Effects of Ozone and/or Excess Soil Nitrogen on Growth, Needle Gas Exchange Rates and Rubisco Contents of Pinus densiflora Seedlings

Abstract

The effects of ozone (O₃) and excess soil nitrogen (N), singly and in combination, on growth, needle gas exchange rates and ribulose-l,5-bisphosphate carboxylase/oxygenase (Rubisco) contents of Pinus densiflora seedlings were 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 O₃ (8 hours a day) in naturally-lit phytotrons for 173 days from 22 May to 11 November.

The exposure to O₃ or high N supply to the soil caused a significant reduction in the dry weights of the seedlings. Although no significant interactive effects of O₃ and excess soil N were detected on the dry weight growth of the seedlings, the whole-plant dry weight of the O₃-exposed seedlings grown in the soil treated with 300 mg N-L^-1 was greatly reduced compared with the control value. Ozone reduced net photosynthetic rate at 350 μmol mol^-1 CO₂ (A₃₅₀), carboxylation efficiency (CE) of photosynthesis and Rubisco content without a significant change in the gaseous phase diffusive conductance 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 O₃ and/or excess soil N was caused by reduction in the net photosynthetic rate mainly due to the decrease of Rubisco quantity in the chloroplasts.