Abstract: By using opentop chambers (OTCs), this paper studied the effects of elevated O₃ concentration on the growth and photosynthesis of Catalpa ovata, a common greening tree species in Shenyang City. The results showed that elevated O₃ concentration significantly inhibited the growth of C. ovata. Compared to the ambient air control, relative water content, dry weight, fresh weight, specific leaf weight, dry weight increment and fresh weight increment were significantly decreased in leaves of C. ovata after 20 days exposure to O₃ (160 nmol·mol^-1). The net photosynthetic rate (P_n) of C. ovata gradually decreased with the extension of time. The decrease in photosynthesis of C. ovata was related to nonstomatal factors. The change trends of intercellular CO₂ concentration (C_i) and transpiration rate (T_r) and stomatal conductance (G_s) were consistent, with decline first and then increase. The maximal photochemistry efficiency of PSⅡ (F_v/F_m) and quantum yield of PSⅡ electron transport (Φ_PSⅡ) decreased significantly over the time (P<0.05), the decline of C. ovata photosynthesis correlated markedly with the excess of light energy, and photoinhibition phenomenon under O₃ stress. Superoxide dismutase (SOD) and ascorbic acid peroxidase (APX) activities showed no significant change during the whole O₃ stress, which implied that it was quite difficult for C. ovata to tolerate excessive O₃ concentration, and two antioxidant enzymes did not make positive physiological response to the acute stress of O₃.

Key words: organic fertilizer, acetylene inhibition, N₂O reduction, isotope characteristic value, nitrification