Relationship between photosynthesis and protein synthesis in maize. I. Kinetics of translocation of the photoassimilated carbon from the ear leaf to the seed

To gain a better understanding of the biochemical basis for partitioning of photosynthetically fixed carbon between leaf and grain, a 14CO2 labeling study was conducted with field-grown maize plants 4 weeks after flowering. The carbon flow was monitored by separation and identification of 14C assimilates and 14C storage components with each tissue during the chase period (from 4 to 96 hours) following a 5 minute 14CO2 pulse. In the labeled ear leaf, the radioactivity strongly decreased to reach, at the end of the experiment, about 12% of the total incorporated radioactivity, mostly associated with sucrose and proteins. Nevertheless, an unexpected reincorporation of radioactivity was observed either in leaf starch or proteins, the day following the pulse. Conversely, the radioactivity in the grain increased to attain 66% of the total incorporated 14C after a 96 hour chase. The photosynthates, mostly sucrose, organic and free amino acids, rapidly translocated towards the developing seeds, served as precursors for the synthesis of seed storage compounds, starch, and proteins. They accumulate in free form for 24 hours before being incorporated within polymerized storage components. This delay is interpreted as a necessary prerequisite for interconversions prior to the polycondensations. In the grain, the labeling of the storage molecules, either in starch or in storage protein groups (salt-soluble proteins, zein and glutelin subgroups), was independent of their chemical nature but dependent on their pool size