Evaluating photosynthetic models and their potency in assessing plant responses to changing oxygen concentrations: a comparative analysis of An–Ca and An–Ci curves in Lolium perenne and Triticum aestivum

Accurate determination of photosynthetic parameters is essential for understanding how plants respond to environmental changes. In this study, we evaluated the performance of the Farquhar-von Caemmerer-Berry (FvCB) model and introduced a novel model to fit photosynthetic rates against ambient CO₂ concentration (<i>A</i> _n <i>-C</i> _a) and intercellular CO₂ concentration (<i>A</i> _n <i>-C</i> _i) curves for <i>Lolium perenne</i> and <i>Triticum aestivum</i> under 2% and 21% O₂ conditions. We observed significant discrepancies in the FvCB model's fitting capacity for <i>A</i> _n <i>-C</i> _a and <i>A</i> _n <i>-C</i> _a curves across different oxygen regimes, particularly in estimates of key parameters such as the maximum carboxylation rate (<i>V</i> _cmax), the day respiratory rate (<i>R</i> _day), and the maximum electron transport rate for carbon assimilation (<i>J</i> _A-max). Notably, under 2% and 21% O₂ conditions, the values of <i>V</i> _cmax and <i>R</i> _day derived from <i>A</i> _n <i>-C</i> _a curves using the FvCB model were 46.98%, 44.37%, 46.63%, and 37.66% lower than those from <i>A</i> _n <i>-C</i> _i curves for <i>L. perenne</i>, and 47.10%, 44.30%, 47.03%, and 37.36% lower for <i>T. aestivum</i>, respectively. These results highlight that the FvCB model yields significantly different <i>V</i> _cmax and <i>R</i> _day values when fitting <i>A</i> _n <i>-C</i> _a versus <i>A</i> _n <i>-C</i> _i curves for these two C₃ plants. In contrast, the novel model demonstrated superior fitting capabilities for both <i>A</i> _n <i>-C</i> _a and <i>A</i> _n <i>-C</i> _i curves under 2% and 21% O₂ conditions, achieving high determination coefficients (<i>R</i> ²≥ 0.989). Key parameters such as the maximum net photosynthetic rate (<i>A</i> _max) and the CO₂ compensation point (<i>Γ</i>) in the presence of <i>R</i> _day, showed no significant differences across oxygen concentrations. However, the apparent photorespiratory rate (<i>R</i> _pa0) and photorespiratory rate (<i>R</i> _p0) derived from <i>A</i> _n <i>-C</i> _i curves consistently exceeded those from <i>A</i> _n <i>-C</i> _a curves for both plant species. Furthermore, <i>R</i> _pa0 values derived from <i>A</i> _n <i>-C</i> _a curves closely matched observed values, suggesting that <i>A</i> _n <i>-C</i> _a curves more accurately reflect the physiological state of plants, particularly for estimating photorespiratory rates. This study underscores the importance of selecting appropriate CO₂-response curves to investigate plant photosynthesis and photorespiration under diverse environmental conditions, thereby ensuring a more accurate understanding of plant responses to changing environments.