Abstract: Plastic pots with sand and vermiculite were used to grow Citrus grandis saplings,and 0.05mmol P (normal P level,P) and 0.1 mmol P (higher P level,2P) nutrient solution were supplied every week. The saplings were grown under atmospheric CO₂ (about 39 Pa) and doubled CO₂ partial pressure (81±5Pa) for 45d. The Pn-Pi curve was measured under higher irradiation (1150 μmol·m^-2 ·s^-1) by CI-301PS Photosynthesis System (CID,Inc),and the Pn-PAR curve was measured under higher CO₂ partial pressure (PCO₂ 56 Pa).The results showed that the maximum rate of photosynthesis for 2P plants was 13.3% higher than that of P plants under atmospheric CO₂ partial pressure. Regardless of P and 2P plants,the maximum photosynthetic rate under doubled CO₂ partial pressure was lower than that under atmospheric CO₂ partial pressure,but the photosynthetic rate of 2P plants was higher than that of P plants under doubled CO₂ partial pressure. 2P plants had a higher apparent quantum efficiency and light energy conversion efficiency (P<0.05) than P plants,but no change in Γ^*,Rd and the ratio between carboxylation rate (Vc) and oxygenation rate (Vo) of Rubisco (P>0.05) wwere found. The V_cmax and J_max of 2P plants were 8.3% and 12.5% higher than those of P plants under atmospheric CO₂ partial pressure,respectively,and 2P plants also had higher V_cmax and J_max than P plants under double CO₂ partial pressure. The partitioning coefficient of leaf nitrogen in Rubisco and in thylakoid light-harvesting component was varied in CO₂ acclimation,but it was not in bioenergetics.It may suggest that an increasing phosphorus supply would promote the phosphorus turnover in photosynthetic carbon cycle,and increase the photosynthetic rate under doubled CO₂ partical pressure.Increaing phosphorus supply may be a way to regulate the photosynthetic parameter of plants in CO₂ acclimation.

Key words: Citrus grandis, Phosphorus supply, Doubled CO₂ partial pressure, Photosynthetic parameter