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

Abstract

Immediate export in leaves of C3-C4 intermediates were compared with their C3 and C4 relatives within the Panicum and Flaveria genera. At 35 Pa CO2, photosynthesis and export were highest in C4 species in each genera. Within the Panicum, photosynthesis and export in 'type I' C3-C4 intermediates were greater than those in C3 species. However, 'type I' C3-C4 intermediates exported a similar proportion of newly fixed 14C as did C4 species. Within the Flaveria, 'type II' C3-C4 intermediate species had the lowest export rather than the C3 species. At ambient CO2, immediate export was strongly correlated with photosynthesis. However, at 90 Pa CO2, when photosynthesis and immediate export increased in all C3 and C3-C4 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 14C-labelling into sugars and starch among the photosynthetic types within each genus. However, during CO2 enrichment, C4 Panicum species accumulated sugars above the level of sugars and starch normally made at ambient CO2, whereas the C4 Flaveria species accumulated only additional starch.