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Physiological and biochemical bases of sprout inhibition in stored onion Allium cepa L. cv. Red Creole bulbs by pre- or postharvest application of ethylene
Dissertation Abstract:
The physicochemical and physiological bases of sprout inhibition with pre- and postharvest application of ethylene were investigated in ‘Red Creole’ onion bulbs during storage at 6–8ºC and under ambient conditions (25–30ºC).
Ethylene as ethephon solution (3,000 ppm) was applied to the onion plants two weeks before harvest. After harvest, the bulbs were again treated with various concentrations of ethylene at different exposure periods as follows: control (0 ppm ethylene), 10 ppm for 36 h, 10 ppm for 72 h, 20 ppm for 36 h, 20 ppm for 72 h, 30 ppm for 36 h, and 30 ppm for 72 h. The treated bulbs were stored under ambient condition and at low temperature for six months. Bulb samples were then withdrawn every month for evaluation.
After six months of ambient storage, the shortest sprout lengths, 0.30 cm in preharvest ethylene treated onion bulbs and 0.52 cm in non-pretreated bulbs, were observed in postharvest ethylene treatment with 30 ppm for 72 h exposure. The control bulbs (no postharvest ethylene treatment) produced the longest sprouts, 4.40 and 5.71 cm, in preharvest non-ethylene treated and ethylene treated bulbs, respectively. Under cold storage condition, when the bulbs were treated with preharvest ethylene, the shortest (5.10 cm) sprout length was recorded in bulbs with 10 or 20 ppm + 72 h treatment and the longest (19.01 cm) sprout length was shown in bulbs treated with 30 ppm ethylene for 72 h. In preharvest non-ethylene treated bulbs under cold storage, the sprout lengths were significantly shorter in 10 ppm + 72 h and 20 ppm + 36 or 72 h treatments than in the others.
Percent sprouting was also measured in terms of bulbs with green leafy structure appearing from the neck of the bulbs. Onion bulbs showed visible sprouting as early as three months in all postharvest ethylene treatments under cold storage except in 10 or 20 ppm + 72 h treatment in preharvest ethylene treated bulbs and in 10 ppm + 72 h and 20 ppm+ 36 or 72 h treatments in the bulbs which were not treated with preharvest ethylene. Under ambient storage of preharvest ethylene treated bulbs, no sprouting was observed in all postharvest ethylene treatment until 4 MAS while sprouting started in control and 10 ppm + 36 h treatments in preharvest nonethylene treated onion bulbs. It was also observed that ethylene concentration used as postharvest treatments was highly and negatively correlated with percent sprouting in both preharvest non-ethylene and ethylene treated onion bulbs only under ambient storage condition. At four months after storage, percent sprouting was significantly higher (12.24% in pretreated bulbs and 6.7% in non-pretreated bulbs) in cold storage than that in ambient storage.
In this study, preharvest application of 3,000 ppm ethephon coupled with 30 ppm + 72 h postharvest ethylene treatment resulted in the shortest sprout length and 0 percent visible sprouting in bulbs stored for six months at ambient. The same preharvest treatment coupled with 10 or 20 ppm ethylene + 72h exposure resulted in 0 percent sprouting in bulbs stored for three months in cold storage.
Sprout inhibition exhibited by bulbs exposed to pre- and postharvest ethylene treatment (30 ppm + 72 h stored at ambient; 10 or 20 ppm + 72 h stored in the cold) was accompanied by higher levels of total, reducing, and non-reducing sugars. With these increased levels was a concomitant reduction of respiration rate. Sprout inhibition in these treatments was also accompanied by reduced rate of ethylene production and lower pungency levels apparently due to reduced activities of 1-aminocyclopropane-1-carboxylic acid synthase and alliinase, respectively.