Simulation of citrus foliar gas exchange across diverse meteorological conditions: application of the optimal stomatal regulation method

Introduction: The optimal stomatal regulation theory provides an eco-evolutionary framework for interpreting the trade-off between CO2 uptake and water loss. This theory postulates that the marginal water cost of carbon gain (λ=∂E/∂A) remains approximately constant over short timescales, thereby offering a mechanistic basis for predicting stomatal behavior and gas exchange.Methods: In this study, leaf-level meteorological variables and gas exchange parameters of orchard citrus were measured throughout the entire phenological period during 2021–2022. We developed a family of optimal stomatal conductance-based models (OSCMs), comprising six forms: Rubisco-limited forms (OSCvc and OSCvcd), RuBP-regeneration-limited forms (OSCvj and OSCvjd), and combined forms that dynamically select the prevailing biochemical limitation (OSC and OSCd).Results: The key parameter λ was estimated daily and averaged over the entire phenological period. Using daily λ inputs, the three models produced stomatal conductance (gs) with accuracies ranked as OSCvjd (R2 = 0.73) > OSCd (0.63) > OSCvcd (0.40). When a long-term constant λ was applied, model performance declined with accuracies ranked as OSCvj (0.66) > OSC (0.52) > OSCvc (0.38).Discussion: The OSC model also produced intercellular CO2 concentration (ci) and photosynthesis (A) reasonably well (R2 = 0.78 and 0.48, respectively). Under moderate meteorological conditions (air temperature 30–40 °C and vapor pressure deficit 1–2 kPa), the OSC model showed its best performance with a mean absolute relative error of 35.2% for gs estimation. Overall, the OSCMs provided a mechanistic approach to simulate citrus leaf gas exchange requiring minimal species-specific traits and routine meteorological inputs. This modeling strategy supports rapid assessment of plant physiological status and estimation of foliar carbon-water fluxes in orchard management under subtropical climates.