Transpiration and Photosynthesis in Soybean

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Abstract
Continuous and simultaneous measurements of CO2 exchange and transpiration rates of whole soybean plants were made under contrasting, controlled environmental conditions for periods of 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/12 mb, and 22.5 °C/5 mb. Night temperatures were 5°C lower than day temperatures and night VPD was 2.7 mb and 3.5 mb at the higher and lower temperature respectively. The experimental conditions were virtually the same as those under which the plants had been grown. Transpiration rates were higher at the higher VPD but were also influenced by temperature. At 12 mb VPD the rates were 16 per cent lower at 22.5 °C than at 27.5 °C. Temperature had no effect on the transpiration rate at 5 mb VPD. Photosynthesis rates were lower at 5 mb VPD than at 12 mb VPD at both temperatures: the difference was substantially greater (c. 70 per cent) at 22.5 °C. Under all treatments meso-phyll resistance (r'm) appeared to have a major effect on the photosynthetic rate, and varied more than twofold between treatments. r'm was highest in plants grown at 22.5 °C/5 mb VPD and lowest at 27.5 °C/12 mb VPD.

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