Early Season Patterns of Carbohydrate Partitioning in Exposed and Shaded Apple Branches

The partitioning of photosynthates labeled by (CO2)-C-14 in exposed and shaded 'Empire' apple Borkh.) branches was examined at 1, 3,5, and 10 weeks after bloom. Extension shoots, nonfruiting spurs, or fruiting spurs were labeled separately to examine which shoot types exported to the fruit at each time. The general partitioning patterns were observed with autoradiography, while label accumulation in fruit was determined by oxidation and scintillation counting. At each treatment time, half of the branches was preconditioned with artificial shade (to 35% full light) for 48 hours before labeling and returned to the shade for a 2-day translocation period. One and 3 weeks after bloom, extension shoots showed little export to fruit; nonfruiting and vigorous fruiting spurs exported label to weak spurs and extension shoot tips. Shade had no major effect on partitioning patterns at 1 and 10 weeks, but essentially eliminated export from extension shoots at 3 weeks and greatly reduced export to fruit 5 weeks after bloom, as observed on the autoradiograms. At 5 weeks after bloom, the shading effect was equal to a 2-week delay in export. By 10 weeks after bloom, all shoot types were exporting most of the C-14 fixed to fruit. The photosynthate support of the fruit before fruit set seemed to strongly depend on the spur canopy, especially when the extension shoots were exposed to low light.