Objective  The antioxidant enzyme activities and respond mechanism of Chinese fir (Cunninghamia lanceolata) under acid and/or aluminum stresses were investigated.

  Method  A pot experiment was conducted to determine the MDA content and SOD, POD, CAT, and PPO activities in one-year-old leaves of Chinese fir seedlings under the stress for 15, 30 or 45 d. At the rate of 0.24 g·kg^-1, AlCl₃ 6H₂O was applied to the pots to simulate an aluminum stress, a pH 4.0 acid solution added to simulate an acid stress, and the combination of the two incorporated to simulate an acid-aluminum stress.

  Result  Without the presence of acid or aluminum, the potted seedlings had a low content of MDA in the leaves. However, the addition of acid, aluminum or acid-aluminum combination induced excessive accumulation of MDA with the severities ranking as acid-aluminum stress > aluminum stress > acid stress. The addition also raised the POD and SOD activities in leaves with a rank of acid-aluminum stress > acid stress > aluminum stress, and the CAT and PPO activities in the order of acid-aluminum stress > aluminum stress > acid stress. A prolonged treatment caused an initial increase on the POD and SOD activities followed by a decline; and on the CAT activity similarly under the aluminum or acid-aluminum stress but a steady increase under the acid stress; whereas, a gradual increase on the PPO activity under any of the 3 treatments.

  Conclusion  The acid, aluminum or acid-aluminum stress affected the POD, SOD, CAT and PPO activities in the leaves to varying extents and raised MDA accumulation in the leaves of Chinese fir seedlings. There appeared to be a synergistic effect on the activities between acid and aluminum. But alone, the acid stress exerted a greater effect on POD and SOD than did aluminum, whereas, aluminum seemed to be a more formidable adversary than acid for the seedlings on CAT and PPO.