Reduction in Sink-Mobilizing Ability following Periods of High Carbon Flux.

Sink tissues may play a significant role in determining photosynthetic rates through their ability to mobilize assimilates. The objective in this study was to determine if the mobilizing ability of taproot sink tissues of sugarbeet (Beta vulgaris) could become limiting when assimilate supply was maintained at a high level for an extended period of time. Assimilate supply was either enhanced by CO(2) enrichment or reduced by shading.Field-grown sugarbeet plants were exposed to ambient CO(2) and one of five photosynthetically active radiation (PAR) durations: 10-hours PAR; 6-hours PAR; 3-hours PAR; 1-hour PAR; and continuous 80% shade conditions or 1,000 microliter per liter CO(2) and 10-hour PAR. Taproots were harvested at 1600 hours on the day following the initiation of the treatments. The sucrose-uptake capacity of excised tissue discs was determined in 30 millimolar morpholinopropane sulfonic acid (pH 7.0) containing 40 millimolar [(14)C]sucrose.Rates of sucrose uptake were inversely related to the supply of photosynthate during the preceding light period. CO(2) enrichment reduced uptake capacity relative to the control. In contrast, reducing the duration of PAR increased uptake over the control. Leaf starch accumulation was correlated with reduced uptake capacity. The results indicate that, under the conditions employed here, the mobilizing ability of sinks may limit carbon flux from source and to sink during periods of high photosynthetic rates.