Translucent and leathery
scales in bulb onions
Svein Øivind Solberg
The Norwegian Crop Research Institute,
Apelsvoll/Kise
N 2350 Nes, Norway
e-mail: svein.solberg@planteforsk.no
tel: +47 623 51700
Introduction
The present review is based on my thesis with
the same title at Agricultural University of
Norway (Solberg 1997). Part of the work is
published in two articles in Journal of
Vegetable Crop production (Solberg &
Dragland1999, Solberg & Bøe 1999). For
details please look at these documents.
What is translucent and leathery scales?
During the 1980’s a new post-harvest problem was reported by farmers in Norway. Onions with outer
leathery scales, and inner translucent scales, could be seen after harvesting. The symptoms developed
further during storage. The situation for many farmers was critical, and onions with poor quality were sent
to the marked. Research priority was given to the topic (Hoftun 1991). In Europe, the disorder is otherwise
only sparsely reported (Lancaster 1988, Weichmann 1990, Bjørn 1997), as well as in the USA (Ceponis et
al. 1986) and New Zealand (Watson & Hale 1984). ‘Watery scales’ is a term that has been applied to the
disorders, and the term includes both translucent and leathery scales.
The primary cause of translucent and leathery scales
Watson & Hale (1984) suggested that a disorder resembling translucent scales was induced by a pathogen,
most probably a bacteria. The hypothesis was enfeebled, first by the same persons, and again by Arild
Sletten at the Norwegian Crop Research Institute, Ås, Norway (unpublished).
Experiments at the Agricultural University of Norway during 1990-1991 showed a clear relationship
between choking treatments of onions, using either Vaseline, oil, or water, and translucent scales (Hoftun
1993). High internal CO2 concentration in combination with low internal O2 was regarded as the primary
cause of translucent scales. Several reports of an onion disorder similar to translucent scales were found in
experiments with controlled atmosphere storage. (Ogata & Inous 1957, Ogata 1961, Chawan & Pflug
1968, Adamicki & Kepka 1973, Smittle 1988). In those experiments onions were exposed to enriched CO2
levels in combination with low O2 levels. Internal break-down of onion tissue was observed after a certain
period of time. However, the disorders were not classified. Most likely, the disorder observed in these
experiments are the same as observed by Hoftun (1993), and which is called translucent scales in the
present publications.
The biochemical causes of high CO2 and low O2 disorders in plant tissues is widely studied, but not
fully understood (Striphanich & Kader 1985, Kader 1985, Lougheed 1987, Nanos et al. 1994). High CO2
levels are known to increase pH (Fife & Framton 1935) and lignification of plant tissue (Sadik & Minges
1966, Herner 1987), leading to increased membrane permeability (Ginka & Reinhold 1962) and higher
susceptibility to physiological stress (Eaks 1956, Saltveit 1985).
Nanos et al. (1994) proved that storing fruits in a very low O2 atmosphere for some days resulted in a
clear rise in activity of succinic acid dehydrogenase and some other mitochondria enzymes, indicating
disturbances in the citric acid cycle (Matthews & van Holde 1990). Accumulation of succinic acid is
frequently reported at high CO2 concentrations (Biale 1940. Frenkel & Petterson 1973, Shipway &
Bramlage 1973, Monning 1983, Kerbel et al. 1988, Nanos et al. 1994).
Ke et al. (1993) showed that the pyruvate decarboxylase and alcohol dehydrogenase gradually
increased at low O2 and high CO2 atmosphere, indicating accumulation of acetylaldehyde and ethanol
under such conditions.
Onion crop management and translucent and leathery scales
Already 30 years ago some research was done on translucent scales (Lipton & Harris 1965), describing a
relationship between translucent scales and conditions late in the growing season. The increasing problem
with translucent and leathery scales in Norway during the 1980’s took place at a time with rapid changes
in onion crop management. Harvesting machines and new cultivars were introduced in this period. The
harvesting machines made it possible to harvest large quantities of onions in a short period of time. This
left challenges to the following artificial drying process. The rise in the occurrence of disordered onions
during the 1980’s suggested a relationship between crop management and the occurrence of translucent
and leathery scales. On-farm experiments carried out by the local extension services in Toten and
Hedmark, as summarised by Dragland (1994), describe a relationship between late harvesting and
translucent and leathery scales development.
