Method to Calculate Net CO2 Exchange Rate of Whole Plants under Continuously Increasing or Decreasing CO2 Concentrations in a Greenhouse Using a Real-time Photosynthesis and Transpiration Monitoring System

Abstract

We developed an open-chamber system to monitor photosynthesis and transpiration of whole plants with high time resolution. The system may be promising for environmental controls based on plant stress diagnoses; however, the conventional method used to calculate net CO2 exchange rate (NCER) should be improved to avoid overestimation and underestimation of NCER under greenhouse conditions. To calculate NCER, the conventional method uses the difference in CO2 concentration between inlet and outlet air of the open-chamber system. The CO2 concentrations are alternately recorded by a single sensor every 5 min. With this method, inherently delayed response of outlet CO2 concentration causes overestimation and underestimation of NCER especially when CO2 concentration in a greenhouse continuously increases or decreases. We developed a calculation method considering the number of air exchanges, and then compared the NCER calculated with the new method (NCERex) to that calculated with the conventional method (NCERconv). During the night, when the CO2 concentration in the greenhouse continuously increased, NCERex (with the average NCERexs being -0.78, -0.71, and -0.96 mmol s-1 at different timeframes) was lower than NCERconv (with the average NCERconvs being -0.30, -0.32, and -0.65 mmol s-1). The higher values of NCERconv indicated overestimation by the conventional calculation method during the night. The total amount of night respiration calculated using NCERex was 2.2 times greater than that calculated using NCERconv. The new calculation method is useful to measure the NCER of whole plants validly under greenhouse conditions.