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United States Department of Agriculture

Agricultural Research Service

Title: Transpiration from Sorghum and Soybean Growing under Ambient and Elevated Co2 Concentrations

Authors
item Dugas, W - TEXAS A & M UNIVERSITY
item Prior, Stephen
item Rogers Jr, Hugo

Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 28, 1996
Publication Date: N/A

Interpretive Summary: Increasing levels of atmospheric carbon dioxide may affect plant water use and this effect may be different among crop species. Our goal was to determine hourly and daily whole-plant transpiration (i.e., water use) for two different crop species (i.e., grain sorghum ('Savanna 5') and soybean ('Stonewall') when grown under a present day CO2 level (359 ppm) and a projected future level of CO2 (705 ppm). Water use patterns for these two crops were made in August 1994 for about 3 weeks at Auburn, Alabama, using stem flow gauges placed on plants grown at these two specified CO2 levels in open top field chambers. Average leaf area was lower for sorghum (about 0.1 m2 / plant) compared to soybean (about 0.2 m2 / plant). In general, water use (i.e., both 15 minute and daily averages) of both crops was lower under elevated CO2; reduction in water use due to more CO2 was greater for soybean than for sorghum. For example, average daily water use was 1128 and 772g m2 / day for sorghum and 731 and 416g m2 / day for soybean under ambient and elevated CO2 conditions, respectively. Our results suggest that farmers may expect a decrease in crop water use per unit leaf area in a future CO2 enriched world if increases in atmospheric CO2 continue to occur.

Technical Abstract: The increasing concentration of carbon dioxide in the atmosphere may have several direct effects on plants and this effect may be different for C3 and C4 plants. Our objective was to measure hourly and daily whole-plant transpiration from the C4 plant grain sorghum (Sorghum biclor [L.] Moench) and the C3 plant soybean (Glycine max [L.} Merr.) grown under ambient and elevated CO2 (359 and 705 ppm), respectively. Transpiration measurements were made for 22 days in August 1994 at Auburn, Alabama, USA, using stem flow gauges on plants growing in open top field chambers. Leaf area averaged slightly more than 0.1 m2 / plant for sorghum and about 0.2 m2 / plant for soybean. Fifteen-minute and daily averages of transpiration, per unit leaf area, were consistently greater from plants growing under the ambient conditions for both species. Average daily transpiration from plants growing under the two CO2 levels was significantly different on all but two days for soybean and on nine of the 22 days of measurements for sorghum. Average daily sorghum transpiration was 1128 and 772g m2 / day from plants growing under ambient and elevated CO2, respectively. Corresponding soybean averages were 731 and 416g m2 / day. The transpiration reduction under elevated CO2 was greater for the C3 plant soybean than for the C4 plant sorghum. These results support previous studies showing that transpiration, per unit leaf area, from sorghum and soybean will both be reduced if atmospheric CO2 concentrations continue to increase.

Last Modified: 12/19/2014