Roles of hydroxyl radical generating/scavenging mechanisms in pseudo polluted dew in reducing the foliar CO2 assimilation rate and biomass production of Japanese red pine (Pinus densiflora Sieb. et Zucc.) seedlings

Hydroxyl radical (OH) is one of the most highly reactive of all active oxygen species. Aqueous-phase OH is photochemically generated in polluted dew droplets on needle surfaces of Japanese red pine (Pinus densiflora Sieb. et Zucc., an evergreen coniferous tree) in declining pine forests. In former studies, we examined the effects of OH-generating mist solutions, which simulate polluted dew droplets, on needle ecophysiological traits of pine seedlings. Two types of solution with different OH-generating sources, i.e. the photo-Fenton reaction (H2O2-Fe-oxalate) and photolysis of HONO and NO2− [N(III)], had contrasting functional effects on needle gas exchange characteristics even though they had similar OH photoformation rates. In the present study, we investigated the effects of OH-generating mist solutions containing mixed-sources of OH (photo-Fenton and N(III)) on needle gas exchange and the biomass production of potted Japanese red pine seedlings. The N(III) and H2O2 concentrations of the OH-generating mist solutions were regulated to the same concentration (0, 25, 50, 75 or 100 μM, pH 5.2-5.3). Treated needles with the lowest photoformation rate and scavenging rate constant of OH had the smallest CO2 assimilation rate (Amax) and needle conductance (gn) of all treated pine seedlings. This suggests that the photoformation rate and scavenging rate constant do not always explain the ecophysiological disorders of pine needles subjected to OH-generating wet deposition. On the other hand, the calculated steady-state concentration of OH in the mist solutions was significantly negatively correlated with Amax. In addition, pine seedlings with reduced Amax showed suppressed biomass (dry weight) production. These results suggest that the OH concentration in the mist solutions is the decisive factor in explaining the negative effects of pseudo dew droplets not only on needle but also plant productivity. We propose that the deposition of polluted dew with chemical components and systems that result in a high OH concentration is one of the causes of growth decline of Japanese red pine.