Translocation of C in the sugarcane plant during the day and night

The time-course of translocation of ¹⁴C from the blades of the sugarcane plant was investigated by analysis and radioactive counting of successive samples punched from a single blade. In 1 experiment, the time-course was studied by determining the specific activity of the carbon dioxide respired by the roots.

The rate of translocation, expressed as percentage, was highest immediately after the application of the radioactive carbon dioxide. Morning-made photosynthate translocated a higher percentage during the morning than during the afternoon in 90-minute periods in the light. Afternoon-made photosynthate translocated as well or better than morning-made photosynthate for the first hour in the light.

The leaf-disk data and the specific activity of the carbon dioxide respired by the roots corresponded by showing lower rates of translocation by night than by day for several successive days. Also, the translocation of ¹²C sucrose was slower at night.

The ¹⁴C sucrose translocated by day was made primarily by photosynthesis; the sucrose translocated by night was made primarily by the conversion of other labeled compounds, e.g. organic acids, organic phosphates, and insoluble residue.

The radioactive constituent of the residue, which was converted to sucrose, was tentatively identified as a glucose-xylose-glucuronic acid hemicellulose, with most or all of the ¹⁴C in the glucose moiety.

Translocation of sucrose may be triggered by different mechanisms during the night than the day. The conversion of insoluble residue to sucrose by increasing the osmotic potential at the source would favor a pressure-flow mechanism for nocturnal translocation; whereas translocation by day is thought to be a process of phototranslocation, a photoactivation of the translocation mechanism.