Importance of dormancy and sink strength in sprouting of onions (Allium cepa) during storage

Abstract

In onion (Allium cepa L.) postponement of sprouting is necessary to achieve long term storage. We studied the factors determining sprouting during dry storage at 16°C. The period to visible sprouting depends on the length of the dormancy period, if present, and on the growth rate of the sprout. In the three cultivars tested, sprouts were initiated within 2 weeks after harvest indicating the absence of a real dormancy period. Sprout length increased linearly during storage. The mitotic activity of the apex decreased before harvest, was low at the transition from scale to leaf formation, and increased again when the sprout was initiated. From a few weeks before harvest, the initially high fructan content of the scales decreased, leading to a large increase in fructose. The sprout always contained enough carbohydrates for growth (between 50 and 60 mg g−1 dry weight, of which 30% was fructan). The activity of sucrose synthase (EC 2.4.1.13) increased as the sprout grew, indicating an increase in sink strength. Invertase (EC 3.2.1.26) was absent in all bulb organs, during the various developmental stages. Although carbohydrates and enzymes were available for fast sprouting, sprout growth was still linear instead of exponential during dry storage at temperatures favorable for growth (16°C). The relative importance of factors determining sprouting are discussed.

Phytochemical composition of onion during long-term storage

“…In the dormant phase, supposedly started Acta Agriculturae Scandinavica, Section B -Soil & Plant Science 3 near harvest (Miedema 1994;Yasin & Bufler 2007), many physiological processes are slowed down, although not ceased (Pak et al 1995;Yasin & Bufler 2007;Brewster 2008). After the dormancy breaking, onion bulbs enter the regrowth phase that is accompanied by gradually increasing metabolic activity.…”

“…A similar conclusion was drawn by Kaack et al (2004), who observed a variation in DM from 9.2 to 14.4 g 100 −1 g as a function of cultivar, whereas a six month storage at 5°C had no measurable effect on this quantity. On the contrary, Pak et al (1995) have observed an approximately 9% decrease in DM of onion bulbs cv. Hysam.…”

“…From a marketing point of view, the later can be defined as the capability of the bulb to retain firmness and integrity of the outer layer and to persist to sprouting and rooting. Onset of sprouting is commonly preferred to characterize the storage life of onions because of its clear relationship to physiological processes in bulbs (Pak et al 1995;Yasin & Bufler 2007;Chope et al 2007a;Bufler 2009). In the present work, the development of inner sprouts was observed on longitudinally cut bulbs at the 10th week of storage, both with those stored in the dark room and in the glasshouse under sunlight.…”

“…After the initiation of sprouting, fructan in the scales is hydrolyzed and the products of hydrolysis are transported to the bulb base to be used for growth processes (Pak et al 1995). Therefore, a redistribution of sugars over the bulb during storage is expected and was documented by some authors (Rutherford & Whittle 1982;Pak et al 1995). On the contrary, Yasin & Bufler (2007) did not notice any qualitative change in the saccharide profile even after the onset of sprouting eight weeks after harvest.…”

“…Glucose usually demonstrated the same pattern as fructose except the work of Rutherford & Whittle (1982), where it fluctuated within 10% of the average level. After the initiation of sprouting, fructan in the scales is hydrolyzed and the products of hydrolysis are transported to the bulb base to be used for growth processes (Pak et al 1995). Therefore, a redistribution of sugars over the bulb during storage is expected and was documented by some authors (Rutherford & Whittle 1982;Pak et al 1995).…”