Sulfur nutrition affects cellular sulfur, dry weight distribution, and bulb quality in onion

Three onion (Allium cepa L.) cultivars, 'Southport White Globe', 'Grano', a nd 'Pukekohe Longkeeper' were grown at low to high 8 (at 0.5, 1.8, 3.0 or 4 .0 meq(.)L(-1)) in hydroponic culture. Differential solvent extractions of bulbs were used to isolate quantitatively cell contents, cell wall proteins , and cell wall residue. The weight of the cell fractions, their S content, and the S content of intact bulbs were determined. Bulb characteristics of fresh weight (FW), firmness, soluble solids concentration (SSC), and solub le sugars were also determined. For all three cultivars, bulb FW increased with S from 0.5 to 4.0 meq(.)L(-1). Sulfur had a significant effect on bulb firmness. Onion bulbs grown with S at 0.5 meq.L-1, the lowest S concentrat ion, were significantly softer than onion bulbs grown at the highest concen tration of 4.0 meq(.)L(-1). Varying the S supply had a major effect on dry weight (DW) allocation to the cell wall residue. Bulbs of all three cultiva rs grown at the lowest S had significantly less DW in the cell walls compar ed to S at 3.0 or 4.0 meq(.)L(-1). In contrast to the effect of S supply on DW allocation, varying S supply had no effect on total bulb S, free SO4-2, and on the S content of the cell contents and the fell wall residue and on ly a minor effect on cell wall proteins. There was no significant effect of S supply on either SSC or soluble sugars. At low S nutrition, which is lim iting to the growth of onion bulbs, cell wall deposition is reduced, with a consequent decrease in bulb firmness. The S composition of the cellular co mponents is maintained at the expense of bulb growth.