Increase in leaf mass per area benefits plant growth at elevated CO2 concentration

An increase in leaf mass per area (MLA) 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 MLA benefits plants by increasing the net assimilation rate through maintaining a high leaf nitrogen content per area (NLA). To test this hypothesis, Polygonum cuspidatum was grown at ambient (370 micro mol mol-1) and elevated (700 micro mol mol-1) CO2] with three levels of N supply. Elevated CO2] significantly increased MLA with smaller effects on NLA and leaf mass ratio (fLM). The effect of change in MLA on plant growth was investigated by the sensitivity analysis: MLA 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 MLA (observed at elevated CO2]) did not increase R, compared with R for a low MLA (observed at ambient CO2]), whereas at elevated CO2] the high MLA always increased R compared with R at the low MLA. These results suggest that the increase in MLA contributes to growth enhancement under elevated CO2]. The optimal combination of fLM and MLA to maximize R was determined for different CO2] and N availabilities. The optimal fLM was nearly constant, while the optimal MLA increased at elevated CO2], and decreased at higher N availabilities. The changes in fLM of actual plants may compensate for the limited plasticity of MLA.