Modeling the canopy photosynthetic rate of romaine lettuce (Lactuca sativa L.) grown in a plant factory at varying CO₂ concentrations and growth stages

Abstract

Photosynthetic models of crops are essential for predicting the optimum CO₂ concentrations that should be maintained for crop productivity in closed systems throughout the growth period. The objective of this study was to develop a canopy photosynthetic model of romaine lettuce (Lactuca sativa L., cv. Asia Heuk romaine) incorporating CO₂ concentration and plant growth stage. The canopy photosynthetic rates of the plants were measured 4, 7, 14, 21, and 28 days after transplanting using closed acrylic chambers, in which the temperature was maintained at 24°C and a 200 µmol · m ^-2 · s^-1 light intensity was provided by an 8:1:1 ratio of RBW light-emitting diodes. The canopy photosynthetic rate of the lettuce was calculated by measuring the reduction in CO₂ within the chamber over time, from an initial concentration of 2,000 µmol · mol ^-1. The canopy photosynthetic rate became saturated as the CO₂ concentration increased, while it exponentially decreased with the plant growth stage. Among the previously published models available, the Thornley model was suitable for the expression of the canopy photosynthetic rate; however, it had to be adapted to take into account growth stage, resulting in an R² of 0.985. The canopy photosynthetic rates estimated by the models showed good agreement with those actually measured (R² = 0.939). Based on these results, the established model may be helpful in determining the optimum level of CO₂ required for crop production and in calculating the decreasing CO₂ requirements throughout the cultivation period.