Designing a strategy for cost-effective CO<sub>2</sub> enrichment in a ventilated greenhouse based on the photosynthetic photon flux density-net photosynthetic rate curve of cucumber seedlings

In greenhouse cultivation, CO2 enrichment enhances the net photosynthetic rate (Pn) of the plant canopy. Null balance CO2 enrichment (NB-CE) maintains equal CO2 concentrations inside and outside the greenhouse (Cin and Cout), limiting CO2 leakage; however, there may be more cost-effective strategies. This study proposes an alternative strategy termed CO2 enrichment only for high photosynthetic photon flux density (PPFD) hours (HP-CE). In this strategy, Cin is maintained slightly above Cout when Pn is expected to be high, aiming to improve cost-effectiveness. We defined the cost-effectiveness of CO2 enrichment as the increase in integrated Pn over a photoperiod compared to non-CO2 enrichment, divided by the CO2 supply rate (S) integrated over the same period. Since CO2 leakage increases with the number of air exchanges per hour (N), we evaluated the cost-effectiveness of HP-CE at several N levels. In this study, the integrated S of HP-CE was set equivalent to that of NB-CE, and the CO2 supply period of HP-CE was adjusted based on the target Cin (450-700 µmol mol-1) at N of 4, 6, 8, and 10 h-1. Using an open-type assimilation chamber to reproduce ventilated greenhouse environments, we measured the PPFD-Pn and PPFD-S curves of cucumber seedlings. We then estimated the Pn and S over a 12-hour photoperiod for NB-CE and several HP-CE scenarios. The estimation revealed that HP-CE could be more cost-effective than NB-CE for Cin 450-700 µmol mol-1 at N of 4, 6, and 8 h-1. HP-CE was less cost-effective than NB-CE at N of 10 h-1 due to significant CO2 leakage at increased target Cin. These findings suggest that for ventilated greenhouses with relatively low N, CO2 enrichment can be more cost-effective than NB-CE by slightly elevating Cin above Cout only for high PPFD hours.