To clarify the effects of long-term exposure to ammonium sulfate (AS) particles on growth and physiological
functions of forest tree species, seedlings of Fagus crenata, Castanopsis sieboldii, Larix kaempferi
and Cryptomeria japonica were exposed to submicron-size AS particles during two growing seasons from
3 June 2011 to 8 October 2012. The mean sulfate concentration in PM2.5 increased during the exposure
inside the chamber in 2011 and 2012 by 2.73 and 4.32 mg SO4 2 m3, respectively. No significant effects of
exposure to AS particles were detected on the whole-plant dry mass of the seedlings. These results
indicate that the exposure to submicrometer AS particles at the ambient level for two growing seasons
did not significantly affect the growth of the seedlings. No significant effects of exposure to AS particles
were found on the net photosynthetic rate in the leaves or needles of F. crenata, C. sieboldii and
L. kaempferi seedlings. Also, in the previous-year needles of C. japonica seedlings, exposure to AS particles
significantly reduced the net photosynthetic rate, which may be caused by the reduction in the concentration
of ribulose-1, 5-bisphosphate carboxylase/oxygenase (Rubisco). On the contrary, in currentyear
needles of C. japonica seedlings, net photosynthetic rate significantly increased with exposure to
AS particles, which may be the result of increases in stomatal conductance and concentrations of Rubisco
and chlorophyll. Furthermore, exposure to AS particles correlated with an increase in concentrations of
NH4
þ, free amino acid and total soluble protein, suggesting that AS particles may be deliquesced, absorbed
into the leaves and metabolized into amino acid and protein. These results suggest that net photosynthesis
in the needles of C. japonica is relatively sensitive to submicron-size AS particles as compared with
the other three tree species.