Transpiration and Photosynthesis in Soybean: EFFECTS OF TEMPERATURE AND VAPOUR PRESSURE DEFICIT

Continuous and simultaneous measurements of CO₂ exchange andtranspiration rates of whole soybean plants were made undercontrasting, controlled environmental conditions for periodsof up to 3 d. Daytime temperatures and vapour pressure deficits(VPD) were 27.5 °C/12 mb; 27.5 °C/5 mb; 22.5 °C/12mb, and 22.5 °C/5 mb. Night temperatures were 5°C lowerthan day temperatures and night VPD was 2.7 mb and 3.5 mb atthe higher and lower temperature respectively. The experimentalconditions were virtually the same as those under which theplants had been grown. Transpiration rates were higher at the higher VPD but were alsoinfluenced by temperature. At 12 mb VPD the rates were 16 percent lower at 22.5 °C than at 27.5 °C. Temperature hadno effect on the transpiration rate at 5 mb VPD. Photosynthesis rates were lower at 5 mb VPD than at 12 mb VPDat both temperatures: the difference was substantially greater(c. 70 per cent) at 22.5 °C. Under all treatments meso-phyllresistance (r'_m) appeared to have a major effect on the photosyntheticrate, and varied more than twofold between treatments. r'_m washighest in plants grown at 22.5 °C/5 mb VPD and lowest at27.5 °C/12 mb VPD.