NOTE
Effect of nitrous oxyde (N
2O) shocks on sprouting and rotting of onion bulbs (Allium cepa L.)
Noureddine BENKEBLIA 1 2 *, Patrick VAROQUAUX 2, Barbara GOUBLE 2
RÉSUMÉ Effet du choc au protoxyde d’azote sur la germination et les pourritures de l’oignon (Allium cepa L.).
L’effet du protoxyde d’azote appliqué à des concentrations de 25 %, 50 %, 80 % et 100 % durant 2 jours et de 100 % durant 4 jours, sur la germination et les pourritures des bulbes d’oignon conservés ensuite à 18 °C à l’air a été étu- dié. Le protoxyde d’azote n’a pas d’effet significatif sur l’inhibition de la germi- nation des bulbes. Cependant, un effet certain du N2O a été observé sur les pourritures des bulbes, notamment à la concentration de 100 % appliquée durant quatre jours. L’effet observé était très significatif entre bulbes témoins et bulbes traités pour toutes les concentrations et les durées d’applications.
Mots clés : protoxyde d’azote, germination, pourritures, oignon.
SUMMARY
The effect of nitrous oxide at different concentrations (25, 50, 80 and 100% for 2 days, and 100% for 4 days) on sprouting and rotting of onion bulbs stored at 18°C, was investigated. On the first hand, nitrous oxide did not have any signi- ficant effect on sprouting of the bulbs stored at 18°C under air after a N2O shock. On the other hand, N2O reduced the level of rotting, specially for bulbs pre-treated with 100% N2O during 4 days, and significant difference were observed between treated bulbs and controls.
Key words: nitrous oxide, sprouting, rotting onion.
1. Department of Biology, Faculty of Sciences and Technology, Mostaganem University, P.O Box 227, 27000 Mostaganem, Algeria
2. Institut national de la recherche agronomique, Station de technologie des produits végétaux, Domaine Saint Paul, Agroparc, 84914 Avignon cedex 9, France
* Correspondance benkablia@usa.net
1 - INTRODUCTION
Onion bulb is one of the world’s oldest cultivated vegetable and is widely used for culinary purpose. Usually, onion bulbs are held for long periods bet- ween harvesting and marketing so as to fulfil the market demand. In this res- pect, several investigations have been carried out throughout the world on the development of new techniques to extend the shelf-life and quality attributes of the bulbs. The objective of these techniques is to control sprouting which is the main problem encountered during the storage of onions.
Recently there has been a great interest in the potential benefits of using some noble gases for storage of vegetables (SPENCER, 1995). These gases are chemically inert, and recent investigations have indicated that they are bioche- mically active probably due to their enhanced solubility in water (SPENCER, 1995).
Nitrous oxide (N2O) is an anaesthetic gas and was classified as a miscella- neous additive and is permitted for food use. Recent research showed that nitrous oxide caused a reversible partial inhibition of oxygen consumption in mitochondria particles and respiration (SOWA et al., 1987; SOWA and TOWILL, 1991). SOWAand ROOS(1991) reported also that N2O extended shelf-life of litchi and logan seeds, and GOUBLEet al. (1995) reported anti-ethylene effects of nitrous oxide in the ripening and senescence sequences in tomato and avocado fruits. Likewise, THOM and MARQUIS (1984) reported that N2O was a potent growth inhibitor, at low pressure, of Escherichia coli, Saccharomyces cerevisiae and Tetrahymena thermophila. However, numerous biochemical and physiologi- cal aspects of effects of nitrous oxide should be clarified before application of this gas starts out in storage technology.
In this investigation, we report the effect of the application of nitrous oxide at different concentrations on sprouting and rotting of onion bulbs.
2 - MATERIAL AND METHODS
2.1 Onions
Onion bulbs Allium cepa (organic product, free of any preharvest chemical treatments) cv. Rouge Amposta were obtained from a local market (Avignon), sorted for uniformity and absence of defects, packed in commercial plastic (PVC) trays of 12 kg and placed at 18°C prior to treatments.
2.2 N2O treatments
Onion bulbs (5 ±0.05 kg) were placed in 10 L vessels, the lid of which was tightly secured. These onions had been kept at 18°C for 12 h before use. The vessels were flushed with nitrous oxide during different times in order to have desired final concentrations of N2O. The gas composition within the vessels
was checked with a gas chromatograph (MTI) as described below. Onions were treated with 25, 50, 80 and 100% N2O during 2 days and with 100% during 4 days.
2.4 Onion storage
After N2O treatment, vessels were opened, and onion bulbs were stored at 18°C and 55-60% RH.
