Soybean growth response to water supply and atmospheric carbon dioxide enrichment

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Abstract
Growth response of soybean [Glycine max (L.) Merr. ‘Bragg'] grown in open top field chambers at five carbon dioxide (CO2) concentrations ranging from 349 to 946 μLL‐1 and under two water regimes was examined. During reproductive growth, plants grown under CO2 enrichment exhibited increases in total leaf area and dry weight. Water stress inhibited growth at all CO2 levels, but the relative enhancement of growth due to CO2 enrichment under water‐stressed (WS) conditions was greater than under well‐watered (WW) conditions. Water‐stressed plants grown under 946 μLL‐1 CO2 were larger than WW plants grown under 349 μLL‐1 CO2. Reproductive yield increases were represented by increases in seed number rather than larger seeds. Although water stress reduced yield, the relative increase in seed number in response to elevated CO2 was greater for WS plants. Leaf tissue analysis suggested that a phosphorus deficiency may have restricted the seed dry weight response to elevated CO2. The mean relative growth rate (RGR) and mean net assimilation rate (NAR) increased with CO2 concentration in the first interval (5 to 14 days after planting) and diminished with time thereafter for 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 similar under both water treatments. At the third interval (63 to 98 days), the RGR for WS plants remained constant across CO2 treatments, whereas under WW conditions a level response of NAR coupled with a negative response of LAR resulted in a decrease in RGR under CO2‐enriched conditions. The decrease in LAR was attributed to a decrease in specific leaf area. Leaf weight ratio was unaffected by CO2.