Changes of net photosynthesis, antioxidant enzyme activities, and antioxidant contents of Liriodendron tulipifera under elevated ozone

Liriodendron tulipifera was exposed to gradually elevated ozone concentrations of 100-300 mu g kg(-1) in the naturally irradiated environment chamber. During 15 d of exposure to O-3, net photosynthetic rate (P (N)) decreased and there was large difference between the control (C) and treatment with ozone (OT), while there was no significant difference in water use efficiency. Total chlorophyll content as well as the value of fluorescence parameter F-v/F-m decreased, while antioxidant enzyme activities related to ascorbate-glutathione cycle increased after 15 d of OT. Unchanged contents of ascorbate and glutathione indirectly suggest that the species hastened the antioxidant's oxidization/reduction cycle using enzymes instead of expanding their pool against oxidative stress.