Elevated CO2 alleviates the effects of low P on the growth of N2-fixing Acacia auriculiformis and Acacia mangium

Nodulated seedlings of Acacia auriculiformis Cunn. ex Benth and Acacia mangium Willd were grown with different phosphorus (P) regimes for 90 days, and half of them were exposed to elevated CO2 (800 mu l l(-1)) during the last 30 days. Under ambient CO2, plant growth and the amount of N fixed symbiotically in N-2-fixing seedlings decreased with the decrease of supplied P; this relationship did not occur under elevated CO2. The increase in plant biomass by elevated CO2 at low P was accompanied by the increase in internal P use efficiency, the amount of N fixed symbiotically and N use efficiency. Elevated CO2 recovered the low P-induced reduction in leaf dry matter per unit area or unit fresh weight, but it had no effect on the low P-induced increase in partitioning dry matter to roots. These results suggest that elevated CO2 alleviates the low P effect mainly by increasing the use efficiency of internal P for plant growth and symbiotic N-2 fixation, and the source-sink relationship is possibly an important driving force for this effect of elevated CO2 in A. auriculiformis and A. mangium.