Development of photosynthesis and respiration measurement system using mass spectrometry

A closed gas-exchange system was developed to measure gross photosynthesis and respiration discriminately and simultaneously. The system developed in this study included a high performance mass spectral analyzer for gas measurements. The gas-exchange system consisted of a 3L assimilation leaf chamber, a 1L flexible metallic bag, gas supply apparatus, and a lighting system. The lights were turned on and gas measurements were started after the initial 12CO2 concentration level was increased to 500 ppm. The 13CO2 gas was added to the chamber 10 min after the start of the light period. The lights were turned off 15 min after the addition of 13CO2. The 12CO2 and 13CO2 concentrations in the chamber during the light and dark periods were measured for pothos and maize leaves. The 13CO2 absorption rate by the leaves was larger than that for 12CO2 during the light period. It was assumed that the 13CO2 absorbed by photosynthesis was not evoluted by respiration during the first 25 min from the start of the light period. Based on this assumption, gross photosynthetic rate and respiration rate were estimated by calculating the difference in uptake rates of 12CO2 and 13CO2 during the light period.