Assimilate relations in source and sink leaves during acclimation to a CO₂‐enriched atmosphere

Evidence from previous studies suggested that adjustments in assimilate formation and partitioning in leaves might occur over time when plants are exposed to enriched atmospheric CO 2 . We examined assimilate relations of source (primary unifoliolate) and developing sink (second mainstem trifoliolate) leaves of soybean [ Glycine max (L.) Merr. cv. Lee] plants for 12 days after transfer from a control (350 μl l −1 ) to a high (700 μ l −1 ) CO 2 environment. Similar responses were evident in the two leaf types. Net CO 2 exchange rate (CER) immediately increased and remained elevated in high CO 2 . Initially, the additional assimilate at high CO 2 levels in the light and was utilized in the subsequent dark period. After approximately 7 days, assimilate export in the light began to increase and by 12 days reached rates 3 to 5 times that of the control. In the developing sink leaf, high rates of export in the light occurred as the leaf approached full expansion. The results indicate that a specific acclimation process occurs in source leaves which increases the capacity for assimilate export in the light phase of the diurnal cycle as plants adjust to enriched CO 2 and a more rapid growth rate.