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Elevated carbon dioxide (CO2) concentration increases plant photosynthesis, biomass and carbohydrate accumulation. Since plants have grown in low CO2 (200 to 300 µmol mol−1) for the last several million years, how will they use extra photoassimilate as the atmospheric CO2 continues to rise? The objectives were to determine the effects of past, present and projected future levels of CO2 on diurnal and seasonal patterns of total nonstructural carbohydrate (TNC) concentration of soybean [Glycine max (L.) Merr.] tissues. Plants were grown at 160, 220, 280, 330, 660 and 990 µmol mol−1 CO2 in outdoor, sunlit chambers wherein CO2 uptake rates were measured continuously. Early morning and late afternoon plant samples were taken at eight dates. The TNC concentration of leaves, petioles and stems increased as CO2 increased. Canopy photosynthetic rates also increased with increasing CO2, apparently without any negative impact of increased leaf TNC. Concentrations of TNC in all vegetative tissues were lower in the morning than the afternoon, which indicates overnight mobilization and utilization of carbohydrates for growth processes. The concentration of TNC was lowest in all plant components during rapid vegetative growth at V8 to R2 developmental stages. Leaves of all plants, especially those grown in superambient CO2, contained large pools of TNC at plant maturity, which indicated that not all of the reserves were utilized for seed yield. Soybean cultivars for the future should be designed to utilize carbohydrates more readily for seed production so that greater benefit can be realized from rising atmospheric CO2. |
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