Soybean growth response to water supply and atmospheric carbon dioxide enrichment

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.