus-indica under similar conditions (Cui & Nobel, 1994; Cui et al, 1993) but were not present in this particular dataset (Fig. 2c)
Summary
Due to their unique circadian rhythm of nocturnal carbon dioxide uptake and storage, CAM photosynthetic plants regularly achieve a water use efficiency six or more times higher than that of their C3 counterparts (Lambers, Stuart Chapin III, & Pons, 2008), making them promising candidates for food, fodder, and biofuel production in water-stressed ecosystems across the globe. Recent research has set out to answer questions about the potential of these crops for biofuel production and food security in future warming and drying environments (Borland et al, 2009; de Cortazar & Nobel, 1990; Mason et al, 2015; Owen & Griffiths, 2014; Yang et al, 2015) In support of these efforts, a number of modeling approaches have arisen to represent CAM plants at varying levels of complexity.
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