The Essential Factor of Ventilation Rate in Prediction of Photosynthetic Rate Using the CO₂ Balance Method

Abstract

Monitoring photosynthesis is a fundamental process to improve the yield and quality of plants in a greenhouse. The CO₂ balance method is often employed to predict the photosynthetic rate of plants. We reviewed the essential parameters for predicting photosynthetic rates of plants canopy in greenhouses using the CO₂ balance method. Even in a naturally ventilated greenhouse, ventilation rate is an essential parameter for the CO₂ balance method, but it must be measured in real time as it fluctuates with weather conditions. We studied three types of ventilation rates (the tracer gas, heat balance, and water vapor balance methods). Comparing the measuring techniques of ventilation rate provided us an understanding of the strengths and weaknesses of each method. This knowledge can guide us to choosing the best method based on accuracy, device usage, practicality, and the installation budget. Most researchers have measured and controlled CO₂ concentrations in a greenhouse using an infrared gas analyzer and predicted the ventilation rates using the tracer gas method. This method is suitable for the measurement of low and closed ventilation. The estimated ventilation rate by the heat balance method is recommended for large ventilation openings. The water vapor balance method is sufficient for measuring the ventilation rate when there is a large quantity of water vapor due to plant transpiration. The reliability of this method depends on the accuracy of short-term transpiration measurements. Improved water vapor balance techniques can benefit various greenhouse applications with different ventilator configurations, owing to the flexibility and ease of use compared to those of other methods.