CO₂‐Enrichment Effects on Soybean Physiology. I. Effects of Long‐Term CO₂ Exposure¹

Soybeans [ Glycine max (L.) Merr. ‘Kent’ and ‘Ware’], grown under field conditions, were exposed to a CO 2 ‐enriched atmosphere (1232 μL L −1 ) from 22 days after planting to maturity. Seed yield increased 81% in Kent and 56% in Ware in response to CO 2 enrichment. Plants grown at high CO 2 concentrations maintained higher rates of apparent photosynthesis and lower leaf conductances throughout ontogeny than those grown at low CO 2 concentrations. Carbon dioxide‐enriched plants accumulated larger pools of soluble sugars and starch in the leaves compared with control plants. Rapid depletion of the stored carbohydrates was evident at the time flower buds were developing and flowering commenced. Carbon dioxide‐enriched plants also maintained large pools of starch and sucrose in the leaves during the grain‐filling periods, suggesting that these were not utilized for seed development. In contrast, control plants were virtually devoid of sucrose and starch at plant maturity. Examination of leaf chlorophyll, protein and proteolytic enzyme activity revealed that CO 2 enrichment did not delay senescence in soybeans.