Estimating the air exchange rate using water vapour as a tracer gas in a semi-closed growth chamber
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Net photosynthetic rate of whole-plant (P n) can be used to estimate plant growth and CO 2 use efficiency (CUE) in a 'closed system with artificial lighting' (CSAL). CUE is defined as the ratio of the canopy net photosynthetic rate in a CSAL to the CO 2 supply rate. P n can be estimated based on the CO 2 balance in the CSAL if the air exchange rate is known. The air exchange rate is usually estimated using CO 2 as a tracer gas, but this method cannot be applied in the presence of plants. We propose a method for estimating P n in a CSAL based on the air exchange rate estimated using water vapour instead of CO 2 as a tracer gas. In this study, a growth chamber was used as an experimental CSAL. The air exchange rate was defined as the number of air exchanges per hour (N), which is the ratio of the hourly exchanged air volume to the air volume of the CSAL. Firstly, N was estimated using water vapour (N W) and CO 2 (N C) as tracer gases in the absence of plants. Then, P n and CUE of the tomato plants were estimated using the proposed method. The results showed that N W agreed well with N C, and P n was accurately estimated by the proposed method. The estimated CUE decreased with increases in N and CO 2 concentration inside the growth chamber. It was concluded that the method can be used to estimate P n and CUE in a CSAL. 2012 IAgrE.