A modification of the SMPTSB model to improve the simulation of photosynthesis and transpiration rates of citrus trees under temperature stress

Plant photosynthesis and transpiration are basic processes in water and carbon cyclings in ecosystems. Accurate simulation of the net CO2 assimilation rate (Am-c) and transpiration rate (Em-c) of citrus tree canopy under high temperature stress can help to obtain rational planting and irrigation schemes to cope with the impact of environmental stress on the citrus trees. In this study, Am-c and Em-c were measured by the wrapped Stem Flow Meter and Portable Photosynthesis System to analyze the daytime variation characteristics of Am-c and Em-c combined with the observed meteorological data. Then, the Synthetic Model of Photosynthesis-Transpiration based on the Stomatal Behavior model (SMPTSB) into the Photosynthetic-Transpiration coupling Model based on three Temperatures (PTM-3T) by modifying the calculation method of aerodynamic resistance (ra) and canopy resistance (rc) using imitation leaf method and temperature difference method, respectively, to improve the simulation accuracy of Am-c and Em-c under high-temperature condition. The results indicate that: (1) Under high temperature stress condition, there was an "asynchronous" phenomenon where the daytime peak of Am-c appeared earlier than that of Em-c. (2) Compared with the SMPTSB model, the PTM-3T model has higher simulation accuracy when citrus trees are affected by high temperature, where the R2 of the Am-c is increased from 0.79 to 0.82, and the RMSE is reduced from 0.87 to 0.77 μmol m–2 s–1, the R2 of the Em-c is increased from 0.17 to 0.76, and the RMSE is reduced from 0.68 to 0.27 mmol m–2 s–1, respectively. This study can provide a way to more accurately simulate the Am-c and Em-c under environmental stress conditions and can obtain a better understanding of the mass transport and energy exchange of the soil-plant-atmosphere system.