Autumn sown sugar beets (winter beets) are expected to yield markedly higher than spring sown beets. This requires a continuous growth during an extended growing period. So far, bolting‐resistant sugar beet varieties are not available to test winter beets under field conditions in Central Europe. The objective of this study was therefore to analyse yield formation and sugar storage of sugar beet plants during an extended growing period to estimate whether sugar beet has the potential to generate the theoretically expected yield increase. From 2008 to 2012, pot experiments were carried out in the glasshouse with 11 sowing dates spread over the years with sequential harvests. The oldest plants were grown for 859 days (14 242 °Cd). Root fresh matter yield continuously increased till the latest harvest. In contrast, the sugar concentration reached an optimum value between 3400 and 5000 °Cd and then decreased with time. Despite longer growing periods, the number of cambium rings, which are regarded as essential for sugar storage, did not change. This points to an early and genetically fixed determination of the formation of cambium rings. Additionally, the rate of photosynthesis decreased concomitantly with the sugar concentration. In conclusion, there is some evidence that the sugar concentration of the storage root is limited by the sink capacity, which in turn controls the source activity by a feedback regulation of photosynthesis and leaf formation. The dry matter composition of the storage root changed towards lower sugar concentration and concurrent higher concentration of cell wall compounds (marc). The sugar yield still increased beyond a thermal time at which winter beets will probably be harvested in practice. Hence, the theoretical yield increase in autumn sown sugar beets can be realized, provided that the plants show sufficient winter hardiness and bolting resistance.

Good storability of sugar beet is of increasing importance, not only to reduce sugar losses, but also with regard to maintaining the processing quality. Genotypic differences are found in storage losses. However, it is not clear to which extent damage may contribute to the genotypic response. The aim of the study was to quantify the effect of root tip breakage on storage losses of different genotypes. For that purpose, in 2012 and 2013, six sugar beet genotypes were grown in field trials at two locations. After lifting roots were damaged with a cleaning device. They were stored for 8 and 12 weeks, either under controlled conditions in a climate container at constant 8°C, or under ambient temperature in an outdoor clamp. The close correlation underlines that storage losses under controlled conditions (constant temperature) can well be transferred to conditions in practice with fluctuating temperature. The strongest impact on invert sugar accumulation and sugar loss after storage resulted from storage time, followed by damage and growing environment (year × growing site). Cleaning reduced soil tare but increased root tip breakage, in particular for genotypes with low marc content. During storage, pathogen infestation and invert sugar content of the genotypes increased with root tip breakage, but the level differed between growing environments. Sugar loss was closely related to invert sugar accumulation for all treatments, genotypes and environments. Hence, it can be concluded that root tip breakage contributes considerably to storage losses of sugar beet genotypes, and evidently genotypes show a different susceptibility to root tip breakage which is related to their marc content. For long-term storage it is therefore of particular importance to avoid damage during the harvest operations and furthermore, to have genotypes with high storability and low susceptibility to damage.

Storage losses of sugar beets are affected by storage conditions, but may also depend on growing site and genotype. The aim of the present study was to quantify the genotype effect on storage losses and to analyze the reasons for genotypic variability in sugar losses and accumulation of invert sugar. In 2011, 36 sugar beet genotypes and in 2012, 18genotypes were cultivated at two growing sites. After harvest beets were stored for 8 and 12 weeks at 8°C and 20°C in climate containers, respectively. Sugar losses increased with thermal time in store and were closely related to invert sugar accumulation. The growing site strongly affected the storage losses and maximum genotypic differences occurred at growing sites with particularly high level of storage losses. Genotypic differences were primarily caused by differences in the level of infestation with microorganisms, but also by differences in the beets’ carbohydrate metabolism. The infestation with microorganisms after storage was related to the marc content of genotypes before storage pointing to a non-specific resistance. The results underline a marked influence of the genotype on storage losses with a proportion of variance of 12%. Thus, selection of varieties with improved storability seems promising to reduce storage losses of sugar beet. But so far, no criteria are available to select for good storability of sugar beet varieties.

To improve the storability of sugar beets, this study aimed at determining reasons for genotypic variability in sugar losses and invert sugar accumulation during storage, and at identifying indirect criteria to select for varieties with low storage losses prior to storage. In 2011 and 2012, 18 genotypes, and in 2012 and 2013, six genotypes cultivated at two locations were stored for 8 and 12 weeks at 8°C under controlled conditions. The same 18 genotypes were grown under stress conditions in Spain in 2012/2013. Sugar losses were closely correlated with the invert sugar accumulation after storage. Genotypic differences in storage losses were primarily caused by differences in the level of infestation with microorganisms. The invert sugar accumulation was lower for genotypes with high marc concentration before storage, pointing to a non‐specific resistance. Additionally, the sugar concentration in dry matter before storage, and the invert sugar concentration after cultivation under stress conditions correlated with the invert sugar concentration after storage. These parameters are therefore suggested as criteria to select for improved storability of sugar beet genotypes.

During storage, the invert sugar content of sugar beets increases with increasing storage period and storage temperature, thereby decreasing the processing quality of the beets substantially. Invert sugar results from the enzymatic degradation of sucrose to glucose and fructose. The current methods to determine invert sugar in sugar beets have a low sample frequency and are very expensive and are therefore not implemented in the routine analyses of sugar factories. The content of invert sugar could be calculated based on the glucose content. This requires a constant ratio of glucose to fructose in freshly harvested sugar beets as well as in sugar beets stored under different conditions. The objective of the present study was thus I) to analyse the glucose to fructose ratio of freshly harvested beets and of beets stored under different conditions, and II) to develop a formula to estimate the invert sugar content based on the glucose content of sugar beets. The ratio of glucose to fructose in freshly harvested beets and beets stored under different conditions was quite similar. A close linear relationship between glucose and invert sugar content in freshly harvested and stored beets was found. By using the regression function, the invert sugar content of an independent dataset was calculated based on the glucose content. The estimated invert sugar content was closely correlated with the invert sugar content measured by HPLC. The invert sugar content in freshly harvested and stored sugar beets can thus be calculated with the formula developed in this study. This would considerably improve the quality assessment of sugar beets once the new method to measure the glucose content becomes implemented in the routine analysis in sugar factories.