Model-based Calculations of Ventilation Rate, Carbon Dioxide (CO2) Absorption Rate by Plants, and Amounts of Ventilated and Supplied CO2 for Tomato Hydroponic Greenhouse during a Winter Season

This study aimed to calculate ventilated carbon dioxide (CO2) and CO2 absorption by plants in a carbon dioxide (CO2) supplemental greenhouse for tomato cultivation during the winter season using a model equation in order to predict the current CO2 concentration and next CO2 supply. A targeted CO2 concentration (600 μmol·mol-1) was supplied from 10 a.m. to 13 p.m. from January 18, 2022 to March 12, 2022 using a CO2 type of fertilizer operated with LPG (liquefied petroleum gas). The ventilation rate ranged from 9.66 to 26.49 m3·m-2·h-1 at greenhouse temperature of 16 to 34°C and relative humidity (RH) of 65 to 93%. The external temperature was in the range of 3 to 20°C and external RH was 42 to 95%. The calculated net assimilation rate was in the range of 5.6 to 11.1 g·m-2·h-1, and the amount of ventilated CO2 ranged from 1.35 to 3.88 g·m-2·h-1. When the CO2 concentration (Ci) was lower than the target CO2 concentration (Cg) at the starting time of CO2 fertilization, the calculated first CO2 supply was in the range of 17.79 to 28.18 g·m-2·h-1. When the CO2 concentration at a certain time (Ct) was lower than the targeted CO2 concentration (Cg) after t hours of supply, the next CO2 supply was between 3.0 and 32.6 g·m-2·h-1. The photosynthesis rate measured at the 83 DAT showed the maximum increase with the 600 µmol·mol-1 of CO2 supplement under the 400 µmol·m-2·s-1 of PPFD, whereas the 800 µmol·mol-1 of CO2 supplement results into its maximum increase under all the PPFD levels higher than 600 µmol·m-2·s-1. Integrating all of the results suggests that during the winter season given the low light intensity, a CO2 supply exceeding 800μmol·mol-1 does not proportionally increase the net assimilation rate; thus, the CO2 supply efficiency appears to be affected more by the ventilated CO2 loss than by the net assimilation rate. These model equations can be used to control the CO2 fertilization in order to improve the yields of fruit vegetables in greenhouses.