The nature of the photosynthetic pathways of Cyperaceae found in Japan were investigated on the basis of Kranz anatomy, the CO2 compensation concentration and previously reported data. Among 301 species (96% of all cyperaceous species recorded in the region), 58 species were classified as being C4 plants. These C4 species were scattered among the tribes Fimbristylideae, Lipocarpheae, Cypereae and Rhynchosporeae in the subfamily Cyperoideae. The genera Cyperus, Eleocharis and Rhynchospora included, in Japan, both C3 and C4 species within a single genus. Using these data, an analysis was made of the ecological characteristics and geographical distribution of the C3 and C4 species in Japan. Although cyperaceous species grow in markedly different environments, the majority were found in wet and aquatic areas (61%) or shaded areas, such as forest floors (20%). Most of the C3 species were also hygrophytes (58%) and forest-living species (25%), and C3 species growing in mesic and dry areas were relatively rare. The C4 species inhabited wet and aquatic (75%), mesic (13%) and dry areas (6%) and showed marked ecological characteristics with respect to soil-moisture conditions, unlike other C4 plants, although they were absent from shaded habitats. In order to determine the climatic factors that influence the relative floristic abundance of C3 and C4 members of the Cyperaceae in Japan, the ratios of number of C4 species to the total number of members of Cyperaceae (C4 percentage) in 16 representative locales were examined in terms of various climatic variables. There were strong positive correlations between the C4 percentage and temperature. Among the C3 groups of three subfamilies, there were different distributional trends for various temperature regimes. The C3 subfamily Caricoideae increased its relative contribution to the cyperaceous flora with a decrease in mean annual temperature, while the C3 subfamily Sclerioideae exhibited the opposite pattern. The C3 group of the subfamily Cyperoideae did not show any marked change in pattern along temperature gradients, unlike the two other C3 subfamilies, and seemed to be heterogeneous in terms of its response to temperature. The relationships between the C4 biochemical subtypes and ecological characteristics are also discussed.
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