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

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... 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. 280 plant, Shimomoto et al. (2020) developed an open chamber method, using a transparent film supported by thin and light-weight steel frames and two ventilation fans, that measure photosynthesis and transpiration in a semicommercial greenhouse. There are several methods to measure photosynthesis, such as the dry matter accumulation method (Lawlor et al., 1981) , manometric method (Hunt, 2003) , gas exchange method (Schulze, 1972; Takahashi et al., 2001) , and phyto-monitoring application method (Dieleman et al., 2017) . All forms of measurement had been reviewed for their advantages and disadvantages by Millan-Almaraz et al. (2009) . The gas exchange method is the most utilized measurement in individual leaves, whole plants, and plant canopy for commercial equipment and experimental setup (Schulze, 1972; Millan-Almaraz et al., 2009 ). The gas exchange method is a suitable and convenient method for greenhouses as CO2 exchange can be readily measured. The CO2 concentrations, both in the greenhouse and chamber, are determined by the gas supply from the CO2 generator, soil respiration, the photosynthesis/respiration reaction of plants, gas movement in the greenhouse due to ventilation, and exterior CO2 concentration level. These relationships are represented by a CO2 balance equation, which is used to calculate the photosynthetic rate in the chamber method. However, when the photosynthetic rate prediction by the CO2 balance method is applied to a naturally ventilated greenhouse, the ventilation rate becomes an unknown parameter as it fluctuates repeatedly depending on natural conditions. There are several techniques to predict the ventilation rate, such as tracer gas, heat balance, and water vapor balance techniques. If we could measure the parameters of the ventilation rate calculation simultaneously with the parameters of the CO2 balance equation, it would be possible to continuously monitor the photosynthetic rate of plants, even in a naturally ventilated greenhouse. The air exchange rate (or ventilation rate) is an important parameter of the CO2 balance method and subsequently affects the CO2 concentration. Accurate gas exchange rate measurements improve the accuracy of photosynthetic rate estimation. This review focuses on the photosynthetic rate measurements using CO2 gas exchange at the greenhouse scale (including the physical and technical aspects); it also evaluates and compares the ventilation rates across tracer gas, heat balance, and water vapor balance techniques. A sufficiently detailed summary of the literature addressing photosynthesis measurement in greenhouses based on the CO2 gas exchange and ventilation rate method is provided.