Effects of Ozone and/or Simulated Acid Rain on Dry Weight Growth and Gas Exchange Rates of Japanese Cedar, Nikko Fir, Japanese White Birch and Japanese Zelkova Seedlings

Abstract

Seedlings of two coniferous tree species: Japanesse cedar (Cryptomeria japonica), Nikko fir (Abis homolepis) and two deciduous broad-leaved tree species: Japanese white birch (Betula platyphylla var. Japonica), Japanese zelkova (Zelkova serrata) were exposed to ozone and simulated acid rain, alone or in combination, for 20 weeks. Ozone exposure was conducted daily in four environment-controlled greenhouses. The ozone treatments consisted of four simulated profiles with diurnal fluctuation: 0.4 (×0.4), 1.0 (×1.0), 2.0 (×2.0) and 3.0 (×3.0) times of the average profile of ozone concentration in ambient air in 1991 and 1992. Exposure to simulated acid rain was conducted three times per week in the nighttime. The rain pHs were 5.6 and 3.0.
Two deciduous broad-leaved tree species showed white fleck leaf injeries followed by yellowing and defoliation in their early stages at ×2.0 and ×3.0 of ozone. Rain treatment at pH3.0 caused necrotic spots on the leaves of Japanese zelkova. Any visible injury symptoms induced by rain treatments were not observed in the leaves of Japanese white birch and not in two conifers by ozone and rain treatmens. After 20 weeks of exposure, dry weights of leaf, stem, root and total plant per seedling of Japanese cedar and two broad-leaved trees significantly decreased linearly with increasing the level of ozone. The root dry weight of Nikko fir was decreased linearly with increasing the level of ozone. On the other hand, the leaf and total dry weights of fir and zelkova exposed to the rain at pH 3.0 significantly decreased.
Net photosynthetic rates of Japanese cedar and two broad-leaved trees, and carboxylation efficiencies (initial linear slope of A/Ci curves) of two broad-leaved trees decreased linearly with increasing the level of ozone. Dark respiration rates of fr increased linearly with increasing the level of ozone. Moreover, rain at pH 3.0 caused an increase in the dark respiration rates in fir and zelkova. Significant interactive effects of ozone and simulated acid rain were observed in the ratio of top dry weight to root (T/R) in all tree species. The T/R of two conifers and birch increased linearly with increasing the level of ozone when they were exposed to the rain at pH3.0. The T/R of zelkova increased linearly with increasing the level of ozone, and the slope of regression line at the rain of pH3.0 was greater than that at the rain of pH 5.6.