Waning advantages of CO₂ enrichment on photosynthesis and productivity due to accelerated phase transition and source-sink imbalance in sweet pepper

CO2 concentration, which can be raised by CO2 enrichment, has been considered to affect photosynthetic physiology, morphology, and yield of crops in greenhouses. However, inconsistent influence of CO2 enrichment on crop productivity has been demonstrated owing to the complicated and dynamic interaction between physIological and morphological responses of crops. Therefore, integrative analysis on the systemic responses of crops to CO2 enrichment is required for clarifying the reasons for the inconsistency. The objective of this study was to analyze the effect of CO2 enrichment on the photosynthesis and yield of sweet pepper plants according to growth phase and to investigate how photosynthetic physiology and plant morphology contribute to the CO2 enrichment effect via functional structural plant model (FSPM), an integrative plant modeling tool. CO2 was enriched to sweet pepper plants with a daily average concentration of about 900 mu mol mol(-1). Growth, yield, and photo-synthesis were periodically measured during the cultivation. The structural and photosynthetic properties ob-tained through 3D-phenotyping and gas-exchange measurement were applied to the 3D-framework for FSPM. The change in the CO2 enrichment efficiency for photosynthesis and the contribution of photosynthetic and morphological acclimation to CO2 enrichment effect were calculated. The overall influence of CO2 enrichment on the plants changed by growth stage mostly mediated by photosynthate source-sink balance. In early stages, CO2 enrichment accelerated the growth phase transition and increased the fruit yield through the increase in photosynthate partitioning to fruits. In later stages, however, the efficiency of CO2 enrichment on fruit yield gradually decreased due to the restricted leaf area expansion caused by excessive photosynthate partition to fruits. The systemic influence of CO2 enrichment on the plants including photosynthetic acclimation, structural acclimation, and fruit yield could be reasonably elucidated through the method used in this study. Thus, balancing the photosynthate source-sink balance in the early stages with appropriate cultivation practices could improve the long-term productivity of sweet pepper under CO2 enrichment.