THE INDIVIDUAL AND COMBINED EFFECTS OF OZONE AND SIMULATED ACID-RAIN ON GROWTH, GAS-EXCHANGE RATE AND WATER-USE EFFICIENCY OF PINUS-ARMANDIFRANCH

The seedlings of Pinus armandi Franch. were exposed to ozone (O-3) at 300 ppb for 8 h a day, 6 days a week, and simulated acid rain of pH 3. 0 or 2.3, 6 times a week, alone or in combination, for 14 weeks from 1 5 June to 20 September 1993. The control seedlings were exposed to cha rcoal-filtered air and simulated rain of pH 6.8 during the same period . Significant interactive effects of O-3 and simulated acid rain on wh ole plant net photosynthetic rate were observed, but not on other dete rmined parameters. The exposure of the seedlings to O-3 caused the red uctions in the dry weight growth, root dry weight relative to the whol e plant dry weight, net photosynthetic rate, transpiration rate in lig ht, water-use efficiency and root respiration activity, and increases in shoot/root ratio, and leaf dry weight relative to the whole plant d ry weight without an appearance of acute visible foliar injury, but di d not affect the dark respiration rate and transpiration rate in the d arkness. The decreased net photosynthetic rate was considered to be th e major cause for the growth reduction of the seedlings exposed to O-3 . On the other hand, the exposure of the seedlings to simulated acid r ain reduced the net photosynthetic rate per unit chlorophyll a + b con tent, but did not induce the significant change in other determined pa rameters.