Onion landraces for organic farming
Due to its unique taste and scent, the common onion (Allium cepa L.) is the second most widely cultivated vegetable globally after the tomato. In Germany, onions are similarly popular. Onion bulbs contain high levels of plant secondary metabolites and fructans which are associated with health-promoting effects.
Secondary plant metabolites are formed by plants in addition to the basic substances of metabolism, the so-called primary metabolites. Secondary metabolites often fulfil specific tasks in the plant organism, e. g. as flower or fruit pigments or in defence against predators or harmful microorganisms. In contrast to primary metabolites, secondary plant metabolites are usually molecules with a complex chemical structure.
Fructans are oligosaccharides that are composed of sucrose linked to additional fructose units. In plants, fructans function as storage carbohydrates; they are classified as dietary fibres and as such have digestive properties.
As with other vegetables, the demand for organically produced onions is constantly rising and this demand should be met by regionally produced crops. In this regard, one major obstacle is the limited availability of suitable onion cultivars and organic seeds. But why is this the case?
There are many different onion varieties with different characteristics, but consumers usually don't get to see much of this diversity: In the supermarket, a distinction is only made between ‘yellow’ and ‘red’ onions and shallots. Further, modern hybrid varieties are predominantly cultivated and marketed in Germany. Hybrid cultivars are adapted to the conditions in conventional farming and achieve high yields under these conditions. In addition, hybrids are not open-pollinated and cannot be replanted - which means that farmers have to keep buying new seed. The dominant use of hybrid varieties is criticised because it leads to the displacement of other varieties and could endanger biodiversity. There are also occasional reports that vegetables from hybrid varieties have comparatively little flavour and aroma.
Hybrid cultivars
A time-consuming procedure is needed to create a hybrid cultivar: Two inbred lines are established over several years and finally crossed under controlled conditions. The offspring of the first filial generation (F1) are robust, uniform and high-yielding but this effect disappears in subsequent generations. To generate hybrids quickly and effectively, the cytoplasmatic male sterility (CMS) is often exploited. This frequently occurring natural phenomenon is caused by mutations in certain parts of the DNA which result in male sterility: The affected plants are, e.g., unable to produce fertile pollen. This feature is helpful with regard to seed production because it prevents an undesired self-fertilisation of the maternal line. With plants or cultivars where CMS does not occur naturally, it can be evoked in the laboratory by using cell fusion techniques to include genetic material from other closely related species. In this way, CMS hybrids are produced.
In contrast to the modern hybrid cultivars, the so-called landraces have been established by small-scale breeding over hundreds of years. Landraces are open-pollinated, genetically diverse cultivars which are usually of regional origin. The large palette of landraces opens the possibility for the selection of cultivars with an optimal adaptation to different growing sites or regions including their further optimisation by means of organic breeding programmes. The diversity of the landraces is also mirrored by their different flavour characteristics: there are mild or medium-pungent or very pungent cultivar as well as very hot ones.
Landraces are of special interest for organic farming because here the so-called site adaptation plays a larger role: it is especially important to use cultivars that are adapted to the conditions of the respective growing area (soil properties, crop rotation, climate, rainfall, etc.). Hybrids fulfil this requirement not or only to a limited extend. According to current German and EU regulations, hybrids as well as CMS hybrids are allowed and indeed still used in organic farming. However, organic farming organisations are critical of the use of hybrid cultivars and the use of CMS hybrids has now been banned by several organisations. However, because conventional breeding has mainly developed hybrids in recent decades, the range of cultivars that can be used for organic farming is still limited.
Since 2016, the Max Rubner Institute has been researching the quality characteristics of onion landraces together with the University of Hohenheim (Workgroup of Prof. Zörb, Institute of Crop Sciences, Chair of Plant Product Quality). As part of a pilot project, 10 Central and Western European landraces were initially compared with regard to their suitability for domestic organic farming and the storability of the onions produced. The metabolite profiles of the cultivars were very different in some cases; in addition, the composition of the onions changed significantly in all varieties during storage. The highest-yielding, very mild variety Jaune des Cévennes also had the lowest storability, which was clearly related to the metabolite profile (see [1,2]).
