Photosynthesis, immediate export and carbon partitioning in source leaves of C3, C3-C4 intermediate, and C4 Panicum and Flaveria species at ambient and elevated CO2 levels

Immediate export in leaves of C-3-C-4 intermediates were compared with their C-3 and C-4 relatives within the Panicum and Flaveria genera, At 35 Pa CO2, photosynthesis and export were highest in C-4 species in each genera, Within the Panicum, photosynthesis and export in 'type I' C-3-C-4 intermediates were greater than those in C-3 species. However, 'type I' C-3-C-4 intermediates exported a similar proportion of newly fixed C-14 as did C-4 species. Within the Flaveria, 'type II' C-3-C-4 intermediate species had the lowest export rather than the C-3 species. At ambient CO2, immediate export was strongly correlated with photosynthesis, However, at 90 Pa CO2, when photosynthesis and immediate export increased in all C-3 and C-3-C-4 intermediate species, proportionally less C was exported in all photosynthetic types than that at ambient CO2. All species accumulated starch and sugars at both CO2 levels. There was no correlation between immediate export and the pattern of C-14-labelling into sugars and starch among the photosynthetic types within each genus. However, during CO2 enrichment, C-4 Panicum species accumulated sugars above the level of sugars and starch normally made at ambient CO2, whereas the C-4 Flaveria species accumulated only additional starch.