Chilling Effect on Soluble Sugars, Respiration Rate, Total Phenolics, Peroxidase Activity and Dormancy of Onion Bulbs

Besides onions being one of the most cultivated and consumed vegetables, during storage onion bulbs are still affected by many physiological, biochemical and technological factors which can influence their quality. Respiration rate (RR O2 ), soluble sugars (SS), total phenolics (TP), and peroxidase (POD) activity were measured in inner bud tissues during a dormancy break of onion bulbs treated four weeks at 0oC and stored in the dark at 20oC. Control bulbs were stored simultaneously in the same condition. Breakage of dormancy was checked by the appearance of first green internal leaves by cutting longitudinally 30 bulbs. After eight weeks, RR O2 of sprouted onions was 52% higher than that of freshly harvested and dormant bulbs. One week after cooling SS decreased from 15 to 9 mg g fresh weight, and then peaked from 9 to 19 mg g after three weeks. For control bulbs, a similar peak was observed after six weeks. For inner buds of cold-treated onions, a slight increase of TP (from 0.17 to 0.2 mg g; fresh weight) was observed during the first two weeks of cooling, and then a decrease to 0.11 mg g was observed after eight weeks. For inner buds of control bulbs, TP also increased slightly from 0.17 to 0.2 mg g after five weeks, and decreased to 0.15 mg g after seven weeks when bulbs began to sprout. POD activity showed a similar pattern in relation to TP. For cold-treated bulbs, POD activity increased to 1.7 U g fresh weight after two weeks, and decreased to 1.1 U g during the last four weeks. For control samples, POD activity was stable during 4 weeks and decreased progressively by 29% during the last four weeks. This decrease in POD activity coincided with the decrease in TP, and coincided with onset of sprouting. With cold treatment, first sprouts were observed during the third week, while total sprouting was observed after eight weeks. In comparison, only 20% of the control bulbs sprouted after the period of 8 weeks.