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- Materials and Methods
- Results and Discussion
Evaluation of the effects of secondary pollutants generated from NOx on terrestrial ecosystems is an important problem. The possibility of negative effects of nitrogen oxides (NOx) on vegetation has been suggested, but there has been little research aimed specifically at the effects of atmospheric NOx (Berrang et al., 1995). A significant correlation has been observed between atmospheric NOx concentration (which is on average lower than 30 ppb) and tree decline (Naemura et al., 1997). However, when various plants were directly exposed to NO2 in short-term experiments, at least 100 ppb NO2 was needed to detect any effects (Natori & Totsuka, 1984). Even under exposure to 500 ppb NO2 for 6 h d−1 for 66 d, Pinus sylvestris did not show visible injury (Oleksyn et al., 1988). On the other hand, atmospheric nitrogen oxides generates ozone and other peroxides through photochemical reactions. Heber et al. (1995) concluded that the indirect toxic action of NO2 via ozone formation is much more damaging than the direct action of NO2.
The forests of Pinus densiflora on Mt. Gokurakuji (34°23′ N, 132°19′ E, 693 m asl) have severely declined over the last decade. The atmospheric NO2 concentration on the seaward side (an area of forest decline) of Mt. Gokurakuji was significantly higher than that on the inland side (an area free of decline). In the area where decline has occurred, significant correlations were found between the mean concentration of atmospheric NO2 and the mortality of the pines (Naemura et al., 1997) and the ethylene emission from pine needles (Kume et al., 2001). Soil acidification (pH, total N content) was not correlated with the decline (Kume et al., 2000). Kume et al. (2000) showed that maximum net photosynthesis (Pn) in the areas displaying decline was about 30% lower than that in the other areas, and that the lower Pn could be largely explained by the decrease in maximum stomatal conductance (gl). The causes of physiological disorders of needles were speculated to be caused by NOx-related substances but not O3 (Kume et al., 2000, 2001). However, in the most polluted areas of Mt. Gokurakuji, the maximum NOx concentration was about 200 ppb and the annual average NO2 concentration was at most 20 ppb. Therefore, the effects of secondary pollutants or other associated pollutants should be considered.
On Mt. Gokurakuji, the atmospheric NO2 is concentrated at lower altitudes because of the climatic inversion layer that frequently occurs below 300 m (Naemura et al., 1996). The highly humid climate in the Seto Inland Sea area is prone to dew from the night to the morning of clear days throughout the year. The frequency of natural dew formation in the low altitude areas is about 20–30% in summer and 30–50% in winter mornings. Under such conditions, not only do the gaseous pollutants stagnate in the low altitudinal range, but also secondary pollutants are generated from NOx and are accumulated in the morning dews and/or fogs. Peroxides and HNO3 are likely to be deposited on leaf surfaces because of their high solubility in water (Cape, 1997).
Photochemical reactions in the liquid-phase form various oxidants, such as HOOH, HO2, singlet molecular oxygen, and OH (Faust & Allen, 1992; Faust et al., 1993; Anastasio et al., 1994; Arakaki & Faust, 1998). NO2−, NO3−, HOOH, Fe(OH)2+, and the photo-Fenton reaction (a reaction between photochemically reduced iron and HOOH) are all known OH sources in the liquid-phase (Zellner & Herrmann, 1990; Faust & Hoigné, 1990; Zepp et al., 1992). Arakaki et al. (1998, 1999b) observed that OH and NO radicals (·OH and ·NO) were formed from the photolysis of nitrite ions (NO2−) in dew water and estimated ·OH photoformation rate constants (Arakaki et al., 1999a). They suggested that the photolysis of aqueous-phase N(III) (HNO2 and NO2−) plays a significant role in initiating oxidation reactions in dew water. Arakaki et al. (1999b) reported that the average rate of ·OH formation in dew collected on a Teflon sheet (1.25 µM h−1) was 3.6 times greater than that in the rainwater (0.35 µM h−1) during the period 1997–1998 at Higashi-Hiroshima, Japan under conditions of the same solar irradiation. Nakatani et al. (2001) determined ·OH formation rates in dew on the needle surface in the declining pine forest of Mt. Gokurakuji during October–November 1999. They found that the mean rate (3.36 µM h−1) and highest rate (5.18 µM h−1) were much higher than the values reported by Arakaki et al. (1999b). Dew dissolves deposits on the surface of the pine needles, and subsequent evaporation after dawn concentrates the ·OH forming compounds. It is probable that the ·OH formation potential is much greater on the surface of the pine needles than in the dew collected before sunrise. Therefore, free radicals generated in the morning dew on needle surfaces might cause some of the ecophysiological disorders observed in the pine needles at Mt. Gokurakuji.
Another important fact is that the mean and range of pH values in dew formed on the pine needles were 4.54 and 4.23–5.96, and these were same levels as those of rain (Nakatani et al., 2001). Therefore, the pH of dew was not low enough to induce harmful effects on the pine needles.
We investigated the effects of polluted dew water, especially ·OH and ·NO on pine needles because ·OH is considered the most potent oxidant in the atmosphere (Warneck, 1988; Wayne, 1991; Thompson, 1992; Finlayson-Pitts & Pitts, 2000) and ·NO is potentially more phytotoxic than NO2 (Wellburn, 1990). Few studies have reported the effects of ·OH on tree decline, probably due to the lower concentrations (1–10 × 106 molecules cm−3, Prinn et al., 1987) and difficulties in detecting ·OH in the atmosphere (Mount & Eisele, 1992).
We focused on wet deposition, in which gaseous and dry phase substances are transformed to the liquid phase, as a factor affecting the ecophysiological traits of pine needles.
In order to verify the effects of ·OH and ·NO on the needles, we conducted simulated morning dew experiments for various liquid phase oxidants, and the effects on photosynthesis, stomatal conductance, and the light reaction of needles were investigated.