Photosynthesis, immediate export and carbon partitioning in source leaves of C-3, C-3-C-4 intermediate, and C-4 Panicum and Flaveria species at ambient and elevated CO2 levels

Immediate export in leaves of C-3-C-4 intermediates were compared with thei r C-3 and C-4 relatives within the Panicum and Flaveria genera, At 35 Pa CO 2, photosynthesis and export were highest in C-4 species in each genera, Wi thin the Panicum, photosynthesis and export in 'type I' C-3-C-4 intermediat es were greater than those in C-3 species. However, 'type I' C-3-C-4 interm ediates exported a similar proportion of newly fixed C-14 as did C-4 specie s. Within the Flaveria, 'type II' C-3-C-4 intermediate species had the lowe st export rather than the C-3 species. At ambient CO2, immediate export was strongly correlated with photosynthesis, However, at 90 Pa CO2, when photo synthesis and immediate export increased in all C-3 and C-3-C-4 intermediat e species, proportionally less C was exported in all photosynthetic types t han that at ambient CO2. All species accumulated starch and sugars at both CO2 levels. There was no correlation between immediate export and the patte rn of C-14-labelling into sugars and starch among the photosynthetic types within each genus. However, during CO2 enrichment, C-4 Panicum species accu mulated sugars above the level of sugars and starch normally made at ambien t CO2, whereas the C-4 Flaveria species accumulated only additional starch.