Effects of ozone and/or excess soil nitrogen on growth, needle gas exchange rates and rubisco contents of Pinus densiflora seedlings

The effects of ozone (O-3) and excess soil nitrogen (N), singly and in comb ination, on growth, needle gas exchange rates and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) contents of Pinus densiflora seedlings wer e investigated. One-year-old seedlings were grown in 1.5-L pots filled with brown forest soil with 3 levels of N supply (0, 100 or 300 mg N.L-1 fresh soil volume). The seedlings were exposed to charcoal-filtered air or 60 +/- 5 nL.L-1 O-3 (8 hours a day) in naturally-lit phytotrons for 173 days from 22 May to 11 November. The exposure to O-3 or high N supply to the soil caused a significant reduc tion in the dry weights of the seedlings. Although no significant interacti ve effects of O-3 and excess soil N were detected on the dry weight growth of the seedlings, the whole-plant dry weight of the O-3-exposed seedlings g rown in the soil treated with 300 mg N.L-1 was greatly reduced compared wit h the control value. Ozone reduced net photosynthetic rate at 350 mu mol.mo l(-1) CO2 (A(350)), carboxylation efficiency (CE) of photosynthesis and Rub isco content without a significant change in the gaseous phase diffusive co nductance to CO2 (gs) of the needles. The excess soil N reduced the A(350), CE, gs and Rubisco content of the needles. These results suggest that the reduction in the dry weight growth of Pinus densiflora seedlings induced by the exposure to O-3 and/or excess soil N was caused by reduction in the ne t photosynthetic rate mainly due to the decrease of Rubisco quantity in the chloroplasts.