Acclimation of photosynthesis to elevated CO 2 in onion ( Allium cepa ) grown at a range of temperatures

Summary Onion ( Allium cepa ) was grown in the field within temperature gradient tunnels (providing about ‐2.5°C to +2.5°C from outside temperatures) maintained at either 374 or 532 μmol mol −1 CO 2 . Plant leaf area was determined non‐destructively at 7 day intervals until the time of bulbing in 12 combinations of temperature and CO 2 concentration. Gas exchange was measured in each plot at the time of bulbing, and the carbohydrate content of the leaf (source) and bulb (sink) was determined. Maximum rate of leaf area expansion increased with mean temperature. Leaf area duration and maximum rate of leaf area expansion were not significantly affected by CO 2 . The light‐saturated rates of leaf photosynthesis (A sat ) were greater in plants grown at normal than at elevated CO 2 concentrations at the same measurement CO 2 concentration. Acclimation of photosynthesis decreased with an increase in growth temperature, and with an increase in leaf nitrogen content at elevated CO 2 . The ratio of intercellular to atmospheric CO 2 (C 1 /C 3 ratio) was 7.4% less for plants grown at elevated compared with normal CO 2 . A sat in plants grown at elevated CO 2 was less than in plants grown at normal CO 2 when compared at the same C 1 . Hence, acclimation of photosynthesis was due both to stomatal acclimation and to limitations to biochemical CO 2 fixation. Carbohydrate content of the onion bulbs was greater at elevated than at normal CO 2 . In contrast, carbohydrate content was less at elevated compared with normal CO 2 in the leaf sections in which CO 2 exchange was measured at the same developmental stage. Therefore, acclimation of photosynthesis in fully expanded onion leaves was detected despite the absence of localised carbohydrate accumulation in these field‐grown crops.