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 (NCER_ex) to that calculated with the conventional method (NCER_conv). During the night, when the CO₂ concentration in the greenhouse continuously increased, NCER_ex (with the average NCER_exs being －0.78, －0.71, and －0.96 μmol s^-1 at different timeframes) was lower than NCER_conv (with the average NCER_convs being －0.30, －0.32, and －0.65 μmol s^-1). The higher values of NCER_conv indicated overestimation by the conventional calculation method during the night. The total amount of night respiration calculated using NCER_ex was 2.2 times greater than that calculated using NCER_conv. The new calculation method is useful to measure the NCER of whole plants validly under greenhouse conditions.