Objectives Inefficient supply of CO₂ during the growth season of plants is often in winter or spring in solar greenhouse. The suitable CO₂ supply concentration was studied for the greenhouse vegetable production in north China

Methods Field plot experiment was conducted inside greenhouse using tomato cultivar ‘xinghai 12’ as materials. The greenhouse was divided into four independent compartments with plastic film, each had a planting area of 52 m². Taking air CO₂ 400 μmol/mol as control, CO₂ levels of (600 ± 20) μmol/mol, (800 ± 25) μmol/mol and (1000 ± 30) μmol/mol were setup in each compartments throughout the experiment from Nov 2016 to Apr 2017. Using an automatic CO₂ release system, CO₂ supply concentration was regulated through the CO₂ flow meter of the cylinder and spread evenly into each pilot area by assistant of the circulation fan. CO₂ was released twice a day at 9:00–11:00 in the morning and 14:00–16:00 in the afternoon on sunny days. The photosynthetic indexes of tomato plants were measured using the LI-6400 photosynthesis analyzer at the seedling stage, flowering stage, young fruit stage and mature stage, respectively.

Results CO₂ enrichment treatments significantly increased the photosynthetic pigment contents of tomato at all the tested growth stages, with the high increment at flowering and young fruit stages. The highest increment in chlorophyll a and b were obtained in treatment of CO₂ (1000 ± 30) μmol/mol; the highest increment of carotenoid was CO₂ (800 ± 25) μmol/mol at flowering stage and was (1000 ± 30) μmol/mol at the other stages. The net photosynthetic rate, intercellular CO₂ concentration and water use efficiency were significantly increased by CO₂ supply at all the growth stages, and with the largest increment in the treatment of CO₂ (1000 ± 30) μmol/mol, and followed by the CO₂ (800 ± 25) μmol/mol. The stomatal conductance and transpiration rate were significantly reduced in the increase of CO₂ supply concentration. Supplying CO₂ increased the light saturation point, maximum net photosynthetic rate and apparent quantum efficiency, but decreased the light compensation point in tomato leaves to some extent, the best effects were all in the treatment of CO₂ (1000 ± 30) μmol/mol, and followed by CO₂ (800 ± 25) μmol/mol.

Conclusions CO₂ supply could obviously increase the photosynthetic capacities of tomato leaves at the flowering and fruiting stages, effectively enhance the accumulation of photosynthetic pigments and promote photosynthesis in tomato at all the growth stages, and increase the tomato yield. Under the experimental range of CO₂ supply, 800–1000 μmol/mol of CO₂ supply is recommended for high tomato yield.