2.5 N2O GC analysis
The N2O partial pressure was measured using a MTI gas chromatograph analyser (model M200, Fremont, USA). The conditions of N2O analysis were slightly modified comparatively to the conditions used to assess the O2 and CO2partial pressures. The analyser involved two manifolds: one fitted with an MS-5A, 4-meter capillary column set at 80°C with argon as carrier gas at a pressure of 110 kPa, and the other fitted with a capillary Poraplot 4, 6-meter column set at 30°C with helium as carrier gas at a pressure of 193 kPa. Both manifolds were coupled with katharometric (or hot wire) detectors. In such conditions argon does not interfere with O2peaks, and CO2and N2O peaks are separated provided that N2O partial pressure does not exceeds 10 kPa.
Calibration was performed as follows. A 100 mL gas jar is flushed with a 10 kPa CO2, 10 kPa O2and 80 kPa N2 gas mixture. Then 5 mL of N2O are injec- ted with an air tight syringe through a silicone septum set in the lid and the gas mixture was used to calibrate the chromatograph in % of each gas (O29.5%, CO29.5%, N276.2%, N2O 4.8%).
2.6 Statistical analysis
Data were analysed statistically by analysis of variance (at P < 5%) and determination of least significant difference (LSD at P<0.05) using XLStat. Pro® statistical software.
3 - RESULTS AND DISCUSSION
3.1 Effect of N2O on sprouting
Effect of nitrous oxide on sprouting of onion bulbs was not highly effective (figure 1). After 12 weeks at 20°C, bulbs treated with 25% of N2O showed a per- cent of sprouting similar to control bulbs. Whereas other treated bulbs showed a percent of sprouting varying between 33% and 35%, except bulbs treated with 80% of N2O where sprouting was 42% after 12 weeks.
Analysis of variance indicated that no significant difference (at ρ < 5%) appeared between control and 25%, 50% and 80% N2O treated bulbs, howe- ver a significant difference was observed between these lots and 100% treated bulbs. No significant difference was observed between bulbs treated with 100%
N2O for two and four days.
Sprouting of onion bulbs is controlled by hormonal balance (ABDALLAHand MANN1963; THOMASand ISENBERG, 1972) and much influenced by temperature (ABDALLAH and MANN, 1963; BENKEBLIA and SELSELET-ATTOU, 1999a), but no data are available on the effects of noble gases on sprout development or inhi- bition. However, according to BENKEBLIAand SELSELET-ATTOU(1999b) ethylene plays a secondary role in sprouting of onion bulbs, when slowing down of the respiration by nitrous oxide as reported by SOWA et al. (1987) and SOWA and TOWIL(1991) did not seem to affect the development of sprouting. According to CUQ et al. (1993), no effect was noted in the presence of high pressure of nitrous oxide (6 bars) on the inactivation of peroxidase enzyme which is linked to phenolic variation and dormancy of onion bulbs (BENKEBLIA and SELSELET- ATTOU, 1999a).
3.2 Effect N2O on rotting
On the other hand, effect of nitrous oxide on rotting of bulbs at 20°C was more effective and visible and this effect seems to be proportional to dose rate of N2O on the development of moulds (figure 2). After 12 weeks levels of mould were proportional to the dose rate of N2O, varying between 13% and 30%, except for bulbs treated with 80% of N2O where rotting was 26%.
Analysis of variance showed that no significant difference (at ρ < 5%) was observed between the lots except bulbs treated with 100% during two days where rotting level was significantly different.
Nitrous oxide is a potent inhibitor of bacterial growth (FENN and MARQUIS, 1968). According to THOMand MARQUIS(1984), N2O inhibits microbial growth of Escherechia coli, Saccharomyces cerevisiae and Tetrahymena thermophila, and the observed inhibition was proportional to N2O pressure. Nitrous oxide is thought to sensitise microorganisms to antimicrobial agents, and this sensitisa- tion is not yet clarified but may involve alteration of the membrane fluidity of microbial cell walls with subsequent influence on cell function (DAY, 1998).
Figure 1
Effect of nitrous oxide on sprouting of onion bulbs (LSD at P<0.05)
Encouraging results were obtained with application of noble gases including nitrous oxide. Nevertheless more investigations are needed to better unders- tand the potential beneficial effects of these gases. On the other hand the meta- bolic pathways and cellular response to nitrous oxide remain yet unclear and more research is needed to clarify these biochemical aspects.
Received 6 December 2000, accepted 16 February 2001.
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Figure 2
Effect of nitrous oxide on rotting of onion bulbs (LSD at P < 0.05)
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