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Influence of Elevated CO₂ on Growth of Soybean Plants¹

Continuing use of fossil fuels in industrialized countries has created a need to understand growth responses of major crop species to elevated concentrations of CO₂. An analysis was therefore undertaken of the growth of soybeans [Glycine max (L.) Merr. ‘Bragg’] grown in pots in open top field chambers at six CO₂ concentrations ranging from 332 µL L^-1 (ambient) to 910 µL L^-1. Major growth response occurred with the first increments of added CO₂ with a maximum 66% increase in total vegetative dry matter at the 910 µL L^-1 CO₂ level. Dry weight increases were proportionate among vegetative plant parts, although the harvest index was found to decrease slightly. Greater absolute growth rates in elevated CO₂ treatments were associated with greater rates of branch and internode elongation, leaf initiation, and leaf expansion. Yield increases represented greater seed numbers per plant rather than larger seeds. Percentage protein of seed decreased with CO₂ enrichment. In the interval from day 5 to 2 weeks after planting, mean relative growth rate (RGR) increased asymptotically with CO₂ concentration. Of the two components of RGR, the mean net assimilation rate (NAR) increased dramatically and mean leaf area ratio (LAR) decreased. In the intervals from week 2 to 7 and from week 7 to 12, RGR became constant across CO₂ treatments as the positive response of NAR and the negative response of LAR became less pronounced. Both RGR and NAR fell through the vegetative growth phase at each CO₂ level. The adjustment in LAR resulted from a decrease in specific leaf area while leaf weight ratio remained unaffected by CO₂.

Key Words: Growth analysis • Growth characteristics • Morphology

² Plant physiologist, USDA-ARS, Botany Dep., North Carolina State Univ., Raleigh, NC 27650; research scientist, Botany Dep., Duke Univ., Durham, NC 27706; assistant professor, and research assistant, Botany Dep., North Carolina State Univ. Raleigh NC 27650.

Received for publication March 7, 1983.

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