Hypothesis and objectives
By applying the conclusions of Hofun (1993), it was suggested that harmful levels of internal CO2 and O2
concentrations could accumulate under certain field and storage conditions. Harmful levels of the gases
could be reached by high respiration rates and/or low gas exchange rates. It is known that respiration rates
of fruits and vegetables increase with storage temperature and handling (Ryall & Lipton 1978,
Oberbarnscheidt et al. 1995), making these factors relevant. High drying temperature and long duration of
the high temperature treatments were suggested to induce translucent and leathery scales. Mechanical
handling of onions during harvesting and transport was also suggested to give translucent and leathery
scales .
The gas transport pathway in onion bulbs was regarded as important, and studies to quantify the
transport were initiated, based on the hypothesis that the main gas transport takes place through the onion
neck. Pressure from surrounding onions in large bins was suggested to reduce the gas exchange through
the neck and induce translucent and leathery scales.
The superior aim of the work was to develop a crop management strategy to minimise the losses
caused by translucent and leathery scales.
Research methods in brief
The study included a number of different research methods including survey, field experiments,
experiments in temperature chambers and laboratory experiments. Accordingly, different methods of
statistical analysis were applied, including standard deviation calculations, simple regression analysis,
multiple regression analysis, and various methods of variance analysis.
Qualitative descriptions of translucent and leathery scales were carried out, based on the examination of
a large number of bulbs. The visual appearance was related to known diseases and disorders (Ryall &
Lipton 1978, Snowdon 1991). The study was supplemented by measurements of the tissues’ electrical
resistance, the scales’ thickness, pH, and some enzymatic analysis. Both healthy and disordered tissue were
examined. The development of translucent and leathery scales during the storage period and the
susceptibility of different cultivars were studied in field experiments in south-eastern Norway (Paper I).
Oxygen permeability of onion scales and tissues was measured after fastening the samples to annulus
with petroleum jelly on a chamber system. An oxygen probe was fitted in an adapter with an open
chamber above the membrane. The oxygen meter was connected to a strip chart recorder detecting the
change in O2 concentration. The permeability of oxygen was related to the need of the gas in respiring
tissue. Manipulating treatments, including spraying water of the neck and stem base part, were included.
The influence of certain mechanical treatments on internal CO2 and O2 concentrations and the
development of translucent and leathery scales were examined in. Onions were dropped by various
methods, and the neck was clamped by various methods, simulating pressure established by
surrounding onions in storage rooms. Internal atmosphere was measured after one and four days, and
2
translucent and leathery scales evaluated after eight months storage. The internal concentrations of CO2
and O2 were measured by a method described by Saltveit & Strike (1989). A 0.5 ml gas sample was
carefully taken from the onion bulbs with a syringe. The samples were injected into a constant flow of
nitrogen gas leading to the O2-analyser and the CO2-detector. The gas concentration was proportional to
peak height on recorder trace calibrated to gases with known concentrations of CO2 and O2.
The influence of certain harvesting and drying methods on the development of translucent and
leathery scales was examined. Onions were harvested at various dates by a traditional harvesting and
field curing method and by a cutting and direct harvesting method, and dried at certain temperatures and
humidities. Translucent scales, leathery scales, leaf sprouting and other post-harvest problems were
examined after 8 months storage. Internal CO2 and O2 concentrations were measured in autumn.
The relationship between crop management data and watery scales (not specified) was examined in a
survey among farmers in an onion producing district in south-eastern Norway during 1985-1995. The
farmers were asked for details of their crop management practice and storage results. The occurrence of
watery scales were related to crop rotation, fertilisation, irrigation, harvesting and drying methods,
storage conditions, and weather data. The extension group officers in the same district had registrations
of the frequency of watery scales during the last 15 years, and the information was related to weather
data from the same area.
Main results and critical remarks
Descriptions of translucent and leathery scales
The disorder translucent scales is characterised by water-soaked, but firm fleshy scales with a tart odour.
The disorder leathery scales is characterised by thick, dark scales between the dry and fleshy scales.
Translucent and leathery scales are sparsely described in the literature and conflicting terms have been
applied to the disorders. The term watery scales (Wasserhaut in German) covers both translucent and
leathery scales, and the term was applied by Hoftun (1993), Lancaster (1988) and Weichmann (1990).
Translucent scales is described by Lipton & Harris (1965), Smith et al. (1966), and Ceponis et al. (1986).
