Osmotic potential and soluble solids concentration in onion (Allium cepa) bulbs

Non-structural carbohydrate composition of onion (Allium cepa L) bulbs is known to vary between varieties according to soluble solids concentration (SS), which may reduce the effect of SS on osmotic potential. This was confirmed in an experiment where osmotic potential (Ψπ) was determined psychometrically, and SS concentration refractometrically, in juice from entire, mature bulbs of 35 varieties of onion. ‘Fresh market’ varieties ranged in mean SS from 63 to 125 g kg- and in mean Ψπ from −0.97 MPa to −1.17MPa, whereas ‘dehydrating’ varieties ranged in mean SS from 167 to 227 g kg-1 and in mean Ψπ from −1.23 MPa to −1.41 MPa. Compared to sugar solutions of constant composition, with increasing SS, there was a relatively slow decline in Ψπ toward a lower limit at the SS levels of dehydrating varieties. The relationship between Ψπ and SS was explained equally well (r2 = 0.83; DF = 172) by non-linear and bent-stick regression models, the latter indicating breakpoints close to the SS level separating fresh market variety samples from those of dehydrating varieties. Variety differences in the relationship between Ψπ and SS indicated among dehydrating varieties in particular are worthy of further investigation because of their possible physiological and quality implications.