A coupled model of photosynthesis and stomatal conductance for the ice plant (Mesembryanthemum crystallinum L.), a facultative CAM plant

The ice plant (Mesembryanthemum crystallinum L.), a medicinal plant with well-known effects on retarding diabetes mellitus, is increasingly being produced in plant factories in Asia. The ice plant is a Crassulacean Acid Metabolism (CAM) plant, but performs C₃ photosynthesis during the juvenile period. The objective of this study was to develop a photosynthetic model of ice plants growing under plant factory conditions. C₃ photosynthesis was observed in juvenile plants in plant factory growth conditions and a conversion from C₃ to CAM photosynthesis was observed under salt-stressed condition at an electrical conductivity (EC) of 6.0 dS·m^-1. The light saturation and compensation points, determined by a regression analysis of C₃ light curves for the ice plant leaves, were 609.4 and 53.2 μmol·m^-2·s^-1, respectively. The accuracy of the light response was compared between negative exponential and non-rectangular hyperbolic function models. The non-rectangular hyperbola was more accurate with complicated parameters while the negative exponential function was more practical with simple parameters in the light response curves. Measurement of net CO₂ assimilation rate (A) and intercellular CO₂ concentration (C_i) allowed construction of the A-Ci curve and regression analysis of this curve revealed the CO₂ saturation and compensation points as 632.9 and 117.2 μmol·mol^-1, respectively. A coupled photosynthetic model was developed for the simultaneous prediction of photosynthesis, stomatal conductance, transpiration, and temperature of the ice plant leaves. Sharkey’s regression method was used to determine the photosynthetic parameters of maximum carboxylation rate, the potential rate of electron transport, and the rate of triose phosphate utilization, which were 222.3, 234.9, and 13.0 μmol·m^-2·s^-1, respectively. The parameters of minimum stomatal conductance of water vapor at the light compensation point (b) and the empirical coefficient (m) for the sensitivity of stomatal conductance and relative humidity in the Ball, Woodrow and Berry model could be solved as b = 0.0487 and m = 0.0012 by linear regression analysis using the measured A-Ci values. Although the A-Ci curve of the negative exponential function had higher accuracy than the biochemical model, the coupled biochemical model could physiologically explain the photosynthesis of the ice plant leaves under plant factory conditions.