スギ，ヒノキ，サワラの生育に及ぼすオゾンと人工酸性雨の複合影響

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  • Combined Effect of Simulated Acid Rain and Ozone on the Growth of Japanese Conifer Seedlings.
  • スギ ヒノキ サワラ ノ セイイク ニ オヨボス オゾン ト ジンコウ サンセイ ウ ノ フクゴウ エイキョウ

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Decline of Cryptomeria japonica has been reported in the suburban areas of Japan and related field surveys suggested that high concentration of photochemical oxidants might be linked with its decline. For assessing combined effects of rain acidity and ozone on the growth of trees, seedlings of Cryptomeria japonica, Chamaecyparis obtusa, and Chamaecyparis pisifera were exposed to simulated acid rain (SAR) and ozone for 23 months in the controlled chambers under the natural light condition. The pH of SAR was 5.6 (deionized water) or 3.0 containing sulfate, nitrate and chloride at the equivalent concentration ratio of 5: 2: 3. Plants were exposed to SAR at a rate of 2.5 mm/hr×8 hr/day, 3 times a week from April to September, and twice a week at a rate of 2.5 mm/hr×8 hr from october to March in the first year and 2.5 mm/hr×4 hr in the second year. Total precipitation was 4300 mm for 23 months. Dairy ozone exposure has conducted at a constant concentration of 0, 60, 120 or 180 ppb between 09: 00 to 15: 00 with no exposure between 16: 00 to 08: 00.<BR>Chamaecyparis obtusa and Chamaecyparis pisifera developed needle chlorosis and early defoliation in the older needles exposed to higher ozone concentrations, however, Cryptomeria japonica did not show any visible symptoms for 23 months exposure to ozone. No visible symptoms by the exposure to SAR at pH 3.0 and 5.6 were observed in these three species.<BR>Plant height and stem diameter in plants exposed to SAR at pH 3.0 were greater than those in plants exposed to SAR at pH 5.6. Dry weight of the plants exposed to SAR at pH 3.0 was greater than that of the plants at pH 5.6. Ozone exposure did not modify the total dry weight of the plants, however, dry weight of leaves in plants exposed to higher ozone concentrations was significantly greater than that of plants exposed to clean air. In contrast, high ozone exposure significantly reduced root dry weights. Thus, top/root dry weight ratio in the plants exposed to higher ozone increased and the exposure to SAR at pH 3.0 enhanced this trend.<BR>These results suggested that higher ozone concentration interfered with carbon allocation and increased rain acidity, probably associated with nitrate, exacerbated this effects. This may lead to increase susceptibility to drought stress in these species. As Cryptomeria japonica is more sensitive to drought stress than Chamaecyparis obtuse, current results may support the hypothesis that high concentration of ozone and high input of nitrogen in suburban areas may have a possible linkage with the decline of Cryptomeria japonica. This study also sggested that changes in dry matter distribution of assimilates might be a good indicator for assessing a long-term chronic effect of ozone on the growth.

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