Partitioning within the Taproot Sink of Sugarbeet: Effect of Photosynthate Supply1

Abstract

Genotypes of sugarbeet (Beta vulgaris L.) differ widely in partitioning of photosynthate between sucrose and nonsucrose dry matter in the taproot sink. Differences in root (sink) morphology between various genotypes and agronomic practices (cell size and distance between vascular rings) have been found to be related to photosynthate partitioning. The objective of this research was to determine the effect of photosynthate supply on partitioning within the root sink. It was hypothesized that altering the photosynthate supply would not affect partitioning, but would rather affect the growth rate of the root with no change in relative root morphology. Photosynthetic rates of field‐grown sugarbeets were increased by CO2 enrichment and reduced by shading. Carbon dioxide levels within the canopy were increased to 600 to 850 ppm using a propane‐fired CO2 generator with dry ice. In the shading treatments, light levels were reduced by 50% with a nylon mesh screen suspended above the canopy. Shading reduced total dry matter production by 29%, but had no effect on either the root:shoot ratio or the sucrose content of the taproot on a dry weight basis. Although the size of root was altered in the two treatments, the differences were compensated by increased or decreased frequency of vascular rings, resulting in no change in vascular density or in cell size. These results are consistent with previously proposed models for photosynthate partitioning within the taproot of sugarbeet and indicate that the partitioning mechanism maintains a constant partitioning ratio between sucrose and nonsucrose dry matter independent of photosynthate supply.