Ozone-induced stomatal closure in Siebold's beech during early summer reduces ozone influx and allows the maximum photosynthetic capacity to be reached, but is not sufficient in older leaves to protect the photosynthetic system.

We set up a free-air ozone (O 3 ) exposure system for determining the photosynthetic responses of Siebold's beech (Fagus crenata) and oak (Quercus mongolica var. crispula) to O 3 under field conditions. Ten-year-old saplings of beech and oak were exposed to an elevated O 3 concentration (60 nmol mol -1 ) during daytime from 6 August to 11 November 2011. Ozone significantly reduced the net photosynthetic rate in leaves of both species in October, by 46% for beech and 15% for oak. In beech there were significant decreases in maximum rate of carboxylation, maximum rate of electron transport in photosynthesis, nitrogen content and photosynthetic nitrogen use efficiency, but not in oak. Stomatal limitation of photosynthesis was unaffected by O 3 . We therefore concluded photosynthesis in beech is more sensitive to O 3 than that in oak, and the O 3 -induced reduction of photosynthetic activity in beech was due not to stomatal closure, but to biochemical limitation. Capsule:Photosynthesis of beech is more sensitive to free air ozone exposure than that of oak

We investigated the effects of ozone and leaf senescence on steady-state stomatal conductance and stomatal response to light variation. Measurements were carried out in a free-air ozone exposure experiment on a representative deciduous broadleaved tree species in Japan (Fagus crenata). Both steady-state and dynamic stomatal response to light variation varied intrinsically with season due to leaf senescence. Ozone induced the decrease in steady-state leaf gas exchange and the sluggish stomatal closure progressively. These findings suggest that ozone reduces the ability of plants to adapt to a fluctuating light environment under natural conditions, and therefore impairs plant growth and ability to control water loss.

We studied the effects of elevated ozone ([O 3 ]) and CO 2 concentrations ([CO 2 ]) on the growth and photosynthesis of the hybrid larch F 1 (F 1 ) and on its parents (the Dahurian larch and Japanese larch). F 1 is a promising species for timber production in northeast Asia. Seedlings of the three species were grown in 16 open top chambers and were exposed to two levels of O 3 (⁄10 ppb and 60 ppb for 7 h per day) in combination with two levels of CO 2 (ambient and 600 ppm for daytime) over an entire growing season. Ozone reduced the growth as measured by height and diameter, and reduced the needle dry mass and net photosynthetic rate of F 1 , but had almost no effect on the Dahurian larch or Japanese larch. There was a significant increase in whole-plant dry mass induced by elevated [CO 2 ] in F 1 but not in the other two species. Photosynthetic acclimation to elevated [CO 2 ] was observed in all species. The net photosynthetic rate measured at the growing [CO 2 ] (i.e. 380 ppm for ambient treatment and 600 ppm for elevated CO 2 treatment) was nevertheless greater in the seedlings of all species grown at elevated [CO 2 ]. The high [CO 2 ] partly compensated for the reduction of stem diameter growth of F 1 at high [O 3 ]; no similar trend was found in the other growth and photosynthetic parameters, or in the other species.

We examined a performance of the multiplicative stomatal conductance model to estimate the stomatal ozone uptake for Fagus crenata. Parameterization of the model was carried out by in-situ measurements in a free-air ozone exposure experiment. The model performed fairly well under ambient conditions, with low ozone concentration. However, the model overestimated stomatal conductance under enhanced ozone condition due to ozone-induced stomatal closure. A revised model that included a parameter representing ozone-induced stomatal closure showed better estimation of ozone uptake. Neglecting ozone-induced stomatal closure induced a 20 % overestimation of the stomatal uptake of ozone. The ozone-induced stomatal closure was closely related to stomatal ozone uptake, rather than AOT40 (accumulated concentrations of ozone exceeding 40 nmol mol^[-1]). Our results suggest that ozone-induced stomatal closure should be implemented to stomatal conductance model for estimating ozone uptake for F. crenata and the implementation will contribute to adequate risk assessments of ozone impacts on F. crenata forests in Japan

To determine the effects of ozone (O3) on the canopy carbon budget, we investigated photosynthesis and respiration of leaves of Siebold's beech saplings under free air O3 exposure (60 nmol mol(-1), during daytime) in relation to the within-canopy light gradient; we then calculated the canopy-level photosynthetic carbon gain (PCG) and respiratory carbon loss (RCL) using a canopy photosynthesis model. Susceptibilities of photosynthesis and respiration to O3 were greater in leaves of upper canopy than in the lower canopy. The canopy net carbon gain (NCG) was reduced by O3 by 12.4% during one growing season. The increased RCL was the main factor for the O3-induced reduction in NCG in late summer, while contributions of the reduced PCG and the increased RCL to the NCG were almost the same in autumn. These results indicate contributions of changes in PCG and RCL under O3 to NCG were different between seasons.

Betula platyphylla var. japonica (white birch) has heterophyllous leaves (i.e., early and late leaves) and is a typical pioneer tree species in northern Japan. Seedlings of white birch were exposed to ozone during two growing seasons, and measurements were carried out in the second year. Early leaves did not show an ozone-induced reduction in photosynthesis because of lower stomatal conductance resulting in higher avoidance capacity for ozone-induced stress. Also, an ozone-related increase in leaf nitrogen content may partly contribute to maintain the photosynthetic capacity in early leaves under elevated ozone in autumn. On the other hand, late leaves showed an ozone-induced decline of photosynthesis and early defoliation of leaves occurred. Also, smaller leaf size and higher stomatal density in late leaves were observed under elevated ozone. Differences in stress resistance to ozone may be related to differing functional roles of early and late leaves for birch species.

We examined the effects of ozone and elevated CO2 concentration in summer on the growth and photosynthetic traits of three representative birch species in Japan (mountain birch, Monarch birch, and white birch). Seedlings of the three birch species were grown in 16 open-top chambers and were exposed to two levels of ozone (6 nmol mol^[-1] and 60 nmol mol^[-1] for 7 h per day) in combination with two levels of CO2 (370-380μmol mol^[-1] and 600 μmol mol^[-1] for daytime) from July to October. No adverse effects of ozone were found in the Monarch birch or the white birch, but elevated ozone in summer reduced branch biomass and net photosynthesis, and accelerated leaf abscission, in the mountain birch. Elevated CO2 promoted root development and thereby reduced the ratio of shoot dry mass (stem + branch) to root dry mass (S/R ratio) in the mountain birch and white birch. In contrast, there was no difference in dry mass between ambient and elevated CO2 for the Monarch birch, due to downregulation of photosynthesis. Studies of the combined effect of CO2 and ozone revealed that elevated CO2 did not ameliorate the effect of ozone on mountain birch in late summer. In considering the ameliorating effect of CO2 on ozone damage, it is necessary to take account of the species and the season