Abstract: Pinus tabulaeformis seedlings were planted in opened top chambers (OTCs) and exposed to 80 nmol·mol-1 of O3, 700 μmol·mol-1 of CO2, and 80 nmol·mol-1 of O3 plus 700 μmol·mol-1 of CO2 for four months to study the effects of elevated O3 or/and CO2 on their antioxidant system. Elevated O3 increased the H2O2 content in P. tabulaeforims needles significantly, which in turn depressed the activities of antioxidant enzymes such as SOD and APX, and exhausted the important antioxidant ASA, leading to a serious lipid peroxidation in the late exposure period. Elevated CO2 lowered the antioxidant enzyme activities and enhanced the ASA content in P. tabulaeformis needles, possibly due to its promotion effect on the synthesis of ASA or its reduction effect on ASA consumption, and thus, alleviated the oxidative stress, which was validated by the significantly lower H2O2 content and lipid peroxidation product (MDA) in the needles in the late exposure period. Comparing with those exposed to elevated O3, the P. tabulaeformis needles exposed to elevated O3 plus elevated CO2 had higher antioxidant enzyme activities and ASA content, illustrating that elevated CO2 abated the depression of antioxidant enzyme activities and the exhaustion of ASA content in P. tabulaeformis needles caused by elevated O3, and subsequently, alleviated the O3-induced oxidative damage in P. tabulaeformis.

Key words: Two-line hybrid rice, Water retaining capacity, Chlorophyll content, Yield, Correlation