To examine the effects of decreased CO 2 and O 2 partial pressure on leaf-level photosynthesis in alpine plants at high altitude, we compared the maximal carboxylation efficiency, CE , of Reynoutria japonica Houtt. var. japonica growing in a highland with one growing in a lowland. CE under the native atmospheric conditions (native CE ) of the highland population was significantly lower than that of the lowland one. The O 2 dependency of CE was significantly less in the highland population than in the lowland. Using theoretical analysis, we explained that O 2 dependency of leaf-level photosynthesis became less as internal conductance ( g i ) decreased. We also showed that g i and the content of active Rubisco ( E ) could be estimated from the O 2 dependency of leaf-level photosynthesis. By applying the analysis, a severe limitation of CO 2 diffusion in the leaf was estimated in the highland population, whereas almost the same values of E were estimated in both populations. A reasonable explanation for the difference in the native CE is the smaller O 2 dependency and photosynthetic capacity caused by a smaller g i in the highland population in addition to the differences in the partial pressures of CO 2 and O 2 .

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The root respiration rate often shows an exponential or a linear relationship with temperature under laboratory conditions. However, under intact conditions in the field, the root respiration rates of some tree species decreased around midday despite an increment of the root temperature (Bekku et al. 2009). To clarify the cause of midday depression, we examined the relationships between the intact root respiration and parameters of leaf gas exchange through the simultaneous field measurement of the gas exchange in the leaf and root of Quercus crispula and Chamaecyparis obtusa, which are canopy trees. There were no significant relationships between the root respiration rates (R r ) and the parameters of leaf gas exchange in the field. However, in C. obtusa, the relationships between R r and the transpiration rates (E) at 1 h before the measurement of R r were fitted by logarithmic function with a determination coefficient of 0.60-0.89. In the light-manipulation experiments using saplings, R r had significant positive correlations with E at 20 min before the measurement of R r , root temperature (T r ), and the photosynthesis (P n ) at 20 min before the measurement of R r . We examined which factor, P n or E, affects the root respiration rate through a manipulation experiment using a growth chamber regulating the ambient CO 2 concentration and relative humidity independently under constant air temperature and photosynthetic photon flux density. As a result, the root respiration rates changed corresponding to E and not P n . These results suggest that the root respiration rate of trees changes significantly in the daytime and is affected by the leaf transpiration rate as well as the temperature.

To identify the leaf characteristics associated with the difference in altitudinal distribution of Polygonaceae plants, we investigated the seasonal changes in leaf characteristics of co-occurring Aconogonum weyrichii and Reynoutria japonica at 2250 m a.s.l. on Mt. Fuji. Aconogonum weyrichii is distributed in the alpine region and R. japonica from the lowlands to the highlands. At the beginning of the growing period, leaves of A. weyrichii had larger amounts of nitrogen, Rubisco and higher photosynthetic capacity than R. japonica. The relationship between the amounts of nitrogen and Rubisco did not significantly differ between the species, but a higher Rubisco activity per unit Rubisco protein content was found in A. weyrichii, and was thought to be responsible for the high photosynthetic capacity of this species in summer. Activity of ascorbate peroxidase, a key enzyme of the hydrogen peroxide scavenging system against oxidative stress under chilling and light conditions, increased significantly in R. japonica late in the growing period, but did not significantly change in A. weyrichii. In A. weyrichii, rapid maturation of leaves and a high photosynthetic capacity are thought to be adaptive features for the short growing season in alpine regions. On the other hand, the slower accumulation of nitrogen and Rubisco during the early stage of the growing period in R. japonica resulted in insufficient utilization of the growing period for photosynthetic production and is thought to be one of the factors restricting the upper limit of its distribution.

We observed the phenomenon of midday depression in the rate of tree root respiration. Diurnal changes in the root respiration rate of Quercus crispula and Chamaecyparis obtusa were measured under intact conditions using a closed chamber method and a soil respiration measurement system (LI-6400 with a root respiration chamber) in a forest in the foothills of Mt. Fuji. After the measurement of intact root respiration in the field, the root was excised and taken to the laboratory, and the temperature dependence on the respiration rate of the detached root was measured using an openflow gas exchange system with an infrared gas analyzer (LI-6252). The measurement was conducted in September 2003, August and November 2005, and June 2006. Whereas the root respiration rate of both species under intact conditions increased with increasing soil and root temperatures from dawn to early morning, the respiration rate decreased around midday from 10:00 to 15:00 despite an increment of soil and root temperatures. There was no clear relationship between the intact root respiration rate and root temperature in either species, although the detached root respiration rate of both increased exponentially with the temperature. The amount of the CO 2 efflux estimated using the temperature dependence of detached root respiration tended to underestimate the actual measurement value (intact respiration rate) by 20-50% in both species. These results indicate that evaluating midday depression in root respiration would be important for a more accurate estimation of the carbon cycle or net ecosystem production in forests.