The following ZwiebÖL project aims at directly comparing the suitability of landraces and hybrids for organic farming. Core of the projects are field trials at two different sites in three years (2020/2021) under organic farming conditions with 4-6 cultivars each (landraces and hybrids). Yield and quality parameters, the sensory properties, the microbiological quality as well as the storability of the bulbs will be determined. Further, the alleged positive effect of the so-called condensation drying on bulb storability will be evaluated. Fresh as well as stored bulbs will be analysed comprehensively to characterise the cultivar-specific profiles of volatile and non-volatile onion constituents including their storage-related changes. For this, especially untargeted GC-MS, GCxGC-MS und LC-MS platforms will be used. In a separate work package, the onion microbiota, which is made up of many different types of bacteria and fungi, is also being analysed. A current sub-project is dedicated to the question of whether organically and conventionally produced onions can be differentiated from each other using various analytical methods, e.g. metabolome analysis, stable isotope analysis, elemental analysis or high-throughput sequencing of the microbiota. In parallel to the field trials, a survey was performed in 2023 to describe the current state of onion cultivation in Germany. A specific aim was here to find out which onion cultivars are typically grown in different German growing regions in the conventional as well as the organic sector. Finally, in collaboration with the farming associations, the results are currently communicated to the farmers in order to put the acquired knowledge into practice.
Initial results: The field trials showed that, on average, yields comparable to those of the hybrid cultivars can be achieved with the landraces. At the same time, the landraces have good quality parameters. The storage trials showed that the landrace “Birnenförmige” can be stored for longer than the other varieties. An online survey showed that currently only 25% of the onion cultivars grown are open-pollinated. For onion growers, characteristics such as shelf life, yield, field resistance and external quality continue to be the most important factors when selecting varieties - flavour and aroma, on the other hand, were mentioned less frequently. Many onion producers showed interest in open-pollinated cultivars, but did not feel sufficiently informed about landraces - there is simply a lack of experience in handling these varieties [3]. In trials on soil salinisation, onions unexpectedly proved to be largely insensitive to higher salt levels, with the landrace “Birnenförmige” again proving to be particularly resistant [4,5]. The diversity of bacteria and fungi on the onions was surprisingly high and depended on the growing location. Several previously unknown bacterial species were found and are currently being characterised in detail [6,7]. In particular, the data from the extensive metabolite analyses are currently being evaluated and published.
The ZwiebÖL project (term 2025-2025) contributes to the implementation of the Strategy for the Future of Organic Farming of the German Federal Ministry of Food and Agriculture (BMEL). The project is funded by the BMEL according to a decision of the German Bundestag within the scope of the Federal Scheme for Organic Farming and Other Forms of Sustainable Agriculture (BÖLN, funding code 2819OE019).
Scientific publication(s)
- Romo-Pérez, M. L., Weinert, C. H., Häußler, M., Egert, B., Frechen, M. A., Trierweiler, B., Kulling, S. E., Zörb, C. (2020). "Metabolite profiling of onion landraces and the cold storage effect." Plant Physiology and Biochemistry 146: 428-437.
(https://doi.org/10.1016/j.plaphy.2019.11.007) - Romo Pérez, M. L., et al. (2018). "Quality aspects in open-pollinated onion varieties from Western Europe." Journal of Applied Botany and Food Quality 91: 69–78.
(https://ojs.openagrar.de/index.php/JABFQ/article/view/8535) - Romo-Pérez, M. L., „Starke Zwiebel-Landsorten – haltbar und voller Geschmack“ bioland 12/2023: 32-33
- M. L. Romo-Pérez, C. H. Weinert, B. Egert, B. L. Franzisky, S. E. Kulling and C. Zörb, Sodium accumulation has minimal effect on metabolite profile of onion bulbs. Plant Physiology and Biochemistry 2021 Vol. 168 Pages 423-431.
(https://doi.org/10.1016/j.plaphy.2021.10.031) - M. L. Romo-Pérez, C. H. Weinert, B. Egert, S. E. Kulling and C. Zörb, The tale of two Ions Na+ and Cl−: unraveling onion plant responses to varying salt treatments. BMC Plant Biology 2024 Vol. 24 Issue 1 Pages 1022.
(https://doi.org/10.1186/s12870-024-05719-9) - D.A. Stoll, C. Grimmler, B. Hetzer, S. E. Kulling and M. Huch (2023), „ Rathayibacter rubneri sp. nov. isolated from Allium cepa var. Rijnsburger, an onion landrace“ International Journal of Systematic and Evolutionary Microbiology 73:4 , 005811.
(https://doi.org/10.1099/ijsem.0.005811) - D. A. Stoll, C. Grimmler, B. Hetzer, A. Masoura, S. E. Kulling and M. Huch (2024), „Bosea rubneri sp. nov. Isolated from Organically Grown Allium cepa “, Current Microbiology 81:7 212.
(https://doi.org/10.1007/s00284-024-03717-6)
Further Information
