Transpiration from sorghum and soybean growing under ambient and elevated CO 2 concentrations

Abstract

The increasing concentration of carbon dioxide in the atmosphere ([CO 2]) has several direct effects on plants and these effects may be different for C 3 and C 4 plants. Our objective was to measure hourly and daily whole-plant transpiration rates from the C 4 plant grain sorghum ( Sorghum bicolor (L.) Moench) and the C 3 plant soybean ( Glycine max (L.) Merr.) grown under ambient (359 μmolCO 2 mol −1 dry atmospheric air) and elevated (705 μmol mol −1)[CO 2] values. Transpiration measurements were made for 22 days in August 1994 at Auburn, Alabama, USA, using stem flow gauges on plants growing in open top chambers, n = 8 for each [CO 2] and species. Leaf area averaged slightly more than 0.1 m 2 per plant for sorghum and about 0.2 m 2 per plant for soybean. Averages (15 min and daily) of transpiration, per unit leaf area, were consistently greater from plants growing under the ambient [CO 2] for both sorghum and soybean. Average daily transpiration from plants growing under the elevated [CO 2] was significantly smaller ( P = 0.05) on all but 2 days for soybean and on 9 of the 22 days of measurements for sorghum. Average daily sorghum transpiration was 1128 gm −2 day −1 and 772g m −2 day −1 from plants growing under an ambient and elevated [CO 2], respectively. Corresponding soybean averages were 731g m −2 day −1 and 416g m −2 day −1. The transpiration reduction under elevated [CO 2] was greater for the C 3 plant soybean than for the C 4 plant sorghum. These results support previous studies showing that transpiration, per unit leaf area, from sorghum and soybean will both be reduced if atmospheric [CO 2] continues to increase, although the reduction may be greater for C 3 plants.