Effects of CO₂ enrichment on photosynthesis and photosynthate partitioning in soybean (Glycine max) leaves

The short‐term and long‐term effects of CO, enrichment on certain aspects of photosynthesis and leaf carbohydrate metabolism were studied with soybean [Glycine max (L.) Merr.] plants. In general, both long‐term and short‐term CO₂ enrichment (300 ppm CO₂ above ambient) of soybean plants resulted in increased rates of photosynthesis (per unit leaf area) and starch accumulation. Leaf sucrose concentrations were increased slightly, but the rate of assimilate export and activity of sucrose phosphate synthase (EC 2.4.1.14) were usually not increased. Plant N‐status affected the response of vegetative soybean ‘Ransom’ plants to short‐term CO, enrichment. When plants were N‐stressed (2 mM NO₃‐ supplied), CO₂ enrichment resulted in increased rates of both starch accumulation and export. As N‐supply was increased, partitioning of carbon into starch in CO₂‐enriched atmospheres increased at the expense of assimilate export. When plants were grown with high‐N (20 mM NO₃), the rate of assimilate export from CO₂‐enriched leaves was reduced below the rate observed with plants maintained at ambient CO₂. The reduction in export rate was associated with decreased activities of sucrose phosphate synthase in leaf extracts. The activity of sucrose phosphate synthase in leaf extracts was closely associated with partitioning of carbon between starch and sucrose in leaves, and with the rate of assimilate export.