Annals of Botany

An increase in leaf mass per area (M_LA) of plants grown at elevated [CO₂] is often accompanied by accumulation of non-structural carbohydrates, and has been considered to be a response resulting from source-sink imbalance. We hypothesized that the increase in M_LA benefits plants by increasing the net assimilation rate through maintaining a high leaf nitrogen content per area (N_LA). To test this hypothesis, Polygonum cuspidatum was grown at ambient (370 µmol mol^-1) and elevated (700 µmol mol^-1) [CO₂] with three levels of N supply. Elevated [CO₂] significantly increased M_LA with smaller effects on N_LA and leaf mass ratio (f_LM). The effect of change in M_LA on plant growth was investigated by the sensitivity analysis: M_LA values observed at ambient and elevated [CO₂] were substituted into a steady-state growth model to calculate the relative growth rate (R). At ambient [CO₂], substitution of a high M_LA (observed at elevated [CO₂]) did not increase R, compared with R for a low M_LA (observed at ambient [CO₂]), whereas at elevated [CO₂] the high M_LA always increased R compared with R at the low M_LA. These results suggest that the increase in M_LA contributes to growth enhancement under elevated [CO₂]. The optimal combination of f_LM and M_LA to maximize R was determined for different [CO₂] and N availabilities. The optimal f_LM was nearly constant, while the optimal M_LA increased at elevated [CO₂], and decreased at higher N availabilities. The changes in f_LM of actual plants may compensate for the limited plasticity of M_LA.