Translucent and leathery scales in bulb onions
Svein Øivind Solberg
The Norwegian Crop Research Institute, Apelsvoll/Kise
N 2350 Nes, Norway
e-mail:
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enfeebled, first by the same persons, and again by Arild Sletten at the Norwegian Crop Research Institute, Ås, Norway (unpublished).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 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).
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.