An energy-saving glasshouse film reduced light, but elevated CO₂ could not fully mitigate light-related yield losses in capsicum

Glasshouse covering materials such as light-blocking film (LBF) can reduce energy consumption but adversely affect yield depending on the crop, cultivar, and growth season. Elevated CO₂ concentration (eCO₂) is a well-adopted growth strategy deployed to boost the growth and yield of horticultural crops. Here, we investigated the impact of eCO₂ and its potential to mitigate yield losses due to light limitations from LBF. We assessed stomatal morphology, physiology, mesophyll ionic status, and crop yield in a capsicum cultivar grown under LBF or control glass at ambient (500 μmol mol^-1) or elevated (800 μmol mol^-1) CO₂ concentration. eCO₂ significantly reduced stomatal size and pore area under LBF and control conditions, but stomatal density was not affected by eCO₂ under LBF. The combination of LBF and eCO₂ induced more prompt stomatal opening rates on the light transition from low (100 µmol m^-2s^-1) to high PAR (1500 µmol m^-2 s^-1), which relatively enhanced water use efficiency. eCO₂ significantly increased fruit yield (weight and number) in the control but not LBF bays. eCO₂ did not affect the V_cmax and J_max of crops under LBF but strongly enhanced J_max under the control glass. In summary, despite some positive physiological characteristics generated by LBF, it limited yield enhancement by eCO₂ of the orange capsicum cultivar.