Differing responses to carbon dioxide enrichment by a dwarf and a normal-sized soybean cultivar may depend on sink capacity

Responses to CO2 enrichment were determined for a dwarf (MiniMax) and a normal-sized (Fiskeby) soybean [Glycine max (L.) Merr.]. Plants were grown in growth chambers with ambient (36 Pa) or elevated (98 Pa) CO2 using a 2 × 2 × 2 factorial design. Harvests for MiniMax and Fiskeby were increased 6 and 26 d, respectively, by CO2 enrichment. At final maturity, mean biomass for MiniMax and Fiskeby was 53 and 197 g, respectively, in the ambient CO2 treatment and these values were 48 and 199% greater, respectively, in response to CO2 enrichment. Root development was constrained in MiniMax and, unlike Fiskeby, all of the biomass attributed to CO2 enrichment was in the shoot. Cultivar differences were not detected for CO2 assimilation rates, stomatal conductance or substomatal CO2 concentrations. Foliar sucrose was 43% greater (P < 0.05) in MiniMax than in Fiskeby. Pods per plant, total seed mass per plant, seed number per plant and seed oil content were greater for Fiskeby than MiniMax. Seeds of Fiskeby were 75% greater by mass than those of MiniMax. Seed mass of Fiskeby increased about 50% in response to CO2 enrichment, whereas MiniMax was unchanged. Root growth, seed size, seeds per pod and starch accumulation rates for MiniMax were unaffected by CO2 enrichment. The above findings suggested that growth of the dwarf genotype under elevated CO2 was sink limited.Key words: Elevated CO2, atmospheric change, photosynthate partitioning, harvest index, soybean genotypes