Integrating Rapid Light- and CO2-Response Curves with Low–High–Low Irradiance Kinetics Under Acute Red:Blue Spectra in Leaf Chicory

Rapid response curves and transition kinetics describe complementary dimensions of leaf photosynthesis, yet they are rarely evaluated together under acute spectral contrasts. We quantified net CO2 assimilation (A), gas exchange and pulse-amplitude-modulated chlorophyll a fluorescence in white-light-grown leaf chicory (Cichorium intybus L. cv. Pão-de-Açúcar) under R100B0, R90B10, R50B50 and R0B100 chamber conditions, with randomised spectral measurement order. Rapid stepwise assimilation responses to photosynthetic photon flux density (A-PPFD) and intercellular CO2 (A-Ci) were followed by low–high–low irradiance measurements at reference CO2 mole fractions (Ca) of 400 and 2000 µmol mol−1. R90B10 produced the highest normalised A-PPFD area under the curve (8.23 ± 1.04 µmol CO2 m−2 s−1), apparent quantum yield, induction amplitude and integrated carbon gain. R0B100 produced the lowest normalised A-Ci area under the curve (14.45 ± 1.43 µmol CO2 m−2 s−1), empirical CO2-response plateau and operational high-Ci triose-phosphate utilisation (TPU) proxy. At Ca = 400 µmol mol−1, the median time to 90% of the transition amplitude (t90) was 6.1–12.0 min for A and 17.1–24.4 min for stomatal conductance and transpiration. At Ca = 2000 µmol mol−1, A approached its final state earlier, with smaller stomatal and transpiration amplitudes under the reported chamber conditions. After the irradiance decreased, A reached 90% of its response within the first 60–120 s observation interval, whereas stomatal conductance and transpiration required 11.5–15.6 min to reach the same fraction of their responses, sustaining postillumination water loss. These measurements reveal that acute spectral quality affects the recorded photosynthetic response and carbon–water coordination and that response capacity, amplitude and fractional speed are complementary physiological traits.