Soybean seedlings were grown in controlled environment chambers with CO2 partial pressures of 38 (ambient) and 72 (elevated) Pa. Five or six shoot apices were harvested from individual 21- to 24-day-old plants. Metabolites were analyzed by gas chromatography and, out of 21 compounds, only sucrose and fructose increased in response to CO2 enrichment. One unidentified metabolite, Unk-21.03 decreased up to 80% in soybean apices in response to elevated CO2. Levels of Unk-21.03 decreased progressively when atmospheric CO2 partial pressures were increased from 26 to 100？Pa. Reciprocal transfer experiments showed that Unk-21.03, and sucrose in soybean apices were altered slowly over several days to changes in atmospheric CO2 partial pressures. The mass spectrum of Unk-21.03 indicated that this compound likely contained both an amino and carboxyl group and was structurally related to serine and aspartate. Our findings suggested that CO2 enrichment altered a small number of specific metabolites in soybean apices. This could be an important step in understanding how plant growth and development are affected by carbon dioxide enrichment. 1. Introduction Atmospheric CO2 partial pressures are increasing due to human activities that include industrialization, fossil fuel combustion, and deforestation . Since CO2 is an important substrate for photosynthesis, elevated atmospheric CO2 has the potential to alter the productivity of terrestrial plants and that of natural or managed ecosystems . Single leaf gas exchange rates of higher plants were affected by CO2 enrichment, and this often resulted in larger plants with increased reproductive capacity [3–5]. Due to accelerated rates of net CO2 assimilation, concentrations of various leaf components including starch, soluble carbohydrates, amines, organic acids, pigments, and important photosynthetic proteins were affected by plant growth in CO2-enriched atmospheres [6–8]. Increased biomass accumulation in response to CO2 enrichment impacted the demand for soil nutrients, and in some cases this resulted in nutritionally limited growth conditions . Nutrient limitations under CO2 enrichment also decreased leaf photosynthetic capacity and further altered leaf constituents . In comparison to source leaves, much less attention has been given to the effects of elevated CO2 on the growth and development of sinks. Sink organs are dependent upon source leaves for assimilates to provide the carbon, nitrogen, and energy needed for growth and development. In general, metabolite levels in sink tissues were altered in concert
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