SOYBEAN GROWTH-RESPONSE TO WATER-SUPPLY AND ATMOSPHERIC CARBON-DIOXIDE ENRICHMENT

Growth response of soybean [Glycine max (L.) Merr. 'Bragg'] grown in o pen top field chambers at five carbon dioxide (CO2) concentrations ran ging from 349 to 946 mu LL(-1) and under two water regimes was examine d. During reproductive growth, plants grown under CO2 enrichment exhib ited increases in total leaf area and dry weight. Water stress inhibit ed growth at all CO2 levels, but the relative enhancement of growth du e to CO2 enrichment under water-stressed (WS) conditions was greater t han under well-watered (WW) conditions. Water-stressed plants grown un der 946 mu LL(-1) CO2 were larger than WW plants grown under 349 mu LL (-1) CO2. Reproductive yield increases were represented by increases i n seed number rather than larger seeds. Although water stress reduced yield, the relative increase in seed number in response to elevated CO 2 was greater for WS plants. Leaf tissue analysis suggested that a pho sphorus deficiency may have restricted the seed dry weight response to elevated CO2. The mean relative growth rate (RGR) and mean net assimi lation rate (NAR) increased with CO2 concentration in the first interv al (5 to 14 days after planting) and diminished with time thereafter f or each CO2 level. At the second interval (14 to 63 days), the direct effect of NAR was offset by lower leaf area ratio (LAR). However, the LAR was greater for WS plants but the response of RGR to CO2 was simil ar under both water treatments. At the third interval (63 to 98 days), the RGR for WS plants remained constant across CO2 treatments, wherea s under WW conditions a level response of NAR coupled with a negative response of LAR resulted in a decrease in RGR under CO2-enriched condi tions. The decrease in LAR was attributed to a decrease in specific le af area. Leaf weight ratio was unaffected by Co-2.