Modelling nonlinear dynamics of Crassulacean acid metabolism productivity and water use for global predictions

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.

From diel photosynthesis to crop growth in the crassulacean acid metabolism (CAM) orchid Phalaenopsis

“…It can predict gs, carbon assimilation, and transpiration under well-watered and drought conditions. Very recently, the model was expanded with an EPI section to predict biomass accumulation at a monthly time scale (Hartzell et al, 2021). While this is an important improvement, assumptions for biomass accumulation are simplified and still suffer from the low resolution of EPI.…”

“…Different approaches are used when modelling leaf conductance, such as using a species-specific maximum of gs and gm that scales based on pCO2 and pH2O (Owen and Griffiths, 2013), a simplified gas-diffusion model that indirectly accounts for stomatal conductance and can predict water-loss on leaf-level (Töpfer et al, 2020), or a combined value for gs and gm based on vapour-pressure deficit (Hartzell et al, 2021). In case of the latter, they used this model to calculate WUE based on a leaf under drought, and extrapolated it to the whole plant.…”

“…In most models, the tonoplast switch is set to occur at a fixed time (Owen and Griffiths, 2013). Both Bartlett et al (2014) and Hartzell et al (2021) model in-and efflux of the vacuole with a cubic function, based on a circadian order variable that depends on temperature and maximum storage concentration of the vacuole, that in return deactivates proton pumping and allows for malic acid release. This function also takes into account PEPC activity.…”

“…This reduced export of sugars towards the roots, which affected shoot:root ratio of the plant. Assuming one fixed value for shoot:root ratio as done by Hartzell et al (2021) might be too simply put, as this balance is subject to change with varying environmental conditions, as well as with the amount of CAM expression (Haider et al, 2012).…”

“…(Winter, 2019). Hartzell et al (2021) were the first to combine short and long-term growth within one model. This approach seems to function in areas that are well-watered and where light is not limiting.…”