A system dynamics model integrating physiology and biochemical regulation predicts extent of crassulacean acid metabolism (CAM) phases

A system dynamics (SD) approach was taken to model crassulacean acid metabolism (CAM) expression from measured biochemical and physiological constants. SD emphasizes state-dependent 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 Kalanchoe 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 CO2 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.