Correlating source leaf photosynthesis and export characteristics of C3, C3-C4 intermediate and C4 Panicum and Flaveria species with whole-plant relative growth rate

Abstract

Source leaves of 10 Panicum and 7 Flaveria C3, C3-C4 intermediate, and C4 species were fed 14CO2 in the mid-afternoon. Immediate export, calculated when isotopic equilibrium existed between 14CO2 and 14C-translocates, was compared with subsequent 14C-export during afternoon-nighttime chase periods. Immediate and daily export patterns were compared to the relative growth rates (RGR) of the plants calculated from whole-plant daily net carbon exchange. The C4 types fixed and exported the most during the feed within each genus. By comparison, immediate export rates during photosynthesis in Panicum "type I" C3-C4 intermediates were greater than those in C3 species. In contrast, immediate export was much lower in Flaveria "type II" C3-C4 intermediates than in their C3 and C4 relatives. In all species 14C-reserves, primarily starch and sugars, sustained export during the chase periods, but the extent to which subsequent day-night export compensated for lower export during the feed was greater among Panicum than Flaveria species. The Panicum "type I" C3-C4 intermediates still behaved in an "intermediate" manner with respect to daily 14C-export. In contrast, the Flaveria "type II" C3-C4 intermediates still had the lowest total 14C-export by the next morning. Overall, in both genera daily export correlated better with immediate export rates than with photosynthesis itself and other leaf export parameters. Among the Panicum, the correlation between whole-plant RGR and leaf function measured was not strong. However, among the Flaveria the correlation between whole-plant RGR was strongest with immediate export of the source leaf.