Integrating Rapid Light- and CO₂-Response Curves with Low-High-Low Irradiance Kinetics Under Acute Red:Blue Spectra...

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-C_i) were followed by low-high-low irradiance measurements at reference CO₂ mole fractions (C_a) 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-C_i area under the curve (14.45 ± 1.43 µmol CO₂ m^-2 s^-1), empirical CO₂-response plateau and operational high-C_i triose-phosphate utilisation (TPU) proxy. At C_a = 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 C_a = 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.