Environmental and Experimental Botany 60 (2007) 159–169
Roles of hydroxyl radical generating/scavenging mechanisms in pseudo
polluted dew in reducing the foliar CO
2
assimilation rate and biomass
production of Japanese red pine (Pinus densiflora Sieb. et Zucc.) seedlings
Nobutake Nakatani
a,∗
, Sachiko Akane
a
, Masaaki Chiwa
a,b
,
Tsuyoshi Kobayashi
c
, Hiroshi Sakugawa
a
a
Graduate School of Biosphere Science, Hiroshima University, 1-7-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8521, Japan
b
Department of Forest and Forest Products Sciences, Faculty of Agriculture, Graduate School of Kyushu University,
394 Tsubakuro, Sasaguri 811-2415, Japan
c
Faculty of Agriculture, Kagawa University, 2393 Ikenobe, Miki-cho, Kita-gun, Kagawa 761-0795, Japan
Received 8 September 2006; accepted 1 October 2006
Abstract
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 (H
2
O
2
-Fe-
oxalate) and photolysis of HONO and NO
2
-
[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 H
2
O
2
concentrations of the
•
OH-generating mist solutions were regulated to the same concentration (0, 25, 50, 75 or 100 M, pH 5.2–5.3). Treated needles
with the lowest photoformation rate and scavenging rate constant of
•
OH had the smallest CO
2
assimilation rate (A
max
) and needle conductance (g
n
)
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 A
max
. In addition, pine seedlings with reduced A
max
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.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Hydroxyl radical; Mist treatment; Photosynthesis; Steady-state concentration; Tree decline; Wet acidic deposition
1. Introduction
Active oxygen species (AOS), such as superoxide anion rad-
ical (O
2
•-
), hydrogen peroxide (H
2
O
2
), singlet oxygen (
1
O
2
)
and hydroxyl radical (
•
OH), are produced during plant metabolic
processes and are normally kept at a low concentration by vari-
ous enzymatic and chemical scavenging mechanisms (Asada,
1999; Foyer and Noctor, 2000; Apel and Hirt, 2004). How-
∗
Corresponding author. Tel.: +81 82 424 6504; fax: +81 82 424 6504.
E-mail address: nnakatan@hiroshima-u.ac.jp (N. Nakatani).
ever, when these scavenging mechanisms are disturbed and/or
the production of AOS exceeds the capacity to reduce them,
AOS accumulate in plant cells. For example, environmental fac-
tors that induce stressful conditions such as drought (Egert and
Tevini, 2002), cold (Bowers, 1994) and salinity (Gueta-Dahan
et al., 1997) promote the accumulation of AOS in plant cells.
In addition to these factors, gaseous atmospheric pollutants
also induce excess AOS production in plant cells (Morita and
Tanaka, 2002). Air pollutants such as sulfur dioxide (SO
2
) and
ozone (O
3
) are taken into the leaves through the stomata and
dissolved in cellular fluid (Kondo and Saji, 1992). In both enzy-
matic and non-enzymatic processes, AOS are then produced
0098-8472/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.envexpbot.2006.10.006