Dynamic Responses of Photosynthetic Parameters in Lonicera japonica Leaves During Photosynthetic Induction Under Elevated CO2 Concentrations

Photosynthetic induction is a critical physiological process through which plants rapidly adjust their photosynthetic machinery in response to a step increase in light intensity. While steady-state photosynthetic responses to elevated CO2 (eCO2) are well characterized, the characteristics of photosynthetic induction in honeysuckle under varying CO2 concentrations remain underexplored, particularly for ecologically important species such as honeysuckle (Lonicera japonica). In this study, we investigated the photosynthetic induction responses of L. japonica leaves subjected to an abrupt light transition from 100 to 1200 µmol m−2 s−1 under three CO2 concentrations: 420 μmol mol−1 (ambient, aCO2), 620 μmol mol−1 (medium, mCO2), and 820 μmol mol−1 (elevated, eCO2). Key photosynthetic parameters, including net photosynthetic rate (A), stomatal conductance (gs), intercellular CO2 concentration (Ci), and apparent maximum Rubisco carboxylation rate (V′cmax), were continuously estimated throughout the induction process. Results showed that eCO2 significantly enhanced steady-state A while simultaneously reducing gs and V′cmax, indicating that higher CO2 availability drives carbon gain primarily through increased substrate supply rather than enhanced enzymatic performance. Importantly, the time required to reach 90% induction state of steady-state A (tA90) and V′cmax (tV′cmax90) decreased markedly with increasing CO2. Time-integrated limitation analysis revealed that biochemical limitation consistently dominated the induction process regardless of CO2 treatment, while stomatal limitation remained uniformly low. These findings demonstrate that eCO2 substantially accelerated photosynthetic induction in L. japonica, as reflected by faster gas-exchange-derived induction parameters (e.g., tA90 and tV′cmax90), without altering the fundamental limitation hierarchy governing the induction process.