Pre-emergence Growth of Genotypes of Sugarbeet (Beta vulgaris L.) Tolerant to Rhizomania

Growth during pre-emergence of three sugarbeet genotypes, all tolerant to the root disease rhizomania but with poor emergence features, was studied. Dry mass, carbon and nitrogen content, and N isotope composition were measured for the seeds and each seedling part. These results were compared with those obtained for Vega, a genotype with no emergence problems. The depletion of seed reserves and carbon use were similar for all genotypes. Tolerant genotypes had altered nitrogen absorption: the net nitrogen balance (total N in the seedling/N initial mass in the seed) was 20% lower in these genotypes at the end of the experiments. Differences in the natural abundance of 15 N in seeds and nutrient solutions were used to determine the proportion of N originating from seed reserves or from absorption by the organs. N absorbed from the nutrient solution was found mainly in the hypocotyl and radicle, with less in the cotyledons of all genotypes. Tolerant genotypes had less absorbed N in all parts than non-tolerant genotypes. Hypocotyl and radicle elongation was the same in all genotypes, but the emergence forces in rhizomania-tolerant genotypes were smaller. These results suggest that breeding to introduce tolerance to rhizomania has also led to a lowered mineral absorption capacity.