AI+200 and AI+160 were short-term (one growing season) abrupt increase of [CO2] by ~200 μmol mol-1 and ~160 μmol mol-1, respectively. CI: constant increase of [CO2] by ~200 μmol mol-1 based on ambient [CO2] for four generations. SI: stepwise increase of [CO2] for four generations with the [CO2] being ~40, 80, 120 and 160 μmol mol-1 higher than ambient in each generation. Data is mean ± SE (n=12). Different letters in the same CO2 treatment indicate significant difference among growth stages (p <0.05).