Variation in CO₂ assimilation rate induced by simulated dew waters with different sources of hydroxyl radical (·OH) on the needle surfaces of Japanese red pine (Pinus densiflora Sieb. et Zucc.)

The hydroxyl radical (·OH) is generated in polluted dew on the needle surfaces of Japanese red pine (Pinus densiflora Sieb. et Zucc.). This free radical, which is a potent oxidant, is assumed to be a cause of ecophysiological disorders of declining trees on the urban-facing side of Mt. Gokurakuji, western Japan. Mists of ·OH-generating N(III) (HNO₂ and NO₂^-) and HOOH+Fe+oxalate solutions (50 and 100 μM, pH 5.1-5.4) simulating the dew water were applied to the foliage of pine seedlings grown in open-top chambers in the early morning. Needles treated with 100 μM N(III) tended to have a greater maximum CO₂ assimilation rate (A_max), a greater stomatal conductance (g_s) and a greater needle nitrogen content (N_needle), suggesting that N(III) mist acts as a fertilizer rather than as a phytotoxin. On the other hand, needles treated with 100 μM HOOH+Fe+oxalate solution showed the smallest A_max, g_s, and N_needle, suggesting that the combination of HOOH+Fe+oxalate caused a decrease in needle productivity. The effects of HOOH+Fe+oxalate mist on pine needles were very similar to the symptoms of declining trees at Mt. Gokurakuji.