The term leathery scales was first used in Norway (Hoftun 1993, Dragland 1994). Both translucent and
leathery scales were found in the field during the last part of the growing season. Farmers observed the
symptoms of watery scales in the field prior to harvesting. Initial symptoms of translucent scales were first
observed in the middle (54% of the cases) or upper part (39% in the cases) of the outer fleshy scales. The
symptoms were more severe after long-term storage, and discoloration and odours could be observed. A rise
in pH from 5.9 in normal fleshy scales to 6.4 in weak translucent scales was detected. Severe translucent
and leathery scales had very low pH. High CO2 concentrations is known to rise pH of plant tissue (e.g.
Lipton et al. 1967), and the rise in pH in the initial symptoms can be an indication of CO2 damage. As the
translucent and leathery scales got more severe, leakage from the vacuoles into the intercellular spaces
could result in more anaerobic conditions, leading to the accumulation of ethanol and organic acids.
Accumulation of ethanol was detected in typical translucent scales, while no accumulation of organic acids
was found. Further analysis is needed to clarify the biochemical changes taking place in disordered tissue.
Gas transport pathway
Oxygen permeability through the dry scales and epidermis of the fleshy scales was low (11.0 x 10-6 and
6.8 x 10-6 cm s-1 atm-1, respectively). The permeability of oxygen in the dry scales was lower than the
calculated values needed to supply a single 5mm thick onion scale with O2 concentrations at moderate
respiration rates. The permeability in the epidermis of the fleshy scales was even lower than in the whole
dry scales. The radial gas transport from scale to scale in the bulb was regarded as negligible compared to
the transport through the neck and the stem base part of the bulbs. At harvesting, the neck was a more
important transport channel of O2 (10.8 x 10-4 cm3 s-1 atm-1) than the stem base region (1.9 x 10-4 cm3 s-1
atm--1). After the onion leaves were completely dry, the same amount of O2 passed through the neck and
the stem base of the bulbs. Manipulating treatments, applying pressure on the neck, reduced the gas
transport through the neck considerably. Pressure on the neck can cause translucent scales development,
especially if the pressure is established at a time when the gas transport through the neck is the only
significant channel for the bulbs. Accordingly, onions harvested under wet conditions late in the autumn
frequently develop translucent scales (described later).
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Weather conditions and translucent and leathery scales
The results from the survey among farmers in the same area indicate that watery scales related to the
humidity of the drying air (R2=0.40) and to high precipitation the last three days of the field curing period
(R2=0.24). Experiments showed higher numbers of bulbs with both translucent and leathery scales at late
harvesting. At late harvesting, the drying conditions for the onions are poor. Shorter days, lower
temperatures and dew make the drying conditions far from. The occurrence of watery scales varied very
much from farm to farm in the same district. Even in years with high average losses, some farmers had no
watery scales. Obviously, other factors than weather conditions were decisive. Weather conditions should
be regarded as important, but under unfavourable climatic conditions, better harvesting and drying
methods can be the key to reducing the problem of translucent and leathery scales.
Crop management and translucent and leathery scales
Translucent and leathery scales was observed both on small and large onion bulbs. A selection of 13
modern cultivars of the yellow Rijnsburger type showed little variation in the susceptibility to translucent
and leathery scales. Little variation between cultivars is reported in the official testing program in Norway
(Dragland & Berentsen 1989) and Denmark (Bjørn 1997). The modern cultivars all have quite similar skin
quality properties. Small bulbs, internal splitting and skinning make the old cultivars less attractive, but
perhaps the old cultivars were less susceptible for watery scales. Breeding programs for new cultivars,
combining the positive properties of the modern cultivars with less susceptibility to the disorders, could be
a future research task to reduce the problems with translucent and leathery scales.
It has not been possible to link watery scales to crop rotation, the origin of the onion sets, irrigation, and
pesticide use (results from survey). A weak relationship between high nitrogen application and watery
scales (R2=0.23) could be seen in the survey, but the result was questioned due to co-variation between
nitrogen and other variables. The influence of N on translucent and leathery scales has been enfeebled in
field experiments (Bøe 1997), and other factors than fertilisation are regarded as important for the
induction of the disorders.
A relationship between handling treatments and translucent scales was described in the experiments.
