Abstract
The seedlings of Japanese cedar (Cryptomeria japonica D. Don), Japanese cypress (Chamaecyparis obtusa Endl.) and Japanese zelkova (Zelkova serrata Makino) were exposed for 24 weeks per year to four simulated profiles with diurnal fluctuations of ozone in enviironment-controlled greenhouses: 0.4 (×0.4), 1.0 (×1.0), 2.0 (×2.0) and 3.0 (×3.0) times the ambient followed average profile of the concentration observed at the outdoor in last two years. Experiments were repeated in 1993, 1994 and 1995. During the experiments, yearly seasonal averages of daytime 12-hr daily mean (daily 1-hr maximum) ozone concentration at×0.4, ×1.0, × 2.0 and ×3.0 were 16 (22), 39 (52), 74 (102) and 114 (159) ppb, respectively.
Japanese zelkova seedlings at two higher (×0.2 and×3.0) ozone treatments developed white fleck injuries followed yellowing, and defoliated in its early stage. Japanese cedar and cypress did not show any visible injury symptoms by ozone. After 24 weeks exposure in all experiments, total dry weights (TDW) of zelkova seedlings at the two higher ozone treatments significantly reduced compared with the lowest (×0.4) ozone treatment. Stem dry weight of zelkova at the ambient level (×1.0) of ozone treatment in 1995 were significantly less than those at the lowest treatment. TDW of cedar at the highest ozone treatment significantly reduced compared with the lowest treatment in 1993 and 1995. The relative growth rate and net assimilation rate of cedar and zelkova seedlings decreased with increasing level of ozone treatment. The net photosynthetic rates in Japanese cedar and zelkova leaves, measured throughout the 1993 experiment, decreased with increasing ozone. The growth and new photosynthesis of cypress seedlings were hardly affected by ozone.
These results indicated that Japanese zelkova is the most sensitive to ozone compared with less sensitive cedar and tolerant cypress by estimating growth inhibition, and that the ozone-induced reductions in growth was brought about mainly due to the inhibition of net photosynthesis leading to reductions in the efficiency of dry matter production.
