TRANSPIRATION FROM SORGHUM AND SOYBEAN GROWING UNDER AMBIENT AND ELEVATED CO2 CONCENTRATIONS

The increasing concentration of carbon dioxide in the atmosphere ([CO2 ]) has several direct effects on plants and these effects may be diffe rent for C-3 and C-4 plants. Our objective was to measure hourly and d aily 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 mu molCO(2) mol(-1) dry atmospheri c air) and elevated (705 mu molmol(-1)) [CO2] values. Transpiration me asurements were made for 22 days in August 1994 at Auburn, Alabama, US A, using stem flow gauges on plants growing in open top chambers, n = 8 for each [CO2] and species. Leaf area averaged slightly more than 0. 1m(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 [CO2,] for both sorghum and soybean. Average daily transpiration from plants gro wing under the elevated [CO2] 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 772 gm(-2) day(-1) from plants growing under an ambient and elevat ed [CO2], respectively. Corresponding soybean averages were 731 gm(-2) day(-1) and 416 gm(-2) day(-1). The transpiration reduction under ele vated [CO2] was greater for the C-3 plant soybean than for the C-4, pl ant sorghum. These results support previous studies showing that trans piration, per unit leaf area, from sorghum and soybean will both be re duced if atmospheric [CO2] continues to increase, although the reducti on may be greater for C-3, plants.