Dropping onions four times from one meter height onto a wooden floor increased the occurrence of
translucent scales compared to no dropping. Pressure around the neck by a nylon string also increased the
occurrence of the disorder. Combined drop and pressure treatments gave the highest numbers of onions
with translucent scales. The treatments simulated handling by harvesting machinery and pressure from
other onions during drying and storage in large bins. The intensity of onion handling in harvesting
machinery was verified by on-farm measurements, showing no great impact of handling on internal
atmosphere and translucent scales development. Visual symptoms of compression on the onion neck was
observed on approximately 5% of the onions sampled on 1-1.5 m depth in a bin storage during artificial
drying. Immediately after sampling, onions with symptoms of compression had 2% higher CO2
concentration than bulbs without such symptoms. The drop treatment was presumably too hard compared
to normal handling of onions on the farms. nevertheless, the results pinpoint the importance of gentle
handling to ensure optimal post-harvest quality.
Harvesting and drying methods clearly affected the number of onions with translucent and leathery
scales. Translucent and leathery scales were both influenced by harvesting date. The average of five trials
in south-eastern Norway showed that harvesting at 5-10% leaf fall-over gave 9% onions with translucent
scales, while harvesting two and four weeks later resulted in 12 and 21% onions with translucent scales,
respectively. The numbers of onions with leathery scales were 5, 10 and 17%, respectively. A clear
relationship between high drying temperature and a high occurrence of translucent scales was found. The
average of three trials showed that drying at 14, 20, 25, and 30C gave 4, 6 15 and 17% onions with
translucent scales, respectively. Another five trials, comparing drying at 20 and 30, showed average
values of 10 and 17% translucent scales, respectively. Long duration of the high temperature drying gave
the highest number of bulbs with translucent scales. High humidity of the drying air was related to a high
occurrence of translucent scales. No relationships between leathery scales and the drying temperature,
humidity of the drying air, or the duration of the high temperature drying were observed, but long duration
of the field curing period (more than two weeks) gave more onions with leathery scales than a short field
curing period.
Crop management and internal atmosphere
The most critical periods, showing the highest internal CO2 concentrations were observed at the time of
4
field curing and at the end of the artificial drying treatment. Onions standing in the field with no leaves
fall-over had lower internal CO2 and higher internal O2 than mature onions with 100% leaf fall-over.
Late harvesting resulted in higher average internal CO2 concentration than harvesting at 50-80% leaf
fall-over or earlier. Artificial drying at 30°C increased the CO2 levels compared to drying at 14°C, and
the relationship between internal CO2 concentration and temperature can be expressed by a linear model
CO2 (in %) = 0.52 + 0.480Xtemp, (R2=62%), were Xtemp is the temperature in C. The reduction in O2
concentrations followed the increase in CO2, and could be expressed by the regression: CO2 (in %) =
18.7  0.34Xtemp (R2=56%).
On average, a rise in the internal CO2 concentration of around 4% (measured at 25-30C) was detected
one day after dropping the onions four times from one meter onto a wooden floor. Pressure around the
onion neck by a nylon string raised the internal CO2 concentration by the same amount, while combined
drop and pressure treatments raised the internal CO2 concentration by around 8% the first day after the
treatments. An average internal CO2 concentration of 22% was found after one day at 30C after the
combined treatments. Four days after the treatments, the differences in CO2 concentration between the
onions exposed to handling treatments and the control was reduced, but the differences were still
significant. The reduction in internal O2 concentrations was less than the rise in CO2, but the results could
still reflect aerobic respiration due to higher diffusion of CO2 than of O2 (Chapman & Cowling 1970).
A close relationship was observed between the factors leading to high numbers of translucent scales
and the factors leading to high internal CO2 or low O2 concentrations.
Concluding remarks and further tasks
The study emphasise the importance of harvesting and drying methods in a strategy to minimise the
problem of translucent and leathery scales. A strategy should include:
* The onions should not be harvested too late. The optimal harvesting date is
usually at 50-80% leaf fall-over.
* Rough handling of the onions should be avoided.
* The field curing should not exceed two weeks.
* Wet harvesting conditions should be avoided. The onions should be artificially
dried in boxes instead of in large bins after such conditions to avoid pressure
on the onion neck.
* The artificial drying temperature should not exceed 20-25C.
* The artificial drying periods should not continue after the neck is completely
dry.
The induction of translucent scales take place during harvesting and drying of the onions, while leathery
scales is most likely induced in the field prior to harvesting. The initial cause of translucent scales is well
documented, while the mechanism inducing leathery scales is not fully understood. Further research is
needed to clarify the biochemical changes taking place in disordered tissue and to clarify the potential of
breeding to meet the problems of translucent and leathery scales.
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