Relationships between bulb dry matter content, soluble solids concentration and non-structural carbohydrate composition in the onion (Allium cepa)

Relationships between bulb dry matter content (DM), soluble solids concentration (SS) and non‐structural carbohydrate composition were evaluated in 49 onion (Allium cepa L) varieties ranging in mean DM from 74 to 215g kg‐1. The purpose of the study was to determine whether changes in total (free and combined) fructose concentration explain differences between varieties in DM and SS, and whether SS and DM are negatively correlated with reducing sugar concentration. There was a strong linear relationship between bulb SS and DM. Total fructose concentration was strongly related to SS and DM, either as a curvilinear relationship with SS, or with varying linear coefficients among fresh market (SS < 140g kg‐1) varieties and dehydrating (SS > 140g kg‐1) varieties. There was evidence of differences between dehydrating varieties possibly associated with the earliness or lateness of bulb maturation. Reducing sugar concentration tended to decrease with increasing SS, but correlations between reducing sugar concentration and SS or DM were smaller within the table or dehydrating variety groups than over all varieties. The poorness of those correlations was not explained by differences in maturity group, bulb diameter or number of fleshy leaf bases per bulb within either group. In several respects, fresh market and dehydrating varieties behaved differently, though this may simply reflect the relative scarcity of samples with SS between 120 and 150g kg‐1. The results imply increasing polymerisation of sugars into fructans with increasing SS. They support the hypothesis that the general trend in non‐structural carbohydrate composition of the bulb limits the rise in osmotic potential with increasing SS and therefore reduces the concentration and osmotic gradients to the bulb, but they also indicate further differences between varieties, particularly among dehydrating varieties.