The nature of onion dormancy and changes over time

Within the onion bulb, a succession of internal changes take place, preparing it for regrowth (see Section 6.11). In agreement with Brewster (1987), Miedema (1994a, b) and Miedema and Kamminga (1994) showed with Japanese, Dutch and US culti-vars that dormancy exists in bulbs soon after maturation, followed by rest, during which slow internal preparation for rooting and sprouting takes place, unless temperatures are very low (near 0°C) or above 25°C.

Onion dormancy can be rapidly broken under favourable conditions for regrowth (e.g. Abdalla and Mann, 1963; Pak et al., 1995); for example, resumption of root growth is promoted when onions get wet in the field during curing.

Sprout meristems were mitotically active from lifting throughout storage at 4 or 10°C for up to 25 weeks, with greatest activity 5 weeks after harvest (Matejko and Dahlhelm, 1991). Earlier, Abdalla and Mann (1963), in the USA, found that, in cv. 'Excel', the average number of mitoses per apex detected in the weeks before harvesting was 10-13. The number declined to <1-4 by 3 weeks later, with more divisions continuing at 15 than at 0 and 30°C, where division practically stopped; but Abdalla and Mann (1963) found that at no time was the shoot apex morphologically inactive. In northern European cultivars in The Netherlands, mitotic activity of the apex decreased before harvest, was low for the 3 weeks after harvest and increased after that at 4-8 weeks as sprouts were initiated, when onions were stored at 16°C. The pause in mitosis was comparatively short and was not regarded as a true dormant period but rather as a transition between storage-scale and foliar-leaf formation in the bulb (Pak et al., 1995; Fig. 10.3). Mitotic activity was connected with leaf initiation and elongation in the inner bud of bulbs; the extent of sprout growth was dependent on temperature (Abdalla and Mann, 1963; Matejko and Dahlhelm, 1991). Although carbohydrates and enzymes were available for fast sprouting, sprout growth remained linear rather than exponential during dry storage at 16°C, and was considered to be limited by lack of external water (Pak et al., 1995).

Starch has been found in A. cepa in the primary thickening meristem (PTM) during sprouting, but not during dormancy; absence of starch may therefore be useful as a marker for dormancy (Ernst and Bufler, 1994). Ernst et al. (1999) studied four culti-vars stored at 0, 15 and 30°C. Low starch in the PTM indicated primarily root dormancy, and only indirectly sprout dormancy. Starch in the PTM increased before sprouting at the low and intermediate temperatures but

Fig. 10.3. Mitotic activity in the meristems of onion cultivars 'Hysam' (circles), 'Hystar' (squares) and 'Centurion' (triangles). Percentage of dividing cells (mitotic index) starting 3 weeks before harvest (harvested at time 0) and during storage at 16°C. Mean values of five apices. (From Pak et al., 1995, with permission.)

Fig. 10.3. Mitotic activity in the meristems of onion cultivars 'Hysam' (circles), 'Hystar' (squares) and 'Centurion' (triangles). Percentage of dividing cells (mitotic index) starting 3 weeks before harvest (harvested at time 0) and during storage at 16°C. Mean values of five apices. (From Pak et al., 1995, with permission.)

was not detectable at the highest temperature. This provides an interesting clue to the mechanism of high-temperature dormancy. Miedema (1994b) considered that lack of cytokinin, due to root dormancy, was its immediate cause.

7.2 Cultivars

Onion cultivars can be characterized by the toughness of the dry scales, the colour, thickness and number of which are mainly genetically determined. Skin quality is an important factor in determining storability and has a significant role in maintaining dormancy (Fustos, 1997).

In Poland, Adamicki (1998) considers that late-maturing cultivars generally store better than early-maturing cultivars. In Zimbabwe, midseason-maturing SD culti-vars stored better than most early- and all late-maturing ones (R.L. Msika, unpublished data). In general, 'Grano'/ 'Granex' have thinner and fewer skins than traditional LD storage cultivars and, normally, a shorter storage life. In the Republic of Macedonia, bulbs for seed production were stored in non-controlled conditions for 5

months; cv. 'Texas Grano' had the greatest losses in terms of number of affected bulbs (91%) and bulb mass (93.9%), with cv. 'Moldavski' having the fewest losses (59% sprouted, 60.1% mass reduction) (Agic et al., 1997). In Iran, the local cv. 'Dorcheh' stores longer at both low and high temperature than cv. 'Texas Early Grano' (Ramin, 1999). In Holland, the range in time to 50% rooting at 10°C in ten cultivars was from 8 to 63 days, and to 50% sprouting, 40 to 156 days, with considerable bulb-to-bulb variation (Miedema, 1994a). In the tropical countries, storage lives of different types of short-day onions vary considerably (Peters et al., 1994). They averaged 1-2 months for 'Grano/ Granex', 4-5 months for 'Red Creoles' and up to 10 months for local culti-vars in Egypt (means calculated from a survey: Currah and Proctor, 1990).

7.3 Temperature and dormancy breaking

Temperature plays a critical role in the spoilage of onions in stores (Abdalla and Mann, 1963; Komochi, 1990; Mondal and Pramanik, 1992; Tanaka et al., 1996). While both low and high temperatures maintain onion dormancy, intermediate temperatures between about 5 and 20°C are effective in breaking dormancy, with some variation due to cultivars; in many studies, 15°C has been found the optimum temperature for promoting sprouting. At room temperature in Georgia, USA, the quantity of marketable bulbs of 'Granex'-type onions decreased by 12-25% month-1, due to water loss and black mould damage (Smittle, 1988).

Many wild alliums show high-temperature dormancy in hot seasons, and the reaction of bulb onions is probably related to this behaviour. In the tropics, in the absence of refrigerated stores, the storage of onions at 25°C within the range of 50-70% RH produces the least spoilage (Mondal and Pramanik, 1992). Miedema and Kamminga (1994) found that low cytokinin concentrations occurred under high temperature (30°C) conditions (Tables 10.3, 10.4). After 6 or 12 weeks of storage at 30°C, rooting and subsequent sprouting of cv. 'Augusta' (Rijnsburger type) were more rapid than after storage at 5 or 15°C; however, at the latter temperatures, cytokinin activity was six and almost nine times greater, respectively, after 18 weeks than in 30°C storage. Increased levels of cytokinins, probably generated during root initiation, were associated with onion sprouting (Miedema and Kamminga, 1994).

Sprouting in onion bulbs was thought to be inhibited by ABA (Yamazaki et al., 1995, 1999a) and promoted by cytokinins (Kuraishi et al., 1989), with dormant culti-vars having increased sensitivity to ABA compared with non-dormant cultivars (Yamazaki et al., 1999b). The importance of cis- rather than trans-ABA in the breaking of bulblet dormancy has been suggested (Kuraishi et al., 1989).

Roots start to grow within the base plate and do not emerge until sufficient outside moisture is available to support them. Tanaka et al. (1985) described and distinguished 'external' and 'internal' (new) roots within the basal plate and showed that external moisture was the cue to start the internal roots growing at temperatures of 5°C up to 15°C, but that temperatures of 2 or 30°C strongly suppressed their development. Miedema (1994b) found that substituting benzyl adenine (BA) for roots was effective in stimulating sprouting if the roots themselves were trimmed off.

The time lapse between the appearance of visible roots and visible sprouts varies between cultivars. For example, in the Japanese cv. 'Radar', rooting was followed about a month later by visible sprouting, whereas in cv. 'Hyduro' (Rijnsburger storage type) there was a lapse of about 3 months before sprouts became visible, after rooting had started (Miedema, 1994a; Fig. 10.4)

In temperate countries, storage at low temperatures near or even below 0°C is commonly used to keep both onions and pathogens inactive. Ambient air can be used to keep onions dormant during the winter but refrigeration must be used in the spring to further delay sprouting. Onions with relatively high DM content can tolerate temperatures just below 0°C, but those with low DM may be damaged by freezing.

In Algeria, 9°C treatment of cv. 'Rouge d'Amposta' promoted sprouting faster than

Table 10.3. The effects of storage temperature and duration on dormancy characteristics in bulb samples of onion cv. 'Augusta'. Time to rooting and sprouting were estimated on bulbs planted in moist vermiculite at 15°C; three replicates of 20 bulbs were used per temperature and sampling date. Values followed by the same letter are not significantly different at P < 0.05.

Table 10.3. The effects of storage temperature and duration on dormancy characteristics in bulb samples of onion cv. 'Augusta'. Time to rooting and sprouting were estimated on bulbs planted in moist vermiculite at 15°C; three replicates of 20 bulbs were used per temperature and sampling date. Values followed by the same letter are not significantly different at P < 0.05.

Duration of storage (weeks)

Days to 50% rooting

Days to 50% sprouting

15°

30°

15°

30°

0

21.7a

21.7a

21.7a

52.3a

52.3a

52.3a

6

11.3bc

14.0b

7.8c

40.7b

46.2ab

29.1c

12

5.0d

4.9d

3.5de

37.6b

23.9cde

22.1de

18

2.7e

2.5e

3.0e

27.3cd

16.5f

19.6ef

From Miedema and Kamminga (1994), with permission.

From Miedema and Kamminga (1994), with permission.

Table 10.4. The effects of storage temperature and duration on cytokinin activity in bulb samples of onion cv. 'Augusta', estimated with the Amaranthus bioassay. Values are means ± standard error of three bioassays.

Duration of storage (weeks)

Cytokinin activity (nmol zeatin eq. g-

1 fresh weight)

5°C

15°C

30°C

0

0.10 ± 0.00

0.10 ± 0.00

0.10 ± 0.00

6

0.17 ± 0.03

0.23 ± 0.03

0.27 ± 0.03

12

1.80 ± 0.12

3.93 ± 0.15

0.33 ± 0.03

18

2.90 ± 0.10

4.23 ± 0.19

0.50 ± 0.06

From Miedema and Kamminga (1994), with permission.

From Miedema and Kamminga (1994), with permission.

storage at 0°C. When the concentrations of phenolics and peroxidase activity were relatively high, inner bud development was inhibited; sprouting was accompanied by high concentrations of oligosaccharides and glucose (Benkeblia and Selselet-Attou, 1999a). In a further study of cv. 'Rouge d'Amposta' onions during storage at 4 and 20°C, an inverse relationship between phenolic content and the amount of sprouting development of bulbs was observed. Low temperature had a stimulatory effect on phenylalanine ammonia-lyase (which is involved in phenolic metabolism) and peroxidase activity, both of which are highly involved in onion-bulb sprouting (Benkeblia, 2000a).

Cold treatment of A. victoralis L. ssp. platyphyllum Hult. (a wild species used as a food plant in East Asia) at 0°C was more

Time (days)

Fig. 10.4. Time course of rooting (solid symbols) and subsequent sprouting (open symbols) of onion cvs 'Radar' (circles) and 'Hyton' (triangles) at 10°C in moist vermiculite (from Miedema, 1994a, with permission).

Time (days)

effective in breaking dormancy than 5°C (Kanazawa et al., 1997).

The expression of histone 2A has been inversely correlated with dormancy in cv. 'Robusta' (Carter et al., 1999). High levels were found in basal tissues and in the inner, meristematically active parts of bulbs, and expression levels increased throughout storage as onions began to emerge from dormancy. A comparison of early- and late-sprouting onion UK breeding lines showed that histone 2A levels peaked at around the same time of year, irrespective of sprouting time, suggesting that differences in storage longevity are not related to different times of dormancy breakage. Factors controlling the rate of sprout emergence post-dormancy (primarily temperature) are likely to be major determinants of storage capability (Carter et al., 1999).

7.4 Relative humidity

Control of humidity during storage is important for three main reasons. One is concerned with discouraging disease development. Pathogens can attack onion skins when the moisture content rises above a percentage that can be reached when the skin is in equilibrium with air at RH > 80% (see Section 10). The second reason is the prevention of rooting, also encouraged by high air humidity or free water in store (the start of rooting also involves shape changes at the base of the bulb, which can lead to skin cracking). The third, related reason is the need to retain sufficient skins on onion

bulbs out of store. The moisture content of the skin is mainly controlled by the RH of the surrounding air, in equilibrium with moisture from the interior of the bulb. When dry skins are lost, a new equilibrium is reached after higher initial water loss and, for this, manipulation of the air RH may be needed throughout storage (see Section 6.9).

Ideally, the air RH in the store should be between 65 and 70% (Mondal and Pramanik, 1992), though in practice wider limits than these are used. In Brazilian experiments, rates of fresh-weight loss were higher when bulbs were stored at < 55% RH, because very dry onion-skins crack easily, so exposing wetter interior skins until a new equilibrium is reached. The optimum storage conditions in a 30-day trial were 20°C (from a range from 20 to 35°C) with RH between 55 and 70%. The water content of the skins increased dramatically when the air RH rose above 75% and, at high skin moisture content, both skin permeability and rates of fresh-weight loss increased (de Matos et al., 1997).

High RH inside stores encourages root development and therefore may tend to break dormancy in onions that would keep well if dry. Methods of storage that keep onion basal plates dry, e.g. hanging in strings, avoid this difficulty.

7.5 Internal atmosphere

Since the meristem of an onion bulb is surrounded by many layers of bulb scales, it may be subjected to an environment of high CO2 concentrations. After 3 months of storage at 20°C, Ladeinde and Hicks (1988) found that the internal atmosphere in bulbs was 3.1% CO2 and 16.2% O2. In Georgia, USA, shoot growth was inhibited by keeping sweet-onion bulbs under low O2 and high CO2 concentrations in CA similar to that used in apple storage (Smittle, 1988; see also Section 8.4).

In Texas, the effects of internal CO2 atmospheres on shoot growth and respiration rates in cv. 'TG 1015Y' stored at 1, 7, 13, 20, 27 or 34°C for 12 weeks were measured (Yoo et al., 1997). Maximum shoot growth occurred at 13 and 20°C, coinciding with maximum respiration rates during the first 8 weeks of storage. Internal CO2 concentration ranged from 2 to 5%, with the centre scale tissues at 11-17% CO2, a figure that increased with higher storage temperatures, while the internal gas volume decreased (Yoo et al., 1997; Fig. 10.5). Sealing the neck area at 1 or 27°C increased the CO2 concentrations, but had no effect on sprouting, indicating that elevated internal

Storage temperature (°C)

"T 13

Storage temperature (°C)

Fig. 10.5. Changes in internal CO2 concentration (A) and internal gas volume (B) in onion bulbs stored at different temperatures for 4 (circles), 8 (squares) and 12 (triangles) weeks. Vertical bars indicate estimates of the standard deviation of the population (n = 10). Data point for 34°C at 12 weeks is missing due to decay of the bulbs. (From Yoo et al., 1997, with permission.)

CO2 concentrations at higher temperatures were not the sole cause of inhibited shoot growth at high storage temperatures. Respiration was unaffected by concentrations of 10-30% CO2, although 30% CO2 accelerated ethylene evolution, perhaps due to injury (Pal and Buescher, 1993).

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