Effects of Ozone and/or Soil Water Stress on Growth and Photosynthesis of Fagaus Crenata Seedlings

The effects of ozone (O₃) and soil water stress, singly and in combination, on the growth and photosynthesis of Fagus crenata seedlings were investigated. Four-year-old seedlings were exposed to charcoal-filtered air (<5 nmol mol-1 O₃) or 60 nmol mor-1 O₃, 7 hours per day (11:00)-18:00), for 156 days from 10 May to 11 October 1999 in naturally-lit growth chambers at 20/15C (6:00-18:00/18:00-6:00). During the same period, half of the seedlings in each gas treatment received 250 mL of water at the 3-day intervals (wellwatered treatment), while the rest received 175 mL of water at the 3-day intervals (water-stressed treatment). The exposure of the seedlings to O₃ caused reductions in the leaf, stem, root and whole-plant dry weights. The net photosynthetic rate at 350 μmol mol-1 CO₂, the maximum net photosynthetic rate at saturated CO_2- concentration, carboxylation efficiency of photosynthesis and Rubisco content were significantly reduced by the exposure to O₃. The soil water stress induced reductions in the stem, bud and whole-plant dry weights, transpiration rate and leaf water potential during the midday. The additive effects of O₃ and soil water stress were observed on the dry matter production, leaf gas exchange rates and leaf water potential. As a result, the whole-plant dry weight of the seedlings exposed to both stresses was markedly reduced compared with that of the seedlings exposed to charcoal-filtered air and grown in the well-watered treatment.