A system for measuring leaf gas exchange based on regulating vapour pressure difference

A system for measurement of leaf gas exchange while regulating leaf to air vapour pressure difference has been developed; it comprises an assimilation chamber, leaf temperature controller, mass flow controller, dew point controller and personal computer. A relative humidity sensor and air and leaf temperature sensors, which are all used for regulating the vapour pressure difference, are mounted into the chamber. During the experiments, the computer continuously monitored the photosynthetic parameters and measurement conditions, so that accurate and intenstive measurements could be made.When measuring the light-response curve of CO₂ assimilation for single leaves, in order to regulate the vapour pressure difference, the leaf temperature and relative humidity in the chamber were separately and simultaneously controlled by changing the air temperature around the leaf and varying the air flow rate through the chamber, respectively. When the vapour pressure difference was regulated, net CO₂ assimilation, transpiration and leaf conductance for leaves of rice plant increased at high quantum flux density as compared with those values obtained when it was not regulated.When measuring the temperature-response curve of CO₂ assimilation, the regulation of vapour pressure difference was manipulated by the feed-forward control of the dew point temperature in the inlet air stream. As the vapour pressure difference was regulated at 12 mbar, the maximum rate of and the optimum temperature for CO₂ assimilation in rice leaves increased 5 molCO₂ m^–2 s^–1 and 5°C, respectively, as compared with those values obtained when the vapour pressure difference took its own course. This was reasoned to be due to the increase in leaf conductance and the decrease in transpiration rate. In addition, these results confirmed that stomatal conductance essentially increases with increasing leaf temperature under constant vapour pressure difference conditions, in other words, when the influence of the vapour pressure difference is removed.This system may be used successfully to measure inter- and intra-specific differences and characteristics of leaf gas exchange in plants with a high degree of accuracy.Abbreviations A CO₂ assimilation rate - A_max Maximum rate of CO₂ assimilation - A_opt Optimum teperature for CO₂ assimilation - CTWB Controlled-temperature water bath - DPC Dew point controller - E Transpiration rate; gl, leaf conductance - HCC Humidity control circuit - IRGA Infrared gas analyzer - LT Leaf temperature - LTC Leaf temperature controller - MFC Mass flow controller - QFD Quantum flux density - RH Relative humidity - RHC Relative humidity controller - VPD Vapour pressure difference - CO₂ Difference of CO₂ concentration between inlet